WO2000014887A1

WO2000014887A1 - Electronic circuit for correcting cross modulation distortion

Info

Publication number: WO2000014887A1
Application number: PCT/US1999/020613
Authority: WO
Inventors: Martin Regehr
Original assignee: Ortel Corporation
Priority date: 1998-09-09
Filing date: 1999-09-09
Publication date: 2000-03-16
Also published as: AU5913599A

Abstract

An electronic circuit is used for correcting cross modulation distortion. Cross modulation distortion can occur in systems where several independently modulated RF carrier signals are transmitted along a shared transmission path. When one of the several carriers is intentionally amplitude modulated, the other carriers become unintentionally amplitude and/or phase modulated due to non-linearities in the transmission channel. The circuit is used in an RF transmission system (5), and comprises a modulator (16) and RF power monitor (18) coupled along an RF transmission path (14). A signal conditioning circuit (72), which may include an amplifier (20) is coupled on a path between the RF power monitor (18) and the modulator (16). In operation, the RF power monitor (18), detects fluctuations in the RF power and produces a signal that depends on the RF power. The signal is conditioned and fed to the modulator (16), which modulates the RF signal at the appropriate levels to cancel cross modulation distortion produced by other elements of the system.

Description

ELECTRONIC CIRCUIT FOR CORRECTING

CROSS MODULATION DISTORTION

FIELD OF THE INVENTION The present invention relates to predistorters for electronic and optical signals, and more particularly, to an electronic circuit for correcting cross modulation distortion.

BACKGROUND OF THE INVENTION

Cross modulation distortion can occur in systems where several independently modulated RF carrier signals are transmitted along a shared transmission path. When one of the several carriers is intentionally amplitude modulated, the other carriers become unintentionally amplitude or phase modulated due to non-linearities in the transmission path.

Gain compression is one mechanism by which elements in a transmission path can generate cross modulation distortion. Gain compression occurs because an amplifier transmitting an RF signal typically produces less gain at higher RF signal power than at lower RF signal power. The amount of gain compression that occurs depends on the total RF power being transmitted. In a case where the amplifier is transmitting two carriers, one unmodulated at the amplifier input and the other amplitude modulated, unintentional amplitude modulation may occur in the first carrier since the gain it experiences depends on the total RF power which includes the amplitude of the second carrier. Other mechanisms may also generate cross modulation distortion.

The phase delay through an amplifier may also depend on the total RF power being transmitted through it. Accordingly, unintentional phase modulation can also occur as a result of the amplitude modulation on another carrier. Cross modulation is essentially a third order distortion phenomenon, like CTB (composite triple beat). CTB consists of third order intermodulation products of the carriers (three at a time), and so too cross modulation can be viewed as intermodulation between the modulated carrier, a modulation sideband, and the unmodulated carrier. Since cross modulation is a third order process, most circuits which produce CTB also produce cross modulation distortion, and predistorters for reducing or eliminating CTB typically have the effect of also reducing cross modulation distortion.

Certain transmission systems, however, do not exhibit optimal suppression of cross modulation distortion when tuned for suppressing CTB. In such cases, it would be desirable to have a circuit that suppresses cross modulation distortion without substantially contributing to CTB or other types of distortion. SUMMARY OF THE INVENTION

There is therefore provided according to a presently preferred embodiment of the present invention, an electronic circuit for correcting cross modulation distortion in an RF transmission system. The circuit comprises a modulator and an RF power monitor coupled along an RF transmission path. A signal conditioning circuit, which may include an amplifier, is coupled on a path to the modulator and RF power monitor. In operation, the RF power monitor detects fluctuations in the RF power and produces a signal that depends on the RF power. The signal is conditioned and fed to the modulator, which modulates the RF signal at appropriate levels to cancel cross modulation distortion produced by other elements of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will be better understood in light of the following detailed description read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a general block diagram of an exemplary electronic circuit according to the present invention for correcting cross modulation distortion;

FIG. 2 is a circuit diagram of the presently preferred embodiment of an RF power monitor used in the circuit of FIG. 1 ;

FIG. 3 is a circuit model which illustrates the operation of the Schottky diode used in the RF power monitor of FIG. 2; FIG. 4 is a circuit diagram of a presently preferred embodiment of a modulator used in the circuit of FIG. 1 ;

FIG. 5 is a circuit diagram of an alternate embodiment of the modulator for modulating primarily higher frequencies;

FIG. 6 is a circuit diagram of yet another alternate embodiment of the modulator for phase modulating the RF signal for correcting amplitude-to-phase cross modulation distortion; and

FIG. 7 is a block diagram of the presently preferred embodiment of the signal conditioning circuit used in the circuit of FIG. 1.

