CA1073128A - Uhf tuning circuit utilizing a varactor diode - Google Patents
Uhf tuning circuit utilizing a varactor diodeInfo
- Publication number
- CA1073128A CA1073128A CA278,677A CA278677A CA1073128A CA 1073128 A CA1073128 A CA 1073128A CA 278677 A CA278677 A CA 278677A CA 1073128 A CA1073128 A CA 1073128A
- Authority
- CA
- Canada
- Prior art keywords
- uhf
- impedance
- coupled
- tuned circuit
- electrode
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
- H03J3/00—Continuous tuning
- H03J3/24—Continuous tuning of more than one resonant circuit simultaneously, the circuits being tuned to substantially the same frequency, e.g. for single-knob tuning
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
- H03J3/00—Continuous tuning
- H03J3/02—Details
- H03J3/16—Tuning without displacement of reactive element, e.g. by varying permeability
- H03J3/18—Tuning without displacement of reactive element, e.g. by varying permeability by discharge tube or semiconductor device simulating variable reactance
- H03J3/185—Tuning without displacement of reactive element, e.g. by varying permeability by discharge tube or semiconductor device simulating variable reactance with varactors, i.e. voltage variable reactive diodes
Abstract
RCA 69,154 UHF TUNING CIRCUIT
UTILIZING A VARACTOR DIODE
Abstract of the Disclosure In the UHF tuning portion of a television receiver, a varactor tuned circuit is coupled to the gate electrode of a field effect transistor (FET) through an impedance trans-formation network comprising a series connected capacitor and shunt connected inductor. In order to increase the signal power transferred between the tuned circuit and the FET at the lower end of the UHF range, the values of the capacitor and inductor are selected so that the relatively low value of equivalent parallel impedance exhibited by the tuned circuit at the lower end of the UHF range is impedance transformed to a value approximately equal to the value of the impedance exhibited at the gate of the FET. A second impedance transformation network, similar to the first, is coupled between the drain of the FET and a second varactor tuned circuit to further increase the signal power transferred between the FET and the second tuned circuit at the lower end of the UHF band. the values of the capacitor and inductor comprising the impedance transformation networks are also selected so that the signal power gain of the UHF
tuning portion at the upper end of the UHF range is not substantially degraded. Because of the impedance trans-formation networks, resonant points below the lowest frequency in the UHF band are established.
UTILIZING A VARACTOR DIODE
Abstract of the Disclosure In the UHF tuning portion of a television receiver, a varactor tuned circuit is coupled to the gate electrode of a field effect transistor (FET) through an impedance trans-formation network comprising a series connected capacitor and shunt connected inductor. In order to increase the signal power transferred between the tuned circuit and the FET at the lower end of the UHF range, the values of the capacitor and inductor are selected so that the relatively low value of equivalent parallel impedance exhibited by the tuned circuit at the lower end of the UHF range is impedance transformed to a value approximately equal to the value of the impedance exhibited at the gate of the FET. A second impedance transformation network, similar to the first, is coupled between the drain of the FET and a second varactor tuned circuit to further increase the signal power transferred between the FET and the second tuned circuit at the lower end of the UHF band. the values of the capacitor and inductor comprising the impedance transformation networks are also selected so that the signal power gain of the UHF
tuning portion at the upper end of the UHF range is not substantially degraded. Because of the impedance trans-formation networks, resonant points below the lowest frequency in the UHF band are established.
Description
RCA 69,154 i ~3~
. . .
1 The present invention relates to tuning systems suitable for television receivers and particularly to U~F
(ultra high frequency) tuning systems. . .
Recently, much interest has developed in the use of .. 5 varactor tuning diodes in tuning apparatus suitable for use,: .
for example, in television receiversO Since varactor diodes .
; are voltage controlled devices, television receivers employ-.
ing such devices may be tuned electronically rather than by :. mechanical means, thereby improving their performance and ~`
10 reliability. Field effect transistors tFET's),which a~e .
readiIy gain controlled, provide a relatively good noise figure and produce a minimum of cross modulation, have been utilized in conjunction with varactor diodes in VHF
(very high frequency) tuning systems.
Unfortunately, field effect transistors have ~:
~ tended to exhibit rather large parasitic input and output .
; . -capacitances and as a result they have not been widely .: . , . :employed in conjunction with varactor diodes in UHF tuning . . circuits. Specifically, the capacitances associated with ..20 field e~fect transistors have until recently been of the same . order of magnitude as the desired capacitances of varac~or . diodes for use in the UHF range. As a result, a tuning :. circuit comprising a~varactor~diode and an FET could not , .. ~ readily~be arranged to be tuned over a sufficiently wide ~ , .
~and in the UHF range.
Furthermore, because the equivalent parallel .
.
. impedance of a tuned circuit including a typical varactor diode selected for use in ~ UHF tuned circuit is low at the ~low end of. the UHF range while an FET is a relatively high .30 impedance device, the signal power gain of a tuning circuit . -2 ' ~ - RCA 69,154 ~ 3731;~!3 .
employing a varactor tuned circuit coupled! for example, to the gate electrode of an FET will be rather low at the low end of the band. Therefore, it is des,irable to provide - a UHF tuning circuit including a varactor tuned circuit and 5 an FET which has an improved signal power gain at the low' ' :~:
end of the UHF band. Furthermore, since the Q (and the ' ' ~ equivalent impedance) of a varactor tuned circuit increases '' ,:
' as a function of frequency, the UHF tuning system employing a varact~r tuned circuit and an FET should desirably im-p'rove - - .
1O' 'the signal power gain at the low end of the UHF range while not significantly degrading such power gain at the high end of the UHF range.
In accordance with the present invention, a tuned circuit means including a varactor diode is coupled to one electrode of a field effect transistor through an-impedance - :~
transformation means. The impedance transformation means ~ ' -' ' ~ . . ' ' ~. ' comprises a capacitor coupled serially between the tuned '~ -~; circuit meanfi and the electrode and an inductor coupled in .
shunt with the electrode. The values of the capacitor and ;
induc~or are selected so that at the low end of the UHF
range the impedances of the fleld effect transistor at the ' ~ ~`
e~ectrode and the impedance of the'combination of the tuned ~; ~ circuit means and the impeda'nce transormation means at the 'e1ectrode are approx1mately matched. ~ -~' 25 In accordance with still another feature of ~he ~ ; 'present invention, the values of the capacitor and the' induc~or~are selected so that at the high end of the UHF' range the impedances of the field effect"transistor a't the .
elPctrode and the impedance of the combination of the tuned ~- 30 circuit means and the impedance transformation means at the ~ ~ .
: ~ ' . . .
, ~'` ` `
' RCA 69,154 ~ ` 107~128 ;~
e.lectrode are approximately matched.
' In accordance with another feature of the present `
; invention, the capacitor and inductor provide a resonance at ' . a frequency below the lowest frequency in the UHF.range.
In accordance with still another feature of the .
. : ; present invention, a first impedance transformation network '' ~ is coupled between a fi.rst:tuned circuit means includin~ a .
first varactor d1ode and a~gate electrode of the field effect .
' transistor and a second impedance transformation network ..
' 10 ~ 1nclud1ng'a second varactor diode is coupled between the drain electrode of the field effect transistor and a second' t.uned circuit means including a second varactor diode.
These and other features of the present invention ~ ~ may best.be understood by reference to the following~detailed ' 15 description and accompanying drawings in which - , . - .:
- FIGURE l shows partially in block diagram form.and . -part1aily in sahematic diagram form a television tuning system : ~ . . . , , ~ . . . .
