WO1992014325A1 - Transmission of multiple carrier signals in a nonlinear system - Google Patents
Transmission of multiple carrier signals in a nonlinear system Download PDFInfo
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- WO1992014325A1 WO1992014325A1 PCT/US1992/000763 US9200763W WO9214325A1 WO 1992014325 A1 WO1992014325 A1 WO 1992014325A1 US 9200763 W US9200763 W US 9200763W WO 9214325 A1 WO9214325 A1 WO 9214325A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/38—Synchronous or start-stop systems, e.g. for Baudot code
- H04L25/40—Transmitting circuits; Receiving circuits
- H04L25/49—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems
- H04L25/4902—Pulse width modulation; Pulse position modulation
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
- H03F1/32—Modifications of amplifiers to reduce non-linear distortion
- H03F1/3241—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B14/00—Transmission systems not characterised by the medium used for transmission
- H04B14/02—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation
- H04B14/026—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation using pulse time characteristics modulation, e.g. width, position, interval
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L5/00—Arrangements affording multiple use of the transmission path
- H04L5/02—Channels characterised by the type of signal
- H04L5/06—Channels characterised by the type of signal the signals being represented by different frequencies
Definitions
- This invention relates to the field of communications and more particularly it addresses processing a narrow band complex signal, typically consisting of multiple signals, in a manner enabling transmission through a nonlinear system such as a saturated power amplifier.
- a nonlinear system such as a saturated power amplifier.
- a highly nonlinear transmission system is a mo power-efficient and cost effective alternative to a linear system.
- Nonlinear power amplifiers especially when operated in a saturated mode, are a typical example of nonlinear systems which are subject to inherent transmission limitations. These amplifiers can be made inexpensive and power-efficient, however, generally the use of this class of amplifier must be confined to a single isolated angle-modulated or pulse-modulated signal. Where multiple source signals are angle-modulated and sufficiently separated in frequency, they may be amplified in separate nonlinear amplifiers, each dedicated to a channel, and then combined in either a passive high power multiplexer or a power combiner; however such systems tend to be complex and expensive, and would not be applicable to the class containing multiple signals which are closely spaced in frequency within a narrow band and which, in some instances, are amplitude-modulated. These signals are within the class of complex signals addressed by the present invention.
- Linear amplifiers typically require enhancement techniques such as feedback and feed forward, (some systems utilizing active feed forward, that is, amplifiers having pilot tone cancellation). Nevertheless, despite its high cost and low power- efficiency, linear power amplification remains the basic conventional approach for handling multiple signals, and is often utilized due to lack of a viable alternative.
- either the collector voltage on a bipolar transistor, or the drain voltage on a field effect transistor (FET), or the plate voltage on a triode are caused to vary as a function of the envelope in such a way that the modulation is imposed on the carrier.
- the modulation is applied just prior to the input to the final output stage which is assumed to be linear.
- the third approach involves splitting the signal into two paths and phase modulating the two paths such that, when the signal is recombined, the magnitude of the resulting vector is proportional to the -envelope.
- the techniques described in the prior art all suffer from limitations in the practical implementation.
- the techniques which apply modulation through some sort of bias control assume a degree of linear transfer of bias voltage to output envelope modulation; however, in reality, this is not the case.
- there are other problems such as detuning of the output network, AM to PM, and possible amplifier instabilities resulting from changing the bias voltage.
- the techniques which apply the modulation at the input to the last amplifying device assume that this device is linear. Since this is the highest power device in the amplification chain, it is, in fact, the most likely not to be linear.
- the techniques which combine phase modulated paths require a high degree of amplitude and phase tracking in order to achieve high depth of modulation. For instance, a combination of 0.2 dB of amplitude and 3 degrees of phase tracking error will result in a maximum depth of null of 20 dB. This represents a severe limitation on the dynamic range of the modulation.
- Another object of the invention is to convert amplitude dynamic range of a multiple carrier signal into a pulse duty factor dynamic range signal to allow the amplification of wide dynamic range signals by nonlinear device at a considerable increase in efficiency over conventional linear and multiplexing practices.