DETAILED DESCRIPTION Referring to FIG. 1, an electronic circuit 5 according to the present invention for correcting cross modulation is coupled to an RF transmission path 12 that includes one or more non-linear devices, such as RF amplifier 26, that produce cross modulation distortion by gain compression or other mechanisms. The circuit 5 includes a modulator 16 and an RF power monitor 18 coupled to the RF transmission path, and a signal conditioning circuit 20 coupled between the RF power monitor and modulator through a path 22. The non-linear device 26 may be upstream or downstream from the correction circuit or may even be located between the RF power monitor and the modulator as illustrated in this drawing. The modulator is preferably placed at a point in the RF transmission path where the RF signal level is low whereas the RF power monitor is preferably placed at a point in the RF transmission path where the RF signal path is high. In operation, the RF power monitor detects fluctuations in the RF power and generates an RF modulation signal that depends on the RF power; the signal is then conditioned and fed back to the modulator with the appropriate levels to cancel cross modulation distortion. The RF amplifier is illustrated in FIG. 1 merely as an example of a possible non-linear device in the transmission path. Other devices producing cross modulation, several devices collectively producing cross modulation, or other causes of cross modulation, may be compensated by the techniques described herein.

Referring to FIG. 2, the RF power monitor 18 in the preferred embodiment comprises a resistive splitter 30 (consisting of resistors Rl, R2, and R3) for diverting a small amount of the RF signal from the main RF transmission path, a Schottky diode circuit 32 (consisting of capacitors CI , C2, and C3, inductor LI, Schottky detector diode Dl and resistor R6 connected to bias voltage V_B1AS) for detecting the RF power, and a lossy resistive network 34 (consisting of resistors R4 and R5), coupled between the splitter and the diode circuit, for providing further isolation between the main transmission path and the diode.

A directional coupler may be used instead of the resistive splitter 30. A directional coupler which attenuates the throughpath by the same amount as a given resistive splitter will generally provide a larger RF level to the detector diode and smaller coupling of detector intermodulation distortion to the forward propagating RF transmission path. In addition to the cross modulation distortion intentionally injected into the RF signal path by the modulator, non-linear elements in the cross-modulation correction circuit, such as diodes, can generate unwanted intermodulation distortion directly from the RF signals flowing through them. One or more conventional predistorter circuits may be used to cancel unwanted intermodulation distortion. The resistive network of the splitter/attenuator preferably provides a sufficiently large RF level to the detector that the RF power measurement is accurate, while also providing enough isolation so that any intermodulation distortion generated by the detector does not compromise signal quality after propagating back through the resistive network into the main RF transmission path. The diode circuit 32 includes resistor R6 and inductor LI for driving a DC bias current through the diode Dl to set its operating point. Preferably the bias current is selected to provide the maximum sensitivity to changes in RF power.

The RF power monitor 18 according to the present invention preferably has acceptable sensitivity over the frequency range where cross modulation is to be suppressed. The operation of the monitor can be understood qualitatively by considering the three types of current flowing through the Schottky diode Dl . The RF current flows through resistors Rl , R3, R5 and capacitor

CI , through diode Dl, and through capacitor C2 to ground. A DC bias current flows through resistor R6, diode Dl, and inductor LI to ground. The RF modulation signal is generated by the Schottky diode due to its non-linear I-V characteristic and flows through inductor LI to ground, and through capacitor C3 to set up a voltage, at the output 35 of the circuit (corresponding to the input 68 of signal conditioning circuit 20 (FIG. 7)), that depends upon the detected RF power.

Referring to FIG. 3, for the purpose of analyzing the circuit response to the RF modulation signal generated in the Schottky detector diode Dl, the diode can be replaced with a Norton equivalent circuit model, consisting of a current source 40 and an equivalent source impedance 42, which is equal to the impedance of the diode (Z_SCH0TTKY) at the modulation frequency when biased at its operating point (typically of order 1 kohm).