.:'-.employing a UHF tuning circuit constructed in accordance wi'th ' '` ': the pre'sent invention; and : ~ 20'. '' FIGURES 2A and 2B show graphical represéntations of .
:~ ~ amplitude versus '~requency characteristics associated with .
. : the UHF tuning c1rcuit shown in FIGURE 1.
In the television receiver of FIGURE l, UHF.broad- ~
~ :;. cast carriers bearing televlsion information are received by '' :~ 2S~ a UHF antenna network 12. The WHF aarriers are caupled to '. ' . ~an lnductor 14, one end of whiah is coupled through a 9 ` ~ relatively high value res`istor 15 to ground. UHF signal.s are ''~ " . . magnetiaally coupled from inductar 14 to an induator 16. .
:'j' '.. ' One end of inductor 16 is coupled to ~round. Inductor 14 , 30 and inductor 16 are arranged in what is someti~es cal.Led a;~ . ,.
: ~ ' ' 4 ' ' .~, ' . ~.
.
; . ~ RCA 69,154 : :
- :1073~
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1 Balun circuit to convert double ended (i.e., not referenced .to ground) input signals provided via arltenna network 12 ' , - into a single ended ~i.e., referenced to ground) signal.
Inductor 16 is coupled in parall.el with the series 5 :,combination of a varactor 18 and a capacitor 20 to form a , ~parallel resonant tuned clrcuit 22. A capacitor 24 serially ::' couples the ~unction of varactor 18 and inductor 16.to the :. : . gate~Gl of an N-channel field effect transistor.(FET) 26.
. .- "' An induator 28 lS coupled in shunt with the gate Gl'of FET 26. .
io Series capaci,tor 24 and shunt inductor 28 form an impedance .
~transformation network 30 to improve the signal power gain of , ' .. . . . . .
the UHF portion"of the tuning system, as will be eXplained. :
Inductor 28'is provided with bias voltage from a band ' selection unit 33 via a reslstance network comprising ; 15 resistors 32 and 34. A bypass capacitor 36 effectiv~ly:~ '. ' ,- . , - couple~ the junction of inductor 2a and resistor 34't-o ' ,~
- ,~ i ground for signals in the UHF range. , , . ~ ...
'~ ~ ' . The source S of FET ~6 is provided with suppIy .. . . ~ ~ . . .
, ~..., ,voltage from band selection unit 33 by means of.a:resist'ance, ~i ~ ... . . . .
` .~0. network'com~rising resistors 38 and 40. The source is ~ -. , ,.~ .. . , : .
,~ . ef,fectlvely coupled to ground in 'the UHF band by virtue of a ' ;bypass capacitor 42. ' ~ : :
' .The drain D of FET 26.is coupl,ed to a'tuned circuit ' ~. ., , . ~ . . ~, ,.-, . . 56, through an impedance transformation network 50. Network , . ,. 25 5.0 i:s siml1ar to impedanoe transformation.network 30 and .~:, ~! ' ' ' comprises a shunt connected inductor 54 and a serie~ .
'. . connected capacitor 52. Inductor 54. is effectlve,ly coupled ;' . ~ ; ,to ground for frequencies in~the U~F band by a bypass capacitor 55., Supply voltage is coupled to the' drain D of FET 26 from band se.lection Unit 33 through inductor; 54.
~ ' '' ' ' ' -S- , , , . . . .
RCA 69',154 ~ , 73~LZ8 ;
..~
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I Automatic gain control (AGC) voltages generated by an automatic gain control unit 46 are coupled to the second , gate G2 of FET 26 throùgh resistor 44. The second gate G2 .
,is effectively bypassed to ground far signals in the UHF
5 range by a bypass capacltor 48. - ;
" ' - Tuned circuit 56 comprises an inductor 60 coupled ' ' ' in parallel with the series combination of a varactor diode 98 and ~a capacitor 62., Inductor 60 is magnetically,coupled - to~an inductor 64, the latter being co~upled in parallel ~ith ' 10' , the series combinat'ion of a varactor diode 68 and a capacitor , 70 to form another tuned circuit 72. Inductor 60 is also .
electrically coupled to inductor 64, through a capacitor 66~
Tuned circuits 56 and 72 are arranged to form what is common-.
ly called a doubly tuned circuit.
Inductor 60 is in addition magnetically coupled to ' a var,iable inductor 74 to supply amplified UHF signals to a .
, ~HF mixer 76. Local oscillator signals are coupled ~to UHF
,; mixer 76 from ~ UHF local oscillator 80. Local oscillator 80 `i ` , includes another tuned circuit 81 comprising an inductor 8~ ~ ~
20 coupled in parallel with the series combination of a ' ,,`
.
"~ ' '' varactor diode 82 and a UHF CapacitQr 86. Tuned circuit 81' ;, ' determines the frequency of oscillation of local oscillator ' ao.: ~ .
.:.,. ~. .. .
UHF'mixer 76 combines the local osaillator signal , 25 ~ provided,by UHF local oscillator 80 and th amplified UHF
, ~ 'signal pr~vided by the UHF amplifier including FET 26 to ~ .. . . .
' derive an'intermediate freguenay (IF) signal. The IF signal ' ~ is'coupled to an IF processing unit 100 where it i~ clmplified in,accordance with an~AGC voltage provid~d by AGC uni.t 46 and filtered. The output signal of IF processing unit lOO is . ' , ' .
: . RCA 69,I54 ,' ~.~73~LZ8 ~
..
1 coupled to a video processing unit 102',ancl to a sound processing unit 106. Video processing unit 102 processes :' chrominance and luminance and synchronizat:ion signals derived from the IF signal to form an image on kinescope 104 ' 5 while sound processing ùnit 106 processes the sound signals . . .: , ' : - ' -derived from the IF signal which are converted ~o audible signals by speaker 108.
- :: ' Varactor diode 18 of tuned circuit 22l varactor '~
. : . diode 58 of tuned circuit 56, varactor diode 68 of tuned ' 10 circuit 72 and varactor diode 82 of tuned circuit 81 are ., , - ,.
''respectively coupled to varactor control unit 88 through res,istors 90, 92, 94 and 96. Varactor diodes 18, 58, 68 ~ , and 82 are poled to be reverse biased by a control voltag,e, provide~ by varactor control unit 88 so they act as capacitors whose values are controlled in accordance with the control . voltage~ The capacitance of varactor diodes ~18, 58, 68 and 82 incr'eases in an inverse relationship with the control . voltage. ' ~ :
:~ ' VHF (very high frequency) signals are coupled,to the receiver through a VHF antenna network 110, amplified by , ' a VHF amplifier 112'and combined with local oscillator '' signals generated by a VHF local oscillator 116 in a VHF
1 , , : , , .
: : mixer ll~ to form an IF signal. ~he IY signaI derived by VHF , . : 'mixer 114 is proce~sed by IF procesfiing unit 100, video ' pro~cessing unit:102 and sound procesfiing unit 106 in a similax manner to the manner in which the IF signal derived ~ b~ UHF mixer 76 is processed. Band sele~tion, varactor :~ ~ control voltages and AGC volbages are coupled to the VHF
:~ tuning portion of the receiver from band selection unit 33, ' 30 "
.: . varactor control unit 88 and AGC unit 46, respectiveLy.
~ -7-, ' . ' ' , .
~, .