- Another object of the invention is to enable a plurality of nonlinear power amplifiers operating in parallel to provide a combined power output signal replicating a multiple signal source.
- Another object is to enable a communication link of limited dynamic range, such as an optical link, to accommodate multiple signals through a signal process which converts amplitude modulation to pulse duty factor modulation.
- the pulse train is applied as pulse time modulation onto a carrier signal which is derived from the source signal by hard limiting so as to contain the phase information.
- the hard-limited carrier signal keyed on/off by the pulse train, is transmitted through the non ⁇ inear system (such as a saturated amplifier) and then passed through a narrow bandpass filter which eliminates the sidebands introduced by the sampling process.
- the RF amplifier always operates at a constant output level and linearity is not required for proper operation.
- the RF signal is pulse-time modulated such that the DC power is a function of the duty factor.
- pulse-time modulation is applied in parallel channels at a reduced repetition rate but interleaved in time such that the combined output generally represents a pulse train at the higher desired pulse repetition rate. This increases the dynamic range by the square of the number of channels.
- the invention thus relates to a signal processing method and apparatus for enabling a modulated RF source signal to be processed through a nonlinear medium or device, such as a nonlinear power amplifier.
- the method of the invention features the steps of deriving from the source signal an amplitude-limited, angle-modulated carrier signal containing phase information from the source signal; deriving from the source signal an envelope signal containing amplitude information from the source signal; and deriving from the information in the envelope signal a pulse train signal having pulses which preferably occur at a repetition rate at least an order of magnitude higher than a maximum envelope bandwidth and which have a duty factor which is a function of the amplitude of the envelope signal.
- the method further features the steps of modulating the carrier signal by the pulse train signal for providing a pulse modulated carrier signal independent of amplitude, but which contains both angle and envelope information relating to the source signal; passing the pulse modulated carrier signal through a substantially nonlinear process so as to provide a processed output signal; and bandpass filtering the processed output signal to recover a linearly related replica of the original source signal.
- the method and apparatus employ a plurality of nonlinear power amplifiers, operating in parallel, for providing the processed output signal.
- the envelope signal is converted to a plurality of pulse trains, the pulse trains being staggered in time by one pulse period and each pulse train having a duty factor which is a function of the amplitude of the envelope signal.
- the carrier signal is modulated by the pulse trains. amplified, and the result is combined to produce an output signal which is thereafter filtered to recover a linear replica of the original source signal.
- the converting step features the steps of controlling the duty factor and pulse duration of each pulse train over a first region of the envelope dynamic range to have a pulse rate of 1/N of the desired pulse rate for the combined output signal so that the pulses occur at periodic intervals while pulse duration is a function of the amplitude of the envelope signal; inhibiting over a second region of dynamic range, pulses of one or more of the pulse trains so that average output power is reduced in proportion to average duty factor; and increasing pulse duration and duty factor of each of the pulse trains over a third region of dynamic range to generate overlapping pulses in the staggered pulse trains.
- FIG. 1 is a functional block diagram of a processing system for transmitting a multiple component signal through a nonlinear power output amplifier in accordance with the present invention.
- FIG. 2 is a functional block diagram of a processing system for transmitting a multiple component signal through a plurality of parallel nonlinear power output amplifiers in accordance with the present invention.
- FIG. 3a is a graph of the time domain locus of two equal amplitude signals.
- FIG. 3b is a portion of the graph of FIG. 3a expanded around a phase reversal point.
- FIG. 4a is a graph of the envelope of the signal of FIG. 3a.
- FIG. 4b is a portion of the graph of FIG. 4a expanded around a phase reversal point.
- FIG. 5a is a graph of the hard limited carrier containing the phase information, as derived from the signal of FIG. 3a.
- FIG. 5b is a portion of the graph of Fig. 5b expanded around a phase reversal point.
- FIG. 6a is a graph of a duration-modulated pulse train as derived from the envelope of FIG. 4a.