All component values are preferably chosen to minimize the current flow through undesired paths, and to minimize undesired voltage drops. For example, resistor R6 and the modulation-frequency impedance of capacitor C2 should preferably both be sufficiently large (compared to impedance Z_SCH0TTKY) not to load the diode excessively. Inductor LI should preferably be sufficiently large to prevent the RF from shunting to ground but sufficiently small (relative to Z_SCH0TTKY plus the input impedance of the signal conditioning circuit) to allow the modulation current to flow freely. Capacitor CI should preferably be large enough (i.e., its impedance small compared to Z_SCH0TTKY) to allow the RF current to flow to the diode. Similarly, the output capacitor C3 should largely pass current at the modulation frequency. If appropriate, more specific band pass filter networks may be used.

Referring to FIG. 4, the modulator 16 (which may be a voltage controlled attenuator) produces amplitude modulation which ideally exactly cancels the undesirable amplitude or phase modulation (or cross modulation) produced by the non-linear device. For example, if the gain of the device decreases by 0.01 dB when the RF level increases by 1 dB, then at the same time the modulator should be driven by the signal conditioning circuit, in response to the signal produced by the RF power monitor, so that its attenuation decreases by 0.01 dB, thereby maintaining the RF output signal at compensated full strength. In a presently preferred embodiment for CATV systems, which are AM systems, the modulator 16 contains first and second RF current diverting paths 41, 42 connected to the main RF transmission path adjacent to resistors R7 and R8 respectively. The first current diverting path 41 includes a capacitor C4 connected in series to a resistor R9 and a PIN diode D2 to ground. A modulator control signal from the signal conditioning circuit 20 (FIG. 7) is input at node 44 through capacitor C7 and resistor R10 to the first path. The second current diverting path 42 includes a capacitor C5 connected in series to a resistor Rl 1. The resistor Rl 1 is also connected to a parallel arrangement of a PIN diode D3 and a capacitor C6 connected to ground. A second modulator control signal from the signal conditioning circuit 20 (FIG. 7) is input at node 46 through capacitor C8 and resistor R12 to the second path. The modulator circuit 16 can also be understood by considering the three types of current flowing in it. DC bias current flows through resistor R13, resistor R10, and diode D2 to ground, and through resistor R14, resistor R12, and diode D3 to ground. Modulator control current flows through capacitor C7, resistor R10, and diode D2 to ground and through capacitor C8, resistor R12, and diode D3 to ground. The RF signal current flows from RF IN through resistor R7 and resistor R8 to RF OUT. Some of this current is shunted to ground through capacitor C4, resistor R9, and diode D2, and through capacitor C5, resistor Rl 1 , and the parallel combination of diode D3 and capacitor C6. The amount of RF current shunted to ground depends on the impedances of the two current diverting paths to ground. The modulator control current flowing through diode D2 and diode D3 modulates their impedance, thereby modulating the fraction of RF current shunted to ground, or, equivalently, modulating the insertion loss of the modulator 16. Resistors R7 and R8 are used to match the input and output impedance of the modulator to the characteristic impedance of the RF transmission path. Capacitors C7 and C8 are preferably chosen large enough to allow the modulation current to flow through them (i. e, their impedance preferably should be small compared to RIO and R12 respectively). Capacitors C4 and C5 are preferably chosen large enough to allow the RF current to flow through them (i.e., their RF impedance preferably should be small compared to R9 and Rl 1, respectively), but small enough to prevent the modulation current from bypassing the diodes (i.e., the modulation-frequency impedance of C4 + R9 should be large compared to Z_PIN the maximum impedance of the PIN diode D2). Resistors R13 and R14 are similarly chosen large enough to prevent modulation current from bypassing the PIN diodes. The values of R9 and Rl 1 are determined by several factors, including the need for adequate attenuation range, any restrictions on insertion loss, and the need to prevent intermodulation distortion in the PIN diodes from degrading the signal. Larger values of R9 or Rl 1 reduce the minimum insertion loss of the modulator, and reduce the amount of intermodulation distortion contributed to the signal by the PIN diodes, but correspond to a smaller range of attenuations. The DC bias points of PIN diodes D2 and D3 should preferably be chosen to maximize the RF modulation produced by the modulator control signal. The circuit of FIG. 4 contains first and second diversion paths for compensating for different amounts of cross modulation distortion at different carrier frequencies. It has been found that different carriers in the same transmission system may exhibit different amounts of cross modulation distortion. Accordingly, capacitor C6 is used to allow the higher- frequency RF carriers to bypass diode D3, so that the modulator control signal applied to diode D3 will modulate primarily the lower-frequency carriers. The first current diversion path 41 provides correction of cross modulation distortion over all frequencies, while the second current diversion path 42 provides an additional means for adjusting the amount of correction at lower frequencies. Therefore, in some cases, it may desirable to produce different amounts of correction dependent on RF signal frequencies. In a presently preferred embodiment of the present invention for a CATV transmission system, the following values are used: R7_ R8 _ 1.5 ohms