RCA 69,154 ~L~731Z8 ' 1 - Portions of the receiver shown in FIGURE 1 may comprise circuits shown in RCA Television Service Data, File 1975 C-10 for the CTC-74 chassis published by RCA
Corporation, Indianapolis, Indiana.
In operation, when a viewer selects a channel, .
band~selectlon unit 33 derives band control signals in ~accordance with the band in which the selected channel - resides. When a UHF channel has been selected, band seleetlon unit 33 couples a positlve supply voltage ~(e.g., +18 v.d.c.) to the UHF tuner which renders FET 26 conduetive. Furthermore, varactor control unit 88 couples a .
control volta~e in accordance with the selected channel (e.g., in the range of +1 v.d.c. to +26 v.d.c.) to varactor diodes 18, 58, 68 and 82. In response, tuned circuits 22, IS ~ 56 and 72 are tuned to resonate at the frequency of the RF
carrier corresponding to the selected channel (e.g., in the . . . ~
United Statesr tuned cireuits 22, 56 and-72 are tuned in~a range between 470 ~Hz and 890 MHz for channels 14 through 83) while tuned circuit 81 is tuned to the frequency of the ; ,. , . . :appropriate local oscillator signal for the selected ehannel (e.g., in the unlted States, tuned circuit 81 is tuned in a range between 517 MHz and g30 MHz for channels 14 through 83).
For any-particular channel, the resonant frequeney of *uned eireuit 22 is determined by the effective capacitance of the series combination of varactor diode 18 and capacitor 20 and ,:1 . , - .
the induetance of induetor 16. The resonant requeneies of , . . : , .
~ tuned circuits 56, 72 and 81 are simiIarly determined.
~ .
The UHF signals received by U~F antenna network 12 are filtered by tuned eircuit 22 in aeeordanee with the -30 seleeted channel, amplified by FET 26 in aceordanee with the ., .:, . , - ,,, ,-10731Z8 RCA 69,154 , , .
1 AGC volta~e provided by AGC unit 46 and further filtered by, the doubly tuned circuit comprising tuned circuits 56 and 72 also in accordance ,with the selected channel be~ore being `' ,' coupled to UHF mixer 76. Impedance transformation networks ,, ' 30 and 50 are provided to increase the signal power gain of ~"
the UHF amplifier comprising FET 26 at the low end of the UHF
range without significantly degrading the signal power gain at the high end of the UHF range in the following manne~
The equivalent circuit o tuned circuit 22 may be considered as comprising the parallel combination of an ideal inductor (i.eO, being purely inductive), an ideal capacitor '(i.e., being purely capacitive), and an ideal resistance l9 , (i.e., being purely resistive). Although the value of the ' equivalent parallel resistance is a unction of the ' resistances associated with inductor 16 and capacitor 20, it ;
, . . . . .
i5 primarily the resistance associated with varactor dlode 18.
- ~ ~ When tuned circuit 22 is resonant, i.e., when a ' ;, - , , channel in the UHF range is selecte,d, the combination'of the .
, ideal inductor and ideal capacitor has a relativel~ large ,' 2C ~impedance. As a,result, the amount of power transferred ., :
~ , from tuned circuit 22 to the FET 26 is determined by the :- , . . . .
, ' rel'ationship between the equivalent parallel resistance of ,tuned circuit 22 and the input impedance of FET 26 at gate ' .
G1. 'That~ lS,~ the equivalent parallel reslstance of tuned ,, 25 'circuit 22 may be thought of as a source impedance which ~, : sho,uld be at least approximately matched by the impedance ' , '~ at gate Gl of FET 26. The lmpedance of FET 26 at ga~e Gl is relatively large at the low end of the UHF band and decreases ,' as'a function o~ frequency. For example, an SFC-54.!1 d~a1' ' ~àte MOSFET (metal-oxide silicon ield eff0c~ transisl:or) . .
9_ , .
, . .
RCA G9tl54 .:
~L~73~
::
. ,',- ' ' ' ~
l available from Texas Instruments, Incorporated, suitable for : use as FET 26, has an input resistance 25, at its first gate, of approximately l,000 ohms a.nd an input capacitance 27 o appro~imately 2.1 picofarads at the low end of the UHF range, i.è., at:channel 14. The equivalent parallel resistance of tuned circuit 22 (approximately equal to the equivalent . parallel resistance of varactor diode 18) is relatively small .. at the low end of the UHF range and increases with ~requency.
For example, a parallel tuned circuit utilizing a BB-105B
varactor diode has an equivalent parallel impedance of .
200 ohms at the low end of the UHF range. The resistance : . o the equivalent parallel resistance increases as a function of frequency because the control voltage~ which reverse biases varactor diode 18, is increased as a function of lS frequency. Thus, if tuned circuit 22 were directly coupled : . . :
.. ~ to:gate~Gl of FET 26, the power transferred between tuned circuit 22 and gate Gl of FET 26 could be adequate at the high end of the UHF range, where the equivalent parallel : .
::. ' , .
: : . resistance of tuned circuit 22 and the inpu~ impedance of ~ :
20 FET 26 at gate Gl are approximately matched, but couId be ~ - .
intolerable at.the low end of the U~F range, where the ~- : equivalent parallel resistance of tuned circuit 22 and the . ~.input impe~ance of FET 26 and gate Gl are mismatched.
The power transferred between tuned circuit 22 and gatè Gl of FET 26 at the low end of the UHF range (iOeO, - ~ , . .
channel 14) is increased by coupling tuned circuit 22 to , : : .
gate Gl through impedance transformation network 30~ At the low end of the UHF range, impedance.transormation network 30 . effectively raises the impedance associated with tuned ~1rcuit 22 which is coupled to gate Gl to a value -10~
. , .
RCA 69,154 ~73~Z8 :
:`
1 approximate1y matching the input Lmpedance of FET 26 at ga.te . Gl. Alternatively, at the low end of the UHF range, impedance transformation network 30 may be thought of as lowering the impedance associated with FET 26 at ga~te Gl . : . .
.5 which is coupled to tuned circuit 22 to a value approximately matching the impedance of tuned circuit 22. ~.
'~'. . As pre~iously mentioned, while the impedances .~ . . . .. ~
associated with tuned circuit 22 are relatively:mismatched at the low end of the band, as the frequency of operation 10 increases, they get closer to being matched. ~herefore,~
while.the values of series capacitor 24 and shunt inductor 28 : . . . .
: : are'selected to'increase the power gain of the UHF amplifier ~ including FET 26 at the low end of the UHF range, they'are ; ' ' aiso selected so that the power gain at the high end of the .
'- :15 UHF.range is not significantly degraded. It has beeh found ' `. that by selecting the vaLues of capacitor 24 and i'nductor.28 ~,~ . . .
' . . to provide a resonance point at a frequency below the lowest frequenc~ of interest in the UHF band, e.g., in the United : .
~' States, 470 M~Hz, the power gain of the UHF amplifier of : .. 20 FIGURE 1 15 increased at the low end of the UHF range : while providing relatively little degradation'at the high ' ' ' ' . end of the UHF range. By seLecting the value of series ..
capacitor 24 in a range between 5.7 and'6.8'picofarads a'nd .' .~ .the value of shunt inductor 28 in a xange between 17 and 27 25: nanohenries,'the power gain of the UHF amplifier at the low :
.. ; end of the UHF range is increased with~ut significantly :
,. . . .
' ' degradlng the power gain at the high end of the UHF rangeO ' : Within.the ranges stated above, the values of series capacitance 24 and shunt inductor 28 may'be selected for an ; 30 optimum trade-off between power gain, noise figure, resonant . , ., .