- FIG. 6b is a portion of the graph of Fig. 6a expanded around a phase reversal point.
- FIG. 7a is a graph of the carrier of FIG. 5a as pulse duration modulated by the pulse train of FIG. 6a.
- FIG. 7b is a portion of the graph of Fig. 7a expanded around a phase reversal point.
- FIG. 8 is a frequency domain graph showing the power spectrum of the original two carrier input signal of FIG. 3a.
- FIG. 9 is a frequency domain graph showing the power spectrum of the pulse duration modulated carrier of FIG. 7a indicating the restored spectrum in the immediate vicinity of the carriers, and showing the spurious sidebands generated by the pulse sampling process.
- FIG. 10 is a graphical representation of the waveforms of N interleaved pulse trains and their recombination in the multiple processing system of FIG. 2.
- FIG. 11 is a functional block diagram of an embodiment of the invention incorporating an optical communication link following the pulse modulator.
- FIG. 12 is a functional block diagram of an embodiment of the invention having an optical communication link which includes a portion of the pulse modulator. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS
- FIG. 1 is a functional block diagram of a processing system for transmitting a multiple component signal through a nonlinear power output amplifier in a basic illustrative embodiment of this invention.
- An input signal is supplied by a signal source 10 which may include well known conventional frequency conversion means such as a single or double, balanced or active, mixer or other implementations, such as those using single sideband generation techniques, for example to convert to a suitable working frequency band for which circuit components are readily available.
- a signal source 10 which may include well known conventional frequency conversion means such as a single or double, balanced or active, mixer or other implementations, such as those using single sideband generation techniques, for example to convert to a suitable working frequency band for which circuit components are readily available.
- the input signal typically includes, within a narrow bandwidth, multiple carriers which may be angle and/or amplitude modulated. It is divided into two paths in a power splitter 12 which can be a junction of two conductors, a reactive splitter, or a splitter with resistive or magnetic isolation. The power split could be equal or unequal and with any arbitrary phase difference between the two output ports.
- a limiter 14 which hard limits the combined carrier signal from splitter 12 and applies the hard-limited carrier signal to a first input port of a pulse modulator 16.
- the other output port of power splitter 12 feeds an envelope detector 18 which provides a detected envelope to an envelope-to-pulse converter 20.
- Converter 20 receives a sampling signal at a frequency at least ten times greater than the envelope bandwidth, and provides as its output a duration modulated pulse train. This pulse train is applied to the second input port of modulator 16, and on/off keys the hard-limited carrier applied to the first input port.
- modulator 16 is applied to a nonlinear power amplifier 22 which is operated in a saturated mode for high power efficiency.
- the power output from amplifier 22 is passed through a narrow band output filter 24 to strip off the sampling sidebands, and is then typically fed to a transmitting antenna 25. No modification of the typical receiver need be made.
- FIG. 2 is a functional block diagram of a multiple amplifier embodiment illustrating how the principle of the invention can be applied to an application where the required power output is high enough so as to require paralleling multiple RF power output stages.
- it is advantageous to pulse modulate each of a number of parallel channels rather than the common input channel.
- the functional blocks in the input portion including source 10, power splitter 12, limiter 14, envelope detector 18 and envelope-to-pulse- duty-factor converter 20 are as described above in connection with FIG. 1.
- the pulse train output designated the master pulse train, from converter 20 drives a multiple modulation controller 26 which acts as a commutator, applying the pulses of the pulse train sequentially to each modulator 16(1), 16(2), ...16(N) such that each modulator operates at 1/N of the rate of pulse train.
- the inputs to modulators 16(1), 16(2), ..., 16(N) are a plurality (N) of interleaved pulse train signals, staggered in time by one pulse period and having a repetition rate N times lower than that of the master pulse train.
- the limited-amplitude output from limiter 14 is split into N parallel channels in an N-way power splitter 28.
