Predistorter for linearization of electronic and optical signals

Abstract: An electronic circuit provides a linear output from an amplitude modulated transmission device such as a semiconductor laser which has inherent distortion. The distortion of the nonlinear device is compensated by applying a predistorted signal equal in magnitude and opposite in sign to the distortion introduced by the nonlinear device. The input signal is split into two paths with primary part of the signal applied directly to the device, with a time delay to compensate for delays in the secondary path. A distortion generator in the secondary path generates intermodulation signals, the amplitude of which is adjusted to match the amplitude of the distortion of the nonlinear device. A tilt adjustment is made to compensate the amplitude of the predistortion for frequency dependence of the amplitude of the distortion. A adjustment of the predistortion signal is also included so that phase of the distortion signal generated is in proper phase relation with the distortion inherent in the nonlinear device. A push-push amplifier is used as a distortion generator, with bias current unbalanced to produce net odd order intermodulation products as well as even order intermodulation products of the fundamental frequencies.

Description: The invention relates to a predistortion circuit and method as indicated in the precharacterizing parts of claims 1 and 14. Such a circuit and method can be delivered from the content of EP-A-0 078 884 which discloses an equalizer which can be easily adjusted for pre-equalizing frequency modulated signals fed to a travelling wave tube or other non-linear arrangement which generates phase distortion and amplitude distortion. It is not clear whether this prior art circuit can be used with AM signals. The circuit splits the input signal into primary and secondary electrical paths. The circuit comprises a means for suppressing the useful signal in the secondary path and a phase/frequency adjustment block for adjusting the amplitude and phase of the signal with respect to the non-linear device. US-A-4,068,186 discloses a prior art circuit for compensating for non-linear characteristics in high-frequency amplifiers. The circuit includes an input coupler for dividing an input signal into two signal parts distributed to first and second transmission lines respectively. The first transmission line contains a delay line and a corrector for developing non-linear phase-frequency characteristics. The second transmission line includes a travelling wave tube for developing primarily distortion for correcting non-linearities in a high frequency amplifier. The signals from the first and second transmission lines are combined in an output coupler for application to the amplifier. Directly modulating the analog intensify of a light-emitting diode (LED) or semiconductor laser with an electrical signal is considered among the simplest methods known in the art for transmitting analog signals, such as sound and video signals, on optical fibers. Although such analog techniques have the advantage of significantly smaller bandwidth requirements than digital pulse code modulation, or analog or pulse frequency modulation, amplitude modulation may suffer from noise and nonlinearity of the optical source. Distortion inherent in certain analog transmitters prevents a linear electrical modulation signal from being converted linearly to an optical signal, and instead causes the signal to become distorted. These effects are particularly detrimental to multi-channel video transmission which requires excellent linearity to prevent channels from interfering with each other. A highly linearized analog optical system has wide application in commercial TV transmission, CATV, interactive TV, and video telephone transmission. Linearization of optical and other nonlinear transmitters has been studied for some time, but proposed solutions suffer from practical disadvantages. Most applications have bandwidths which are too large for practical use. Feedforward techniques require complex system components such as optical power combiners and multiple optical sources. Quasi-optical feedforward techniques suffer from similar complexity problems and further require extremely well-matched parts. One method employed in the past to reduce distortion inherent in nonlinear devices has been predistortion. In this technique, a modulation signal is combined with a signal equal in magnitude to the distortion inherent in the nonlinear device but opposite in sign. When the nonlinear device modulates the combined signal, the device's inherent distortion is canceled by the combined signal's predistortion and only the linear part of the source signal is transmitted. This predistortion signal is usually in the form of additive and subtractive combinations of the input fundamental frequencies as these intermodulation products constitute the most fertile source of distortion in analog signal transmission. In the distribution of AM signals for cable television, for example, there are often as many as 40 frequencies on a particular band and plenty of opportunities for second order and third order intermodulation products of those frequencies. Current predistortion techniques generally divide an input signal into two or more electrical paths and generate predistortion on one or more of the paths resembling the distortion inherent in the nonlinear transmitting device. The generated predistortion is the inverse of the nonlinear device's inherent distortion and serves to cancel the effect of the device's inherent distortion when recombined with the input signal. Attenuation can be used to match the magnitude of the predistortion to the magnitude of the device's inherent distortion characteristics before the signals are recombined and sent to the nonlinear device for modulation. However, the method suffers from crudeness because nonlinear devices frequently have amplitude and phase distortion characteristics dependent on the frequency of the modulating signal. Present techniques provide no means for compensating for these frequency-dependent nonlinearities. Neglecting to correct for the frequency dependence of the distortion leads to a result which may be quite tolerable for many systems and for signals with relatively narrow bandwidth. However, they become particularly troublesome when converting an electrical TV signal