-. - RCA 69,154 ~L6973~%~
..
I circuit tracking and stability.
-~ . Furthermora, even though semiconductor manufacturers : have.:recently learned how to reduce the parasitic capaci-. .tances associated with field effect transistors, such , ~; . 5 parasitic capacitances are still undesirably high compared ~ : ~
with the~capacitance of varactor diodes useful in UHF tuners... .~ ~.
For example, an SFC-5421 dual gate MOSFET, su1table for use as FET 26, has a capacitance at its first gate (i.e., G1) ` in the range of 2.1 picofarads while the.capacitance of the .
~10 BB-105 varactor diode has a capaci~tance range of 2-14 .picofarads for selected channels in the UHF range. Because ;I capacitvr 24 is in series with the parasitic capacitor.
,, .1 associated with gate Gl of FET 26, represented by a .1 . oapacitor 27, it effectively reduces the effect of the ~
parasitic capacitor associated with gate Gl on the capacitance ::
f varactor diode 18.
still ~further, shunt inductor 28 provides a ~ .
relatively low capacitance and low loss path for applying . . ;.
. .
. . supply voltage to gate Gi~ This is advantageous:since a 20 . resistori which could be used for supplying power to gate Gl, :,,, ' : .' , ; -~ would t~nd to have a higher capacitance and loss assoclated : .- with it than does an inductor.
~ .. . :- .
. .In.order to further improve the gain versus .
frequency characteristics of the UHF amplifier, impedance 25 transformation network 50 comprising series capacitor 92.and : i shunt inductor 54:is coup1ed between the drain D of FET 26 and tuned circuit 56~ Impedance transformation network.50 functions in a similar fashion to impPdance transformation : , . : . : , .ne:work 30~ At the low end of the UHF range, impedance transformation network~ 5b effectively raises the impçdance , ' , ' ' ~;
, - . : .,... . . . .. ,- . : .. . .
: ~073128 RCA 69,154 .
' ' I assoc1ated with tuned circuit 56 which is coupled to drain D
to a'value ap.proximately matching the outpu~ impedance of . FET 26 at drain D. To ensure that ~e power transferred - between drain D and tuned circuit 56 is increased at the low ~ '~
. .
end of the UHF range without significantly reducing the power transferred between drain D and tuned circuit 56 at the high . ~-end of.the UHF 'range, the values of shunt inductor 54 and series capacitor 52 are selected to provide a resonant point ':
below ~hè lowest frequency of interest, e.g., 470 MHz, in the ~':' ' 10 UHF range. For this purpose, shunt inductor 54 may be ' selected to have a value in a range between 25 and 37 .
.' nanohenries and series capacitor 52 may be selected to have ` a value in a range between 2.7 and 3.3 picofara.ds. ..
, ~ Furthermore, series capacitor 52 isolates the~relatively high ..
` 15' parasitic.capacitance'(e.g., l.S pico~arads for an SFC-542l .
. " MOSFET) associated with drain D from varactor diode 58.. In : ~ addition, shunt inductor 54, . ' provides a relatively low capacitancP path for coupling .I :supply.voltage to drain D.
io The voltage versus fre~uency response characteris't-ics for the voltage developed at gate Gl for channels l4 ànd ~, .
. 83 are~graphically represented in FIGURE 2A. It is noted that 'for each-characteristia,'ther~ are two resonant frequency points below the'frequency (~!g., ~70 MHz) assoGiated..wlth . : 25 channel 14. .The first re~onant point at 355 MHz is.due to . ~:
.:
impeda'nce transformation network 30 in the gate circ~it of ~ .FET~ 26.- The second r~esonant point at 390 MH~z is due, in ~ ..
~ e~sence, to a re~lection o~ impedance transormation network ,~
50 in the draln circuit of FET 26. It is'noted that the . 3 gains.for chahnels 14 and a3 are both relatively high...
: ~ ~ ' 13- ' '' ,' ,. ' ' , ' , ' .. : .
RCA 69,154 ~
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~0731Z~3 .
. I Nevertheless,~`the gain for channel 83 is somewhat lower.than ~ the gain for channel 14. However, it is seen from the : response characteristics for the voltage developed at drain - . . D of FET 26 of FIGURE 2B that the gains for channels 14 and .
.
,.' . 5- 83 are approximately the same. Once again, it is nQted that .: there are two resonant points below the lowest frequency of interest.in the UHF band (e.g'., 470 M~z) respectively ' 'corresponding to impedance transformation networks 30 and 50.`. ~.
: 1 , , ~ . ;; ' Furthermore, it is noted that the resonant frequency : .
~; 10 associated with impedance transformation netowrk 50 occurs ~' :~ ~ between the resonant frequency associated with impedance . :
transformation network 30 and the lowest frequency of . . interest in'the UHF range.
.:, . : Since the values of series capacitor 24, shunt '15 inductor 28, shunt inductor 54 and series ~apaci~or 52 are l ~ selected to provide resonant points below the lowest ;.
';. : frëquency of interest in the UHF band,~'as is indicated in .
' ~ FIGURES 2A and 2B, tuned circuits 22, 56, 72 and 81 can be made'to track each Qther through the entire UHF band. If :~.
the impedance transformation elements'were selected to ' provide resonant points above the lowest frequency of ; interest:in the U~F ~ange, tracking between the responses .
. - .: , : .
o resonant cirduits 22, 56,~.72 and 81 could.not be readi1y obtained because tuned circuits 22 and 56 would be affec.ted .
?5 by impedance transformation.networks 30 and 50, respectively,' in.the UHF~range, while tuned circuits 72 and 81 would not ..- be affected by the impedance transformation networ~
, .With the values shown in FIGURE 1 indicated above, .
' an overall gain'improvement of approximately 4 dB (decibel) ;~ ~ 3 may:be obtained. In addition, because the gain of the first :
.
1073~Z8 RCA 69,154 . ~
~I stage of the UHF tuner, i.e., the stage lncluding FET 26, is increased at the low end of the UHF ba:nd by the inclusion of the two impedance transformation circuits, the nolse figure of the U~F tuner is correspondingly improved.
, - ~
; . . . -, ~
' . ' ' .
. .
.
, ' ' .~ ' ., .
. , . : - :
:
, .. .
' .. ~ , ' ~ ' "
.
-~ -15 '
. . .
1 The present invention relates to tuning systems suitable for television receivers and particularly to U~F
(ultra high frequency) tuning systems. . .
Recently, much interest has developed in the use of .. 5 varactor tuning diodes in tuning apparatus suitable for use,: .
for example, in television receiversO Since varactor diodes .
; are voltage controlled devices, television receivers employ-.
ing such devices may be tuned electronically rather than by :. mechanical means, thereby improving their performance and ~`
10 reliability. Field effect transistors tFET's),which a~e .
readiIy gain controlled, provide a relatively good noise figure and produce a minimum of cross modulation, have been utilized in conjunction with varactor diodes in VHF
(very high frequency) tuning systems.
Unfortunately, field effect transistors have ~:
~ tended to exhibit rather large parasitic input and output .
; . -capacitances and as a result they have not been widely .: . , . :employed in conjunction with varactor diodes in UHF tuning . . circuits. Specifically, the capacitances associated with ..20 field e~fect transistors have until recently been of the same . order of magnitude as the desired capacitances of varac~or . diodes for use in the UHF range. As a result, a tuning :. circuit comprising a~varactor~diode and an FET could not , .. ~ readily~be arranged to be tuned over a sufficiently wide ~ , .