C4 _ C5 _ 100 pF

C6 _ 0.5 pF

R9 _ Rl 1 _ 3 kohms RIO _ R12 _ 10 kohms

The values set forth are exemplary, and depend on the RF signal level, the amount of insertion loss that can be tolerated, and the maximum amount of cross modulation correction needed for the specific non-linear devices in the RF transmission path. Referring to FIG. 5, an alternate embodiment of a current diversion path for modulating primarily higher frequency carriers includes a single current diversion path 50 connected between impedance matching resistors R15 and R16 on the main RF transmission path 12. The current diversion path contains a capacitor C9 connected to a resistor R17 and PIN diode D4 to ground. This circuit is substantially similar to the first current diversion path 41 in the modulator of FIG. 4, except that the value of capacitor C9 is preferably chosen to be sufficiently small (e.g., 0.5 pF) that it isolates the PIN diode D4 from the main RF signal path at low frequencies, thereby modulating the lower frequency carriers to a smaller extent than the high frequency carriers. This current diversion path can be used in combination with either or both of the current diversion paths of FIG. 4 to form an alternate embodiment of the modulator. As will be apparent, a number of tuned or band pass type current diversion paths may be combined for obtaining a desired modulation which is different in different frequency bands to better match the cross modulation for a non-linear device or plurality of devices in the main signal path.

Referring to FIG. 6, another alternate modulator circuit is disclosed for cancellation of amplitude-to-phase cross-modulation in which intentional amplitude modulation of one carrier appears as phase modulation on another carrier. The circuit includes an electrical path 60 connected to the main RF transmission path between impedance matching resistors R18 and R19. The electrical path includes a capacitor CIO connected to a resistor R20 and a varactor diode D5 to ground. The circuit accepts a modulation signal from the signal conditioning circuit through resistor R21 and node 62. The varactor can be viewed as a voltage controlled capacitor. The modulation signal modulates the capacitance of the diode, which in turn modulates the phase of the RF carriers. Those skilled in the art will note that this type of circuit is useful elsewhere but is not normally used in CATV applications, since phase modulation does not degrade CATV video signals, which are AM signals. The circuit of FIG. 6 can be combined with any combination of the current diversion paths of FIGS. 4 and 5 to form alternate embodiments of the modulator.

Referring to FIG. 7, the signal conditioning circuit 20 amplifies and filters the RF modulation signal such that when this signal is applied to the modulator 16 (FIG. 1), it produces modulation which ideally exactly cancels the cross modulation produced in other parts of the system. In some cases, amplification may not be necessary. In the preferred embodiment, the signal conditioning circuit is a linear circuit that includes a voltage controlled adjustable gain amplifier which comprises first and second cascaded op-amps 70, 72 and two analog multipliers 74, 76. The first op-amp receives input from the RF power monitor 18

(FIG. 1). The first multiplier 74 has first and second inputs 74a and 74b. The first input 74a is connected to the output of the second op-amp 72, and the second input 74b is adapted for receiving a control voltage VI at node 78. The second multiplier 76 also has first and second inputs, 76a and 76b, in which the first input 76a is connected to the output of the second op- amp 72, and the second input 76b is adapted for receiving a second control voltage V2 at node 79. The signal conditioning circuit has two outputs coupled to nodes 44 and 46 for providing modulator control voltages to the modulator 16 (FIG. 4, preferred embodiment). The signal conditioning circuit can be altered as necessary for attachment to the alternate modulator circuits of FIGS. 5 and 6. In addition, reactive elements such as capacitors can be added to the signal conditioning circuit to affect the frequency response of the circuit.