- Splitter 28 can be isolated or non-isolated (that is, it is optional as to whether there exist dissipative elements within the splitter which guarantee that the power into each of the channels is independent of the input impedance into each channel although it is assumed that the impedance is identical in the instance of no isolation).
- a pulse modulator such as 16(1), 16(2),...16(N)
- drives, respectively, a corresponding nonlinear amplifier such as 22(1), 22(2),...22(N) , whose outputs are combined in an N-way power combiner 32.
- Combiner 32 must be isolated, that is, it must contain dissipative elements that guarantee that the operating load impedance presented to each of the channels is independent of the source impedance or source power from each channel.
- the combined power output from combiner 32 is passed through bandpass filter 24, typically to an antenna 25. As with the embodiment of Figure 1, no modification of the receiver need be made.
- hard limiter 14 maintains the phase angle information but eliminates any envelope modulation.
- Implementation could be a diode limiter, an active limiter, or a combination of both, depending upon the application.
- Another possibility would be a zero crossing detector (that is, a comparator) which develops a positive output for any signal above zero and a negative or zero output for any signal below zero volts (or visa-versa).
- envelope detector 18 develops the basic envelope waveform, for example, a video envelope waveform.
- the resulting envelope signal is either always positive or always negative. Any one of a number of techniques for implementing this function can be employed. A simple diode peak detector could be utilized. Another technique utilizes the hard limited carrier delayed to be in quadrature to the modulated carrier so as to be a local oscillator to a balanced mixer and thus develop the modulation using a homodyne technique. Conversion from the detected envelope to the proper pulse duty factor is accomplished in converter 20 through a number of possible circuit techniques. In a common form of pulse-time modulation, the pulse-duration (or "pulse width”) is varied while the pulse repetition rate is held constant, resulting in corresponding variations in duty factor.
- Convertor 20 outputs a duration-modulated pulse train where the start of the pulse is always at the same repetition rate but the end of the pulse is variable.
- a common approach to achieving this is to have the other input to the comparator be a ramp which attains full height (corresponding to the maximum desired value of the envelope) at the end of the clock period.
- the output of the comparator is a logic high until the ramp exceeds the value of the envelope.
- a common analog technique is to have the a sample and hold circuit hold the value of the envelope at the start of each clock period as one input to a comparator.
- Another technique is to have an analog-to-digital converter sample the signal at the start of every clock period and then have the sampled value address a lookup table which determines the proper pulse duration value. This value can be passed through a digital-to-analog converter whose output can be utilized as an input to the comparator.
- Another type of pulse train allows symmetrical modulation about the pulse repetition frequency such that the start of the pulse and the end of the pulse are both modulated.
- a typical version of this circuit utilizes a sample and hold circuit at a point in time equal to or prior to the earliest possible start of a pulse.
- the sampled signal is input to one side of a comparator circuit which has as its other input an inverted triangle-shaped waveform.
- a logical high is developed when the envelope is higher than the triangle waveform so that a full width pulse results when the envelope is at or above the predetermined highest level.
- the pulse duration is also reduced in a symmetrical fashion around that center point in time corresponding to the constant pulse repetition frequency.
- some form of delay circuit can be provided at the output of limiter 14 to synchronize the hard limited carrier with the processed envelope signal.
- delay circuit can be provided at the output of limiter 14 to synchronize the hard limited carrier with the processed envelope signal.
- the development of a symmetrical pulse duration waveform, as described above requires sampling the envelope at a point in time which is earlier than the actual pulse. This situation requires the carrier to be delayed in order to be synchronized with the pulse signal.
- Such a delay can be implemented in transmission delay lines, surface acoustic wave delay lines, or preferably in a digital memory associated with the limiter 14 itself if the frequencies are low enough.
- the duration-modulated pulse train can then be imposed onto the hard limited carrier in the pulse modulator 16.
- modulator 16 passes the RF signal from limiter 14 when the pulse from converter 20 is in a "high” logic state, and gates the RF signal off when the pulse is in a "low” logic state. It is important that the gating process be instantaneous, that is, the RF signal be immediately brought to zero volts at the cessation of the modulation pulse.