~and in the UHF range.
Furthermore, because the equivalent parallel .
.
. impedance of a tuned circuit including a typical varactor diode selected for use in ~ UHF tuned circuit is low at the ~low end of. the UHF range while an FET is a relatively high .30 impedance device, the signal power gain of a tuning circuit . -2 ' ~ - RCA 69,154 ~ 3731;~!3 .
employing a varactor tuned circuit coupled! for example, to the gate electrode of an FET will be rather low at the low end of the band. Therefore, it is des,irable to provide - a UHF tuning circuit including a varactor tuned circuit and 5 an FET which has an improved signal power gain at the low' ' :~:
end of the UHF band. Furthermore, since the Q (and the ' ' ~ equivalent impedance) of a varactor tuned circuit increases '' ,:
' as a function of frequency, the UHF tuning system employing a varact~r tuned circuit and an FET should desirably im-p'rove - - .
1O' 'the signal power gain at the low end of the UHF range while not significantly degrading such power gain at the high end of the UHF range.
In accordance with the present invention, a tuned circuit means including a varactor diode is coupled to one electrode of a field effect transistor through an-impedance - :~
transformation means. The impedance transformation means ~ ' -' ' ~ . . ' ' ~. ' comprises a capacitor coupled serially between the tuned '~ -~; circuit meanfi and the electrode and an inductor coupled in .
shunt with the electrode. The values of the capacitor and ;
induc~or are selected so that at the low end of the UHF
range the impedances of the fleld effect transistor at the ' ~ ~`
e~ectrode and the impedance of the'combination of the tuned ~; ~ circuit means and the impeda'nce transormation means at the 'e1ectrode are approx1mately matched. ~ -~' 25 In accordance with still another feature of ~he ~ ; 'present invention, the values of the capacitor and the' induc~or~are selected so that at the high end of the UHF' range the impedances of the field effect"transistor a't the .
elPctrode and the impedance of the combination of the tuned ~- 30 circuit means and the impedance transformation means at the ~ ~ .
: ~ ' . . .
, ~'` ` `
' RCA 69,154 ~ ` 107~128 ;~
e.lectrode are approximately matched.
' In accordance with another feature of the present `
; invention, the capacitor and inductor provide a resonance at ' . a frequency below the lowest frequency in the UHF.range.
In accordance with still another feature of the .
. : ; present invention, a first impedance transformation network '' ~ is coupled between a fi.rst:tuned circuit means includin~ a .
first varactor d1ode and a~gate electrode of the field effect .
' transistor and a second impedance transformation network ..
' 10 ~ 1nclud1ng'a second varactor diode is coupled between the drain electrode of the field effect transistor and a second' t.uned circuit means including a second varactor diode.
These and other features of the present invention ~ ~ may best.be understood by reference to the following~detailed ' 15 description and accompanying drawings in which - , . - .:
- FIGURE l shows partially in block diagram form.and . -part1aily in sahematic diagram form a television tuning system : ~ . . . , , ~ . . . .
.:'-.employing a UHF tuning circuit constructed in accordance wi'th ' '` ': the pre'sent invention; and : ~ 20'. '' FIGURES 2A and 2B show graphical represéntations of .
:~ ~ amplitude versus '~requency characteristics associated with .
. : the UHF tuning c1rcuit shown in FIGURE 1.
In the television receiver of FIGURE l, UHF.broad- ~
~ :;. cast carriers bearing televlsion information are received by '' :~ 2S~ a UHF antenna network 12. The WHF aarriers are caupled to '. ' . ~an lnductor 14, one end of whiah is coupled through a 9 ` ~ relatively high value res`istor 15 to ground. UHF signal.s are ''~ " . . magnetiaally coupled from inductar 14 to an induator 16. .
:'j' '.. ' One end of inductor 16 is coupled to ~round. Inductor 14 , 30 and inductor 16 are arranged in what is someti~es cal.Led a;~ . ,.
: ~ ' ' 4 ' ' .~, ' . ~.
.
; . ~ RCA 69,154 : :
- :1073~
. .
1 Balun circuit to convert double ended (i.e., not referenced .to ground) input signals provided via arltenna network 12 ' , - into a single ended ~i.e., referenced to ground) signal.
Inductor 16 is coupled in parall.el with the series 5 :,combination of a varactor 18 and a capacitor 20 to form a , ~parallel resonant tuned clrcuit 22. A capacitor 24 serially ::' couples the ~unction of varactor 18 and inductor 16.to the :. : . gate~Gl of an N-channel field effect transistor.(FET) 26.
. .- "' An induator 28 lS coupled in shunt with the gate Gl'of FET 26. .
io Series capaci,tor 24 and shunt inductor 28 form an impedance .
~transformation network 30 to improve the signal power gain of , ' .. . . . . .
the UHF portion"of the tuning system, as will be eXplained. :
Inductor 28'is provided with bias voltage from a band ' selection unit 33 via a reslstance network comprising ; 15 resistors 32 and 34. A bypass capacitor 36 effectiv~ly:~ '. ' ,- . , - couple~ the junction of inductor 2a and resistor 34't-o ' ,~
- ,~ i ground for signals in the UHF range. , , . ~ ...
'~ ~ ' . The source S of FET ~6 is provided with suppIy .. . . ~ ~ . . .
, ~..., ,voltage from band selection unit 33 by means of.a:resist'ance, ~i ~ ... . . . .
` .~0. network'com~rising resistors 38 and 40. The source is ~ -. , ,.~ .. . , : .
,~ . ef,fectlvely coupled to ground in 'the UHF band by virtue of a ' ;bypass capacitor 42. ' ~ : :
' .The drain D of FET 26.is coupl,ed to a'tuned circuit ' ~. ., , . ~ . . ~, ,.-, . . 56, through an impedance transformation network 50. Network , . ,. 25 5.0 i:s siml1ar to impedanoe transformation.network 30 and .~:, ~! ' ' ' comprises a shunt connected inductor 54 and a serie~ .
'. . connected capacitor 52. Inductor 54. is effectlve,ly coupled ;' . ~ ; ,to ground for frequencies in~the U~F band by a bypass capacitor 55., Supply voltage is coupled to the' drain D of FET 26 from band se.lection Unit 33 through inductor; 54.
~ ' '' ' ' ' -S- , , , . . . .
RCA 69',154 ~ , 73~LZ8 ;
..~
.
.
I Automatic gain control (AGC) voltages generated by an automatic gain control unit 46 are coupled to the second , gate G2 of FET 26 throùgh resistor 44. The second gate G2 .
,is effectively bypassed to ground far signals in the UHF
5 range by a bypass capacltor 48. - ;
" ' - Tuned circuit 56 comprises an inductor 60 coupled ' ' ' in parallel with the series combination of a varactor diode 98 and ~a capacitor 62., Inductor 60 is magnetically,coupled - to~an inductor 64, the latter being co~upled in parallel ~ith ' 10' , the series combinat'ion of a varactor diode 68 and a capacitor , 70 to form another tuned circuit 72. Inductor 60 is also .
electrically coupled to inductor 64, through a capacitor 66~
Tuned circuits 56 and 72 are arranged to form what is common-.
ly called a doubly tuned circuit.
Inductor 60 is in addition magnetically coupled to ' a var,iable inductor 74 to supply amplified UHF signals to a .