In operation, the analog multipliers provide for voltage-controlled adjustment of the gain and polarity of the corrective cross modulation. In a preferred embodiment, the signal conditioning circuit is designed to have adjustable gain and selectable polarity, so that the amplitude and polarity of cross modulation produced by the correction circuit can be manually adjusted, or automatically adjusted by a microprocessor (not shown), by varying control voltages VI and V2. The frequency response of the signal conditioning circuit is preferably designed to provide the desired amplitude and phase of corrective cross modulation over the modulation frequency range of interest. Depending on the mechanisms responsible for cross modulation, low frequency modulation of one carrier may be more strongly transferred to other carriers than high frequency modulation. In this case, the signal conditioning circuit should preferably be designed to have more gain at low frequencies than at high frequencies. The signal conditioning circuit should also preferably compensate for any frequency dependence in the RF power monitor or in the voltage controlled modulator. For example, the PIN diode modulator in the preferred embodiment, when driven with a 10 kHz current, will modulate the RF attenuation perhaps 4 dB more than when driven by a 100 kHz current; moreover, it is believed that the phase of the modulation imposed on the RF signal lags the phase of the modulation current. The frequency response of the signal conditioning circuit should be designed to incorporate a corresponding phase lead and an increase in gain between 10 kHz and 100 kHz. In the preferred embodiment, capacitor CIO and resistor R18 form a passive filter which introduces a phase lead and reduces the gain of the signal conditioning circuit at low frequencies. The capacitor CIO also serves to prevent the DC component of the voltage generated by the RF power monitor from being amplified by the signal conditioning circuit. A frequency-dependent response may also be incorporated into the signal conditioning circuit by using active filters. For example, the high-frequency gain of the signal conditioning circuit may be reduced by connecting a capacitor in parallel with the resistor in the feedback path of one of the op-amp circuits.

The signal conditioning circuit may be either linear or non-linear. It will be understood by those skilled in the art that the above described embodiments are merely illustrative of circuits that may be used to implement the present invention. For example, the relative locations of the modulator 16 and RF power monitor 18 may be varied. Normally, the amplitude modulation produced by the circuit is small, and the effect at the monitor of changes to the signal level due to the modulator is small. Therefore, the circuit will function regardless of whether the monitor precedes or comes after the modulator in the RF transmission path, and regardless of whether other RF transmitting elements are interposed between the RF power monitor and modulator on the RF transmission path. It is preferable, however, to locate the monitor at a point in the transmission path where the signals are large, so the detector diode Dl (FIG. 2) can be adequately isolated from the main signal path while still receiving a sufficiently strong signal. It is also preferable to locate the modulator at a point where the signal is relatively weak, so that it can be strongly coupled to the signal path without introducing excessive distortion. In the preferred embodiment shown in FIG. 1 , this is accomplished by the illustrated placement of the monitor and modulator relative to the RF amplifier.

Claims

1. A distortion correction circuit for correcting cross modulation distortion on an RF transmission path, the circuit comprising: an RF power monitor coupled to the transmission path for detecting RF power of the signal; a signal conditioning circuit coupled to the RF power monitor for generating a modulator control signal as a function of the detected RF power; and a modulator coupled to the signal conditioning circuit and the RF transmission path for modulating the RF signal as a function of the modulator control signal, wherein the modulation has the effect of substantially canceling the cross modulation distortion in the RF signal produced by one or more non-linear devices in the transmission path.

2. The distortion correction circuit of claim 1, wherein the RF power monitor comprises a non-linear element.

3. The distortion correction circuit of claim 2, wherein the non-linear element is a Schottky diode.

4. The distortion correction circuit of claim 1, wherein the non-linear device is coupled to the RF path through a lossy path to reduce the coupling of intermodulation distortion generated by the non-linear device to the RF path.

5. The distortion correction circuit of claim 2, wherein the non-linear element is coupled to the RF path through a directional coupler to reduce the coupling of intermodulation distortion generated by the non-linear device to the RF path in a forward-traveling direction.