- the switching function of modulator 16 can be implemented by any one of a number of techniques. Typical techniques employ a doubly balanced mixer as a switch, or else a diode or active element switch, and preferably the new generation of GaAsFET switches. Other techniques could employ gate or base bias modulation, collector or drain modulation, or some combination thereof. In general, any common technique which allows appropriate pulse durations within the range required is acceptable, however, those techniques which minimize switch driver feed through are preferable.
- an optional frequency translation circuit can be implemented at the output of either amplifier 22 or filter 24.
- the translation technique can be a single sideband generation circuit or a doubly balanced mixer.
- Amplification of the signal is best effected in a nonlinear amplifier. While the amplifier 22 need not be nonlinear, a nonlinear device works best for the application. The only requirement is that the amplifier be capable of amplifying pulse modulated carriers over the range of desired pulse durations.
- a major advantage of a nonlinear amplifier, such as a class B or C amplifier is that very little current is drawn when the input signal is gated off. As a result, average consumption is determined by average duty factor.
- the RF output signal from the amplifier 22 is passed through the narrow band output filter 24, which passes the desired spectrum while rejecting the spectrum centered on either side of the desired spectrum around the pulse train sidebands.
- FIGURES 3a-9 In order to understand how the invention works, it is useful to consider its effect on the well understood problem of amplifying two equal amplitude signals closely spaced in frequency, as illustrated in FIGURES 3a-9, in which waveforms and spectrum response have been plotted from mathematical calculations.
- FIG. 3a shows a waveform 34 as the familiar time domain representation of the two signals having an envelope outline which reduces to zero amplitude at periodic points in time such as ti and t2 each representing a point of phase reversal. -15-
- FIG. 3b shows the waveform 34 expanded around time ti, one of the points of phase reversal, to further facilitate understanding the detailed operation of the invention.
- FIG. 4a and FIG. 4b show the detected envelope 36 of the two signals that would be seen at the output of envelope detector 18 (FIGURES 1 and 2) over the entire time span of FIG. 3a and the expanded time frame of FIG. 3b respectively.
- FIG. 5a and FIG. 5b show a waveform 38 of the hard- limited signal from limiter 14 over the. corresponding time periods. It can be 'seen that the output of the envelope detector 18 (waveform 36, FIGs. 4a, 4b) is always positive while the phase reversal is carried forth in the hard limited carrier signal waveform 38.
- FIG. 6a and FIG. 6b show a waveform 40 of a duration modulated pulse train waveform at the output of converter 20 (FIGURES 1 and 2) over the corresponding time periods.
- the pulse duration is essentially zero at the times where the envelope is zero, such as at tj and t2 and the pulse duration is essentially the full period at the times where the signal is at full power.
- the pulse duration as a percentage of the period of the pulse train, is directly proportional to the amplitude of the envelope relative to its allowable peak amplitude.
- FIGURES 7a and 7b show the output of pulse modulator 16, a time domain waveform 42 of the RF carrier, on/off gated by the pulse train of FIGURES 6a and 6b, respectively.
- FIG. 8 shows the frequency domain power spectrum of the two equal carrier signals in the input signal from source 10 (FIGURES 1 and 2), appearing as two band limited peaks 44 and 46 about the respective carrier frequencies. -16-
- FIG. 9 shows the frequency domain power spectrum of the modulated carrier of FIGURES 7a and 7b. It can be seen that the original power spectrum of the carrier signals is present around the respective original carrier frequencies, appearing as peaks 44a and 46a, and in addition, there is introduced spurious sidebands 48 and 50 due to the sampling process.
- an arbitrary input signal, y(t), can be represented as:
- A(t) is the amplitude function
- ⁇ (t) is the phase modulation function.
- the angle modulated function is reproduced.
- the information content of the envelope function A(t) is preserved by sampling it, converting the sampled values to a pulse duration and multiplying the carrier by the resulting duration modulated pulse train. If it is assumed that the pulse train repetition rate is considerably higher in frequency than the maximum frequency component of the envelope, then a quasi static analysis applies.