, ~HF mixer 76. Local oscillator signals are coupled ~to UHF
,; mixer 76 from ~ UHF local oscillator 80. Local oscillator 80 `i ` , includes another tuned circuit 81 comprising an inductor 8~ ~ ~
20 coupled in parallel with the series combination of a ' ,,`
.
"~ ' '' varactor diode 82 and a UHF CapacitQr 86. Tuned circuit 81' ;, ' determines the frequency of oscillation of local oscillator ' ao.: ~ .
.:.,. ~. .. .
UHF'mixer 76 combines the local osaillator signal , 25 ~ provided,by UHF local oscillator 80 and th amplified UHF
, ~ 'signal pr~vided by the UHF amplifier including FET 26 to ~ .. . . .
' derive an'intermediate freguenay (IF) signal. The IF signal ' ~ is'coupled to an IF processing unit 100 where it i~ clmplified in,accordance with an~AGC voltage provid~d by AGC uni.t 46 and filtered. The output signal of IF processing unit lOO is . ' , ' .
: . RCA 69,I54 ,' ~.~73~LZ8 ~
..
1 coupled to a video processing unit 102',ancl to a sound processing unit 106. Video processing unit 102 processes :' chrominance and luminance and synchronizat:ion signals derived from the IF signal to form an image on kinescope 104 ' 5 while sound processing ùnit 106 processes the sound signals . . .: , ' : - ' -derived from the IF signal which are converted ~o audible signals by speaker 108.
- :: ' Varactor diode 18 of tuned circuit 22l varactor '~
. : . diode 58 of tuned circuit 56, varactor diode 68 of tuned ' 10 circuit 72 and varactor diode 82 of tuned circuit 81 are ., , - ,.
''respectively coupled to varactor control unit 88 through res,istors 90, 92, 94 and 96. Varactor diodes 18, 58, 68 ~ , and 82 are poled to be reverse biased by a control voltag,e, provide~ by varactor control unit 88 so they act as capacitors whose values are controlled in accordance with the control . voltage~ The capacitance of varactor diodes ~18, 58, 68 and 82 incr'eases in an inverse relationship with the control . voltage. ' ~ :
:~ ' VHF (very high frequency) signals are coupled,to the receiver through a VHF antenna network 110, amplified by , ' a VHF amplifier 112'and combined with local oscillator '' signals generated by a VHF local oscillator 116 in a VHF
1 , , : , , .
: : mixer ll~ to form an IF signal. ~he IY signaI derived by VHF , . : 'mixer 114 is proce~sed by IF procesfiing unit 100, video ' pro~cessing unit:102 and sound procesfiing unit 106 in a similax manner to the manner in which the IF signal derived ~ b~ UHF mixer 76 is processed. Band sele~tion, varactor :~ ~ control voltages and AGC volbages are coupled to the VHF
:~ tuning portion of the receiver from band selection unit 33, ' 30 "
.: . varactor control unit 88 and AGC unit 46, respectiveLy.
~ -7-, ' . ' ' , .
~, .
RCA 69,154 ~L~731Z8 ' 1 - Portions of the receiver shown in FIGURE 1 may comprise circuits shown in RCA Television Service Data, File 1975 C-10 for the CTC-74 chassis published by RCA
Corporation, Indianapolis, Indiana.
In operation, when a viewer selects a channel, .
band~selectlon unit 33 derives band control signals in ~accordance with the band in which the selected channel - resides. When a UHF channel has been selected, band seleetlon unit 33 couples a positlve supply voltage ~(e.g., +18 v.d.c.) to the UHF tuner which renders FET 26 conduetive. Furthermore, varactor control unit 88 couples a .
control volta~e in accordance with the selected channel (e.g., in the range of +1 v.d.c. to +26 v.d.c.) to varactor diodes 18, 58, 68 and 82. In response, tuned circuits 22, IS ~ 56 and 72 are tuned to resonate at the frequency of the RF
carrier corresponding to the selected channel (e.g., in the . . . ~
United Statesr tuned cireuits 22, 56 and-72 are tuned in~a range between 470 ~Hz and 890 MHz for channels 14 through 83) while tuned circuit 81 is tuned to the frequency of the ; ,. , . . :appropriate local oscillator signal for the selected ehannel (e.g., in the unlted States, tuned circuit 81 is tuned in a range between 517 MHz and g30 MHz for channels 14 through 83).
For any-particular channel, the resonant frequeney of *uned eireuit 22 is determined by the effective capacitance of the series combination of varactor diode 18 and capacitor 20 and ,:1 . , - .
the induetance of induetor 16. The resonant requeneies of , . . : , .
~ tuned circuits 56, 72 and 81 are simiIarly determined.
~ .
The UHF signals received by U~F antenna network 12 are filtered by tuned eircuit 22 in aeeordanee with the -30 seleeted channel, amplified by FET 26 in aceordanee with the ., .:, . , - ,,, ,-10731Z8 RCA 69,154 , , .
1 AGC volta~e provided by AGC unit 46 and further filtered by, the doubly tuned circuit comprising tuned circuits 56 and 72 also in accordance ,with the selected channel be~ore being `' ,' coupled to UHF mixer 76. Impedance transformation networks ,, ' 30 and 50 are provided to increase the signal power gain of ~"
the UHF amplifier comprising FET 26 at the low end of the UHF
range without significantly degrading the signal power gain at the high end of the UHF range in the following manne~
The equivalent circuit o tuned circuit 22 may be considered as comprising the parallel combination of an ideal inductor (i.eO, being purely inductive), an ideal capacitor '(i.e., being purely capacitive), and an ideal resistance l9 , (i.e., being purely resistive). Although the value of the ' equivalent parallel resistance is a unction of the ' resistances associated with inductor 16 and capacitor 20, it ;
, . . . . .
i5 primarily the resistance associated with varactor dlode 18.
- ~ ~ When tuned circuit 22 is resonant, i.e., when a ' ;, - , , channel in the UHF range is selecte,d, the combination'of the .
, ideal inductor and ideal capacitor has a relativel~ large ,' 2C ~impedance. As a,result, the amount of power transferred ., :
~ , from tuned circuit 22 to the FET 26 is determined by the :- , . . . .
, ' rel'ationship between the equivalent parallel resistance of ,tuned circuit 22 and the input impedance of FET 26 at gate ' .
G1. 'That~ lS,~ the equivalent parallel reslstance of tuned ,, 25 'circuit 22 may be thought of as a source impedance which ~, : sho,uld be at least approximately matched by the impedance ' , '~ at gate Gl of FET 26. The lmpedance of FET 26 at ga~e Gl is relatively large at the low end of the UHF band and decreases ,' as'a function o~ frequency. For example, an SFC-54.!1 d~a1' ' ~àte MOSFET (metal-oxide silicon ield eff0c~ transisl:or) . .
9_ , .
, . .
RCA G9tl54 .:
~L~73~
::
. ,',- ' ' ' ~
l available from Texas Instruments, Incorporated, suitable for : use as FET 26, has an input resistance 25, at its first gate, of approximately l,000 ohms a.nd an input capacitance 27 o appro~imately 2.1 picofarads at the low end of the UHF range, i.è., at:channel 14. The equivalent parallel resistance of tuned circuit 22 (approximately equal to the equivalent . parallel resistance of varactor diode 18) is relatively small .. at the low end of the UHF range and increases with ~requency.
For example, a parallel tuned circuit utilizing a BB-105B
varactor diode has an equivalent parallel impedance of .