6. The distortion correction circuit of claim 1, wherein the signal conditioning circuit comprises means for amplifying the signal from the RF power monitor.

7. A distortion correction circuit for correcting cross modulation distortion on an

RF transmission path, the circuit comprising: an RF power monitor coupled to the transmission path for detecting RF power of the signal; a signal conditioning circuit coupled to the RF power monitor for generating a modulator control signal as a function of the detected RF power, the signal conditioning circuit comprising a plurality of signal conditioning paths each generating a modulator control voltage; and a modulator coupled to the signal conditioning circuit and the RF transmission path for modulating the RF signal as a function of the modulator control signal, the modulator comprising a plurality of current diversion paths, wherein each of the signal conditioning paths is coupled to one or more of the current diversion paths, and wherein the modulation has the effect of substantially canceling the cross modulation distortion in the RF signal.

8. The distortion correction circuit of claim 7, wherein some of the signal conditioning paths in the the signal conditioning circuit comprise means for amplifying the signal from the RF power monitor.

9. The distortion correction circuit of claim 8, wherein the means for amplifying the signal comprises one or more operational amplifier circuits.

10. The distortion correction circuit of claim 9, wherein some of the signal conditioning paths in the signal conditioning circuit comprise means for adjusting the amplitudes of the modulator control voltages.

1 1. The distortion correction circuit of claim 10, wherein the means for adjusting consists of an analog multiplier connected to a control voltage.

12. The distortion correction circuit of claim 7, wherein some of the signal conditioning paths in the signal conditioning circuit comprise means for transmitting different frequencies with different amplitudes and phases.

13. The distortion correction circuit of claim 12, wherein the means for transmitting different frequencies with different amplitudes and phases comprises passive filter networks.

14. The distortion correction circuit of claim 13, wherein the passive filter network comprises a capacitor and resistor wired in series, in order to transmit lower frequencies with smaller amplitude and advanced phase relative to the higher frequencies.

15. The distortion correction circuit of claim 13, wherein the means for transmitting different frequencies with different amplitudes and phases comprises active filter networks.

16. The distortion correction circuit of claim 15, wherein the active filter network comprises an operational amplifier circuit having in the feedback path a resistor and a capacitor connected in parallel, in order to transmit higher frequencies with smaller amplitude and retarded phase relative to the lower frequencies.

17. The distortion correction circuit of claim 7, wherein some of the current diversion paths in the modulator comprises a circuit for modulating primarily the amplitude of the RF signal.

18. The distortion correction circuit of claim 17, wherein the circuit for modulating primarily the amplitude of the RF signal comprises a PIN diode and means for modulating its bias current.

19. The distortion correction circuit of claim 18, wherein the circuit for modulating primarily the amplitude of the RF signal further comprises filter means for modulating the amplitude of different RF frequencies by different amounts.

20. The distortion correction circuit of claim 19, wherein the filter means consists of a capacitor connected in parallel with the PIN diode, to prevent the higher frequency RF signals from being modulated by the PIN diode.

21. The distortion correction circuit of claim 19, wherein the filter means consists of a capacitor connected in series with the PIN diode, to prevent the amplitude of lower frequency RF signals from being modulated by the PIN diode.

22. The distortion correction circuit of claim 7, wherein the modulator comprises a circuit for modulating primarily the phase of the RF signal.

23. The distortion correction circuit of claim 22, wherein the circuit for modulating primarily the phase of the RF signal comprises a reverse-biased varactor diode and means for modulating its bias voltage.

24. The distortion correction circuit of claim 23, wherein the circuit for modulating primarily the phase of the RF signal further comprises filter means for modulating the phase of different RF frequencies by different amounts.

25. The distortion correction circuit of claim 24, wherein the filter means consists of a capacitor connected in parallel with the varactor diode, to prevent the higher frequency RF signals from being modulated by the varactor diode.

26. The distortion correction circuit of claim 25, wherein the filter means consists of a capacitor connected in series with the varactor diode, to prevent the lower frequency RF signals from being modulated by the varactor diode.

27. The distortion correction circuit of claim 7, wherein each signal conditioning path and associated current diversion path operates primarily on different frequencies than another signal conditioning path and associated current diversion path for effecting different modulation at different frequencies.