- the standard Fourier transform is a discrete series of the form:
- ⁇ n is the phase angle of each discrete spectral line.
- the series is of the form of a discrete set of spectral frequency lines at multiples of 1/T in frequency with the familiar envelope of sin(x)/x and with the "DC" component being just the duty factor, CT
- CT When this is multiplied times the carrier waveform in the time domain, it is equivalent to convolution in the frequency domain.
- the resulting Fourier spectrum consists of the Fourier transform of the carrier convolved around not only the carrier, but also around all of the sidebands at multiples of 1/T away from the carrier. If 1/T is much higher in frequency than the maximum bandwidth of the modulation, then the other sidebands are easily filtered out leaving only the spectrum of the carrier with an amplitude directly proportional to ⁇ r. Since this is a quasi-static analysis, the value of ⁇ Tcan be assumed to vary with time directly proportional to the envelope. As a result, the voltage magnitude of the resulting signal is directly proportional to the input envelope amplitude.
- a more exact analysis of the spectrum of the dynamically pulse duration modulated pulse train is required to determine the conditions for which significant interference sidebands will be found in the bandwidth of the desired signal. Since the preceding analysis shows that pulse duration modulation of the carrier in the time domain results in convolution of the carrier signal with the power spectrum of the pulse duration modulated pulse train in the frequency domain, it is only necessary to analyze the frequency spectrum of the pulse duration modulated pulse train.
- a number of references provide exact dynamic analyses of a pulse duration modulated signal (see for instance, "Modulation, Noise, and Spectral Analysis", by P.F. Panter, McGraw- Hill, 1965, pp. 540-543). These exact analyses indicate a complex amplitude distribution for the sidebands but a DC component which is directly proportional to the duty factor.
- the dynamic analysis agrees with -18- the static analysis of the low frequency components of the pulse duration modulated spectrum.
- A constant related to the amplitude of the pulse train
- Jl is a Bessel Function of the first type.
- the low frequency spectrum is directly proportional to the magnitude of the envelope (here assumed to be given by l+sin(wm*t), that is 100% modulation) .
- Expanding this equation in the frequency domain yields a considerable number of spectral lines but interference lines within the bandwidth of the modulation are typically less than -100 decibels below the carrier (dBc) with respect to the modulation in the case of pulse repetition rates which are at least an order of magnitude higher than the highest envelope frequency.
- the previous example thus describes the use of pulse duration modulation as a means of adjusting the quasi static duty factor to correspond to the height of the envelope of the signal.
- This is not the only pulse-time modulation technique that will accomplish this goal.
- Another technique is to maintain the duration of the pulses constant and to vary the pulse repetition rate, that is the pulse interval, in such a manner that the quasi static average duty factor is proportional to the amplitude of the envelope signal.
- a version of this approach is to maintain the pulse repetition rate constant and to pulse duration modulate but to allow the pulse duration to be reduced to some minimum width and thereafter to eliminate some of the periodic pulses. For instance, if every other pulse were eliminated, the resulting duty cycle would be half that of the full rate pulse train and the effective envelope voltage would be half that of the full rate envelope.
- the use of pulse duty factor modulation can be implemented by any mixture of pulse duration modulation and pulse repetition rate modulation to achieve envelope restoration by control of the quasi static duty factor of the modulated RF waveform. Any of these waveforms take advantage of the fact that the signal amplitude resulting from operations upon the pulse modulated waveform is constant and thus requires no dynamic range linearity in the power amplifier (or other nonlinear transmission media).
- waveforms 52, 54, and 56 represent the pulse trains handled by each of the amplifiers 22(1), 22(2) ...22(H) respectively (of FIG. 2).
- the system may be designed with any number of like channels, the number N being determined primarily by the total power requirement, the power capability of each amplifier, and as described below, the dynamic range of the input signal envelope.