200 ohms at the low end of the UHF range. The resistance : . o the equivalent parallel resistance increases as a function of frequency because the control voltage~ which reverse biases varactor diode 18, is increased as a function of lS frequency. Thus, if tuned circuit 22 were directly coupled : . . :
.. ~ to:gate~Gl of FET 26, the power transferred between tuned circuit 22 and gate Gl of FET 26 could be adequate at the high end of the UHF range, where the equivalent parallel : .
::. ' , .
: : . resistance of tuned circuit 22 and the inpu~ impedance of ~ :
20 FET 26 at gate Gl are approximately matched, but couId be ~ - .
intolerable at.the low end of the U~F range, where the ~- : equivalent parallel resistance of tuned circuit 22 and the . ~.input impe~ance of FET 26 and gate Gl are mismatched.
The power transferred between tuned circuit 22 and gatè Gl of FET 26 at the low end of the UHF range (iOeO, - ~ , . .
channel 14) is increased by coupling tuned circuit 22 to , : : .
gate Gl through impedance transformation network 30~ At the low end of the UHF range, impedance.transormation network 30 . effectively raises the impedance associated with tuned ~1rcuit 22 which is coupled to gate Gl to a value -10~
. , .
RCA 69,154 ~73~Z8 :
:`
1 approximate1y matching the input Lmpedance of FET 26 at ga.te . Gl. Alternatively, at the low end of the UHF range, impedance transformation network 30 may be thought of as lowering the impedance associated with FET 26 at ga~te Gl . : . .
.5 which is coupled to tuned circuit 22 to a value approximately matching the impedance of tuned circuit 22. ~.
'~'. . As pre~iously mentioned, while the impedances .~ . . . .. ~
associated with tuned circuit 22 are relatively:mismatched at the low end of the band, as the frequency of operation 10 increases, they get closer to being matched. ~herefore,~
while.the values of series capacitor 24 and shunt inductor 28 : . . . .
: : are'selected to'increase the power gain of the UHF amplifier ~ including FET 26 at the low end of the UHF range, they'are ; ' ' aiso selected so that the power gain at the high end of the .
'- :15 UHF.range is not significantly degraded. It has beeh found ' `. that by selecting the vaLues of capacitor 24 and i'nductor.28 ~,~ . . .
' . . to provide a resonance point at a frequency below the lowest frequenc~ of interest in the UHF band, e.g., in the United : .
~' States, 470 M~Hz, the power gain of the UHF amplifier of : .. 20 FIGURE 1 15 increased at the low end of the UHF range : while providing relatively little degradation'at the high ' ' ' ' . end of the UHF range. By seLecting the value of series ..
capacitor 24 in a range between 5.7 and'6.8'picofarads a'nd .' .~ .the value of shunt inductor 28 in a xange between 17 and 27 25: nanohenries,'the power gain of the UHF amplifier at the low :
.. ; end of the UHF range is increased with~ut significantly :
,. . . .
' ' degradlng the power gain at the high end of the UHF rangeO ' : Within.the ranges stated above, the values of series capacitance 24 and shunt inductor 28 may'be selected for an ; 30 optimum trade-off between power gain, noise figure, resonant . , ., .
-. - RCA 69,154 ~L6973~%~
..
I circuit tracking and stability.
-~ . Furthermora, even though semiconductor manufacturers : have.:recently learned how to reduce the parasitic capaci-. .tances associated with field effect transistors, such , ~; . 5 parasitic capacitances are still undesirably high compared ~ : ~
with the~capacitance of varactor diodes useful in UHF tuners... .~ ~.
For example, an SFC-5421 dual gate MOSFET, su1table for use as FET 26, has a capacitance at its first gate (i.e., G1) ` in the range of 2.1 picofarads while the.capacitance of the .
~10 BB-105 varactor diode has a capaci~tance range of 2-14 .picofarads for selected channels in the UHF range. Because ;I capacitvr 24 is in series with the parasitic capacitor.
,, .1 associated with gate Gl of FET 26, represented by a .1 . oapacitor 27, it effectively reduces the effect of the ~
parasitic capacitor associated with gate Gl on the capacitance ::
f varactor diode 18.
still ~further, shunt inductor 28 provides a ~ .
relatively low capacitance and low loss path for applying . . ;.
. .
. . supply voltage to gate Gi~ This is advantageous:since a 20 . resistori which could be used for supplying power to gate Gl, :,,, ' : .' , ; -~ would t~nd to have a higher capacitance and loss assoclated : .- with it than does an inductor.
~ .. . :- .
. .In.order to further improve the gain versus .
frequency characteristics of the UHF amplifier, impedance 25 transformation network 50 comprising series capacitor 92.and : i shunt inductor 54:is coup1ed between the drain D of FET 26 and tuned circuit 56~ Impedance transformation network.50 functions in a similar fashion to impPdance transformation : , . : . : , .ne:work 30~ At the low end of the UHF range, impedance transformation network~ 5b effectively raises the impçdance , ' , ' ' ~;
, - . : .,... . . . .. ,- . : .. . .
: ~073128 RCA 69,154 .
' ' I assoc1ated with tuned circuit 56 which is coupled to drain D
to a'value ap.proximately matching the outpu~ impedance of . FET 26 at drain D. To ensure that ~e power transferred - between drain D and tuned circuit 56 is increased at the low ~ '~
. .
end of the UHF range without significantly reducing the power transferred between drain D and tuned circuit 56 at the high . ~-end of.the UHF 'range, the values of shunt inductor 54 and series capacitor 52 are selected to provide a resonant point ':
below ~hè lowest frequency of interest, e.g., 470 MHz, in the ~':' ' 10 UHF range. For this purpose, shunt inductor 54 may be ' selected to have a value in a range between 25 and 37 .
.' nanohenries and series capacitor 52 may be selected to have ` a value in a range between 2.7 and 3.3 picofara.ds. ..
, ~ Furthermore, series capacitor 52 isolates the~relatively high ..
` 15' parasitic.capacitance'(e.g., l.S pico~arads for an SFC-542l .
. " MOSFET) associated with drain D from varactor diode 58.. In : ~ addition, shunt inductor 54, . ' provides a relatively low capacitancP path for coupling .I :supply.voltage to drain D.
io The voltage versus fre~uency response characteris't-ics for the voltage developed at gate Gl for channels l4 ànd ~, .
. 83 are~graphically represented in FIGURE 2A. It is noted that 'for each-characteristia,'ther~ are two resonant frequency points below the'frequency (~!g., ~70 MHz) assoGiated..wlth . : 25 channel 14. .The first re~onant point at 355 MHz is.due to . ~:
.:
impeda'nce transformation network 30 in the gate circ~it of ~ .FET~ 26.- The second r~esonant point at 390 MH~z is due, in ~ ..
~ e~sence, to a re~lection o~ impedance transormation network ,~
50 in the draln circuit of FET 26. It is'noted that the . 3 gains.for chahnels 14 and a3 are both relatively high...
: ~ ~ ' 13- ' '' ,' ,. ' ' , ' , ' .. : .
RCA 69,154 ~
~ .
~0731Z~3 .
. I Nevertheless,~`the gain for channel 83 is somewhat lower.than ~ the gain for channel 14. However, it is seen from the : response characteristics for the voltage developed at drain - . . D of FET 26 of FIGURE 2B that the gains for channels 14 and .
.
,.' . 5- 83 are approximately the same. Once again, it is nQted that .: there are two resonant points below the lowest frequency of interest.in the UHF band (e.g'., 470 M~z) respectively ' 'corresponding to impedance transformation networks 30 and 50.`. ~.
: 1 , , ~ . ;; ' Furthermore, it is noted that the resonant frequency : .