- the output amplitude from the bandpass filter 24 is proportional to the duty cycle for a pulse signal having a constant pulse repetition rate. Accordingly, if the minimum amplifier pulse width is 0.1*T, then a single channel system, sampling at a rate (1/T), will result in a dynamic range of 20 dB, as the pulse duration width ranges from 0.1*T to T.
- the dynamic range according to the invention can be extended by varying the pulse modulation technique over different parts of the envelope dynamic range.
- the output signal is sampled at a rate of 1/T with pulse widths reduced from T to the minimum pulse width of 7mi n .
- This allows the amplifier to duplicate an input amplitude envelope having a range from 1 to (TJnin/ T).
- the repetition rate can be decreased, while maintaining the pulse width constant at T ⁇ iin.
- the amplitude range of minimum output signal can then -21- be further reduced to CTmin/ ⁇ increased) where the sampling period, Ti ncrease d, is greater than the original sampling period. This increased sampling period causes the sideband spurious signals to be closer to the desired signal, but because this undesirable side effect only occurs with low signal levels, the amplitude of the closer sidebands are greatly reduced.
- the pulse period can be continuously increased from T to 4*T to transmit amplitude information below the original 20 dB dynamic range (the pulse width remaining constant at 0.1*T). In this manner, the output dynamic range is increased by an additional 12 dB, to a total of 32 dB.
- An alternative embodiment combines varying the pulse width and the sampling period over the lower part of the envelope dynamic range. Use of a look-up table 100 in the envelope to pulse modulation circuitry can be utilized to effect and control this method of dynamic range extension.
- This technique of dynamic range extension by varying the pulse modulation technique over different sections of the input dynamic range, as discussed in the preceding paragraph, can also be extended to the embodiment of staggered pulse modulation used with the multiple parallel channel amplifier illustrated in Figures 2 and 10.
- the dynamic range can be preferably increased by eliminating pulses from some of the parallel channels for very low envelope voltages. If only 1 of the N parallel channels is operated, then the resulting output composite signal is just a pulse train at a rate of 1/(N*T), that is, at 1/N of the basic pulse train rate. At that duty factor, assuming that the pulse duration is maintained at min' tne output power in the signal is reduced by another 1/N 2 . In general, if only M of the N parallel channels are operated, then the signal power in the composite output signal is (M/N) ⁇ of the signal power when all channels are operating. In the example of the -22-
- a look up table 100 can provide a continuously varying pulse duration as channels are eliminated so as to insure that there exists a smooth reduction in output power over the full dynamic range.
- the dynamic range of the output signal can be increased by reducing the duty factor from 100% to some substantially lower number both for a single channel as well as a multiple channel environment.
- the dynamic range can be further extended by increasing the pulse duration on each of the channels to a value greater than T and in the extreme, to a duration equal to N*T.
- the pulse duration on each channel shown at an intermediate value in Figure 10, can vary from the minimum achievable pulse duration 7 " in (limited as noted above, only by implementation factors such as system bandwidth and amplifier rise and fall time considerations) at a pulse repetition rate of 1/(N*T), up to a maximum pulse duration of N*T in each channel (occurring at the same repetition rate 1/ (N*T)).
- the combined output 58 from the overall amplifier will be a continuous wave (cw) signal (that is a continuous signal having a 100% duty factor) having superimposed therein, a time varying pulse envelope.
- the continuous wave signal has the maximum available output power from the amplifier, ma ⁇ ' given by N*P c hannel' where ⁇ channel ⁇ s tne RF power developed on each channel when the channel pulse duration is N*T.
- the composite output signal 58 can be a pulse train that varies in pulse duration from the minimum possible pulse duration, T ⁇ inr occurring at a rate of 1/T up to a cw signal (at a full power output).
- the power contribution from each of the channels to the filtered output of the filter 24 is (T/(N*T)) 2 of the overall maximum available power.
- the additional attenuation is due to the -23- property of isolated combiners that when M of N channels are operating simultaneously (and assumed to have equal and in phase output), the total RF output power in the frequency band of interest is given by (M/N) 2 of the total available power when all N of the channels are operating simultaneously.