~; 10 associated with impedance transformation netowrk 50 occurs ~' :~ ~ between the resonant frequency associated with impedance . :
transformation network 30 and the lowest frequency of . . interest in'the UHF range.
.:, . : Since the values of series capacitor 24, shunt '15 inductor 28, shunt inductor 54 and series ~apaci~or 52 are l ~ selected to provide resonant points below the lowest ;.
';. : frëquency of interest in the UHF band,~'as is indicated in .
' ~ FIGURES 2A and 2B, tuned circuits 22, 56, 72 and 81 can be made'to track each Qther through the entire UHF band. If :~.
the impedance transformation elements'were selected to ' provide resonant points above the lowest frequency of ; interest:in the U~F ~ange, tracking between the responses .
. - .: , : .
o resonant cirduits 22, 56,~.72 and 81 could.not be readi1y obtained because tuned circuits 22 and 56 would be affec.ted .
?5 by impedance transformation.networks 30 and 50, respectively,' in.the UHF~range, while tuned circuits 72 and 81 would not ..- be affected by the impedance transformation networ~
, .With the values shown in FIGURE 1 indicated above, .
' an overall gain'improvement of approximately 4 dB (decibel) ;~ ~ 3 may:be obtained. In addition, because the gain of the first :
.
1073~Z8 RCA 69,154 . ~
~I stage of the UHF tuner, i.e., the stage lncluding FET 26, is increased at the low end of the UHF ba:nd by the inclusion of the two impedance transformation circuits, the nolse figure of the U~F tuner is correspondingly improved.
, - ~
; . . . -, ~
' . ' ' .
. .
.
, ' ' .~ ' ., .
. , . : - :
:
, .. .
' .. ~ , ' ~ ' "
.
-~ -15 '
Claims (7)
1. In a UHF tuner, the apparatus comprising:
a tuned circuit including a varactor diode;
a field effect transistor having a plurality of electrodes including a gate electrode;
an impedance transformation means coupled between said tuned circuit and said gate electrode of said field effect transistor, said impedance transformation means including capacitance means coupled in series between said tuned circuit and said gate electrode and inductance means coupled in shunt with said gate electrode, said capacitance means and said inductance means being selected to provide a resonant frequency below the lowest channel frequency in the UHF range so that the impedance coupled to said gate electrode is approximately matched with the impedance of said field effect transistor at said gate electrode at the low end of the UHF range and the power gain of said apparatus at the low end of said UHF range is approximately equal to the power gain of said apparatus at the high end of said UHF range.
a tuned circuit including a varactor diode;
a field effect transistor having a plurality of electrodes including a gate electrode;
an impedance transformation means coupled between said tuned circuit and said gate electrode of said field effect transistor, said impedance transformation means including capacitance means coupled in series between said tuned circuit and said gate electrode and inductance means coupled in shunt with said gate electrode, said capacitance means and said inductance means being selected to provide a resonant frequency below the lowest channel frequency in the UHF range so that the impedance coupled to said gate electrode is approximately matched with the impedance of said field effect transistor at said gate electrode at the low end of the UHF range and the power gain of said apparatus at the low end of said UHF range is approximately equal to the power gain of said apparatus at the high end of said UHF range.
2. The apparatus recited in claim 1 wherein said tuned circuit includes an inductor coupled in parallel relation-ship with said varactor diode, said capacitance means of said impedance transformation means being coupled between said varactor diode and said gate electrode.
3. In a UHF tuned the apparatus comprising:
a first tuned circuit including a first varactor diode;
RCA 69,154 a field effect transistor having a plurality of electrodes;
a first impedance transformation means coupled between said first tuned circuit and a first electrode of said field effect transistor, said first impedance transformation means including first capacitance means coupled in series between said first tuned circuit and said first electrode and first inductance means coupled in shunt with said first electrode;
a second tuned circuit including a second varactor diode; and a second impedance transformation means coupled between a second electrode of said field effect transistor and said second tuned circuit, said second impedance means including second inductance means coupled in shunt with said second electrode and second capacitance means coupled in series between said second electrode and said second tuned circuit;
said first capacitance and inductance means and said second inductance and capacitance means being selected to provide respective first and second resonant frequencies below the lowest channel frequency in the UHF range so that the impedance coupled to said first electrode is approximately matched with the impedance of said field effect transistor at said first electrode at the low end of the UHF range, the impedance coupled to said second electrode is approximately matched with the impedance of said field effect transistor at said second electrode at the low end of the UHF range, and the power gain of said apparatus at the low end of said UHF range is approximately equal to the power gain of said apparatus at the high end of said UHF range.
RCA 69,154
a first tuned circuit including a first varactor diode;
RCA 69,154 a field effect transistor having a plurality of electrodes;
a first impedance transformation means coupled between said first tuned circuit and a first electrode of said field effect transistor, said first impedance transformation means including first capacitance means coupled in series between said first tuned circuit and said first electrode and first inductance means coupled in shunt with said first electrode;
a second tuned circuit including a second varactor diode; and a second impedance transformation means coupled between a second electrode of said field effect transistor and said second tuned circuit, said second impedance means including second inductance means coupled in shunt with said second electrode and second capacitance means coupled in series between said second electrode and said second tuned circuit;
said first capacitance and inductance means and said second inductance and capacitance means being selected to provide respective first and second resonant frequencies below the lowest channel frequency in the UHF range so that the impedance coupled to said first electrode is approximately matched with the impedance of said field effect transistor at said first electrode at the low end of the UHF range, the impedance coupled to said second electrode is approximately matched with the impedance of said field effect transistor at said second electrode at the low end of the UHF range, and the power gain of said apparatus at the low end of said UHF range is approximately equal to the power gain of said apparatus at the high end of said UHF range.
RCA 69,154
4. The apparatus recited in claim 3 wherein said first electrode is a gate electrode and said second electrode is a drain electrode.
5. The apparatus recited in claim 4 wherein the second resonant frequency is between the first resonant frequency and the lowest channel frequency in the UHF range.
6. The apparatus recited in claim 5 wherein said field effect transistor includes a second gate electrode to which an automatic gain control voltage is coupled.
7. The apparatus recited in claim 6 wherein a first band selection voltage is coupled to said first inductance means and a second band selection voltage is coupled to said second inductance means.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/691,189 US4048598A (en) | 1976-05-28 | 1976-05-28 | Uhf tuning circuit utilizing a varactor diode |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1073128A true CA1073128A (en) | 1980-03-04 |
Family
ID=24775511
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA278,677A Expired CA1073128A (en) | 1976-05-28 | 1977-05-18 | Uhf tuning circuit utilizing a varactor diode |
Country Status (2)
| Country | Link |
|---|---|
| US (1) | US4048598A (en) |
| CA (1) | CA1073128A (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
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| US9401696B1 (en) * | 2015-01-09 | 2016-07-26 | International Business Machines Corporation | Boosting varactor capacitance ratio |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1274204B (en) * | 1967-04-14 | 1968-08-01 | Blaupunkt Werke Gmbh | Input circuit for receiver for high-frequency electrical oscillations |
| US3693096A (en) * | 1970-12-01 | 1972-09-19 | Charles M Dosey | Antenna coupling and r.f. tuning circuit |
-
1976
- 1976-05-28 US US05/691,189 patent/US4048598A/en not_active Expired - Lifetime
-
1977
- 1977-05-18 CA CA278,677A patent/CA1073128A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| US4048598A (en) | 1977-09-13 |
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