- the composite signal becomes a superposition of time coincident signals such that the output waveform repeats every T seconds with part of the period consisting of the superposition of M channels (resulting in instantaneous composition output (M/ N) 2 *P max ) and the other part of the period consisting of the superposition of M-l channels (resulting in instantaneous composite output of ( (M-l)/N) 2 *P max ) ) .
- the composite RF output power from the multiple channel amplifi is given by:
- this technique extends the dynamic range of the pulse duration modulation by a factor of N 2 (as increases from T to N*T) .
- N 2 the dynamic range of the pulse duration modulation
- the amplifier rise time and/or bandwidth limit the minimum amplifier pulse width to 0.1*T
- the dynamic range is increased by (1/4) 2 or 12 db; and further, as noted above, if certain channels are selectively eliminated, an additional 12 dB of dynamic range is achieved (for 4 channel example).
- Fig. 11 is a functional block diagram illustrating how the basic technique of this invention, as described in connection with FIG. 1, can be extended in frequency for utilization in an optical link.
- the RF signal from pulse modulator 16 in FIG. 1, is used to amplitude modulate an -24- optical source (such as a solid state laser) in a modulator 60.
- the optical output is then transmitted across an optical link 62 to an optical detector 64.
- the output of this detector will be the pulsed RF signal which can be passed through bandpass filter 26 whereby the complex signal is again restored.
- FIG. 12 is a functional block diagram illustrating utilization in an optical link similar to that shown in FIG. 11 except that the amplitude limited signal is applied directly to the optical modulator 60 without being pulse modulated.
- the pulse modulation whose duty factor is proportional to the amplitude of the envelope of the original complex signal, is then used to directly modulate the optical beam through an optical pulse gate modulator 66.
- the resulting detected signal out of optical detector 64 is the pulsed RF signal which is passed through bandpass filter 26 and restored to its original complex waveform.
- a complex signal can be sent on an optical link without requiring amplitude linearity.
Abstract
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US07/649,857 US5249201A (en) | 1991-02-01 | 1991-02-01 | Transmission of multiple carrier signals in a nonlinear system |
US649,857 | 1991-02-01 |
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WO1992014325A1 true WO1992014325A1 (en) | 1992-08-20 |
Family
ID=24606512
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/US1992/000763 WO1992014325A1 (en) | 1991-02-01 | 1992-01-29 | Transmission of multiple carrier signals in a nonlinear system |
Country Status (4)
Country | Link |
---|---|
US (1) | US5249201A (en) |
EP (1) | EP0569554A4 (en) |
CA (1) | CA2101071A1 (en) |
WO (1) | WO1992014325A1 (en) |
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WO2011129727A1 (en) * | 2010-04-14 | 2011-10-20 | Telefonaktiebolaget L M Ericsson (Publ) | Power-amplifier arrangement |
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Cited By (5)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN1083643C (en) * | 1996-01-17 | 2002-04-24 | 摩托罗拉公司 | Method and apparatus for average power control |
GB2340325A (en) * | 1998-07-31 | 2000-02-16 | Raymond Etienne Schemel | Reducing distortion in signal path by converting the input into a constant envelope phase modulated signal |
GB2340325B (en) * | 1998-07-31 | 2003-04-16 | Raymond Etienne Schemel | Apparatus and method of achieving and improving linearity of amplifiers and other non-linear paths |
WO2011129727A1 (en) * | 2010-04-14 | 2011-10-20 | Telefonaktiebolaget L M Ericsson (Publ) | Power-amplifier arrangement |
US8659354B2 (en) | 2010-04-14 | 2014-02-25 | Telefonaktiebolaget L M Ericsson (Publ) | Power-amplifier arrangement |
Also Published As
Publication number | Publication date |
---|---|
CA2101071A1 (en) | 1992-08-02 |
EP0569554A1 (en) | 1993-11-18 |
EP0569554A4 (en) | 1993-12-15 |
US5249201A (en) | 1993-09-28 |
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