to an optical signal for cable transmission. Such signals for cable TV may have forty or more input frequencies, all of which need to have high quality amplitude modulated signals. The transmission devices for such signal must have an exceptionally high degree of linearity. It is the object of the invention to improve the circuit and the method as indicated in the precharacterizing parts of claims 1 and 14 such that an input modulation signal having a broad band width can be predistorted so that the transmission of the modulation signal is largely linearized. This object is solved by the features as claimed in claims 1 and 14. Thus, in practice of this invention according to a presently preferred embodiment, a predistortion circuit for reducing distortion in the transmission of analog signals splits an input modulation signal into two electrical paths, one primary and one secondary. A predistortion amplifier on the secondary path generates second order or higher even order intermodulation distortion products of the input signal. The distortion so generated, or predistortion, is adjusted to be substantially equal in magnitude and opposite in sign to the distortion inherent in a nonlinear modulation device to which the signal is applied. The predistortion signal is adjusted in amplitude and phase to match the frequency dependence of the distortion by the nonlinear device. The phase of the signals are synchronized by a delay or phase adjustment element in one of the electrical paths. The primary and secondary signals are then recombined to produce a single modulation signal including intermodulation product distortion. Thus, the predistortion circuit largely linearizes the transmission of modulating signals by cancelling distortion inherent in nonlinear transmitting devices. The invention will be better understood and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: FIG. 1 is a block diagram showing a presently preferred embodiment of a predistortion circuit; FIG. 2 is a block diagram of a push-push amplifier employed in the predistortion circuit according to a preferred embodiment of this invention; FIG. 3 is a schematic diagram exemplifying a practical predistortion circuit; and FIG. 4 is an illustration of the effect of predistortion on the waveforms of a modulation signal. The concept of predistortion is shown abstractly in FIG. 4. An input signal Y 0 is input to a predistortion network 40. The predistortion network has a nonlinear transfer function which deviates from linearity oppositely and inversely from the deviation of a nonlinear transmitter having a known transfer function 41. The signal Y 1 from the predistortion network is a combination of the input source signal Y 0 and predistortion resulting from the nonlinear transfer function 40. Signal Y 1 is fed into the nonlinear transmitter and, after modulation by the transmitter, appears as a substantially linear signal Y 2 as a result of the inherent distortion of the transmitter inversely related to and cancelled by the predistortion of signal Y 1 . Referring now to FIG. 1, an input source signal 12 feeds into a directional coupler 10 and is split into a primary electrical path 13 and a secondary electrical path 14. Typically, the portion of the signal on the primary electrical path is substantially larger in power than the signal on the secondary electrical path. For example, an 11 dB directional coupler may be used to achieve this result. The secondary electrical path comprises in circuit series a distortion generator 15, an amplitude adjustment block 17, a "tilt" or frequency adjustment block 19, and a fine phase adjustment block 21. These elements may be varied in order along the secondary electrical path without departing from the functional purpose of the invention. In one embodiment, the signal on the secondary electrical path feeds first into the distortion generator. The output of the distortion generator comprises intermodulation distortion of the input frequencies. Second order and higher even order distortion are produced. The fundamental frequency is suppressed in the distortion generator by cancellation, filtering or other means. The intermodulation product so generated is opposite in phase to the input signal. This inversion may be accomplished within the distortion generator or with a separate inverter element (not shown). The distorted output from the distortion generator is matched in magnitude to the magnitude of inherent distortion predicted in the transmission device (not shown in Fig. 1) receiving the output signal 25. The matching function occurs in the amplitude adjustment block 17 and this adjustment may be accomplished manually with a variable attenuator or dynamically with an automatic gain control element, for example. The output of the amplitude adjustment block 17, therefore, comprises intermodulation distortion of a small portion of the input signal and is substantially equal in magnitude and opposite in sign to distortion inherent in a nonlinear transmission device receiving the output signal 25 of the predistortion circuit. This output or predistortion signal effectively reduces the frequency independent component of the distortion of the nonlinear device. Generation of the predistortion signal on the secondary electrical path typically involves a time delay relative to the primary electrical path. Before the primary and secondary paths are recombined an adjustment is made to set the relative phase of the primary path electrical signal with respect to the phase of the secondary path electrical signal which results in best cancellation of the distortion inherent in the nonlinear device. This phase matching is done on the primary electrical path by an external delay 23 which receives the primary portion of the signal 13 split by the directional coupler 10. The time delay may be manually or automatically adjusted. An exemplary delay may be simply a transmission line of selected length to introduce a suitable delay. An exemplary transmission device may be a semiconductor laser or LED modulated by the output signal. The inherent distortion of such a device is not independent of frequency. Generally speaking, the distortion is inherently greater at higher frequencies. To adjust for frequency dependent distortion of the nonlinear transmitting device, the output of the amplitude adjustment block is then fed into a frequency adjustment or "tilt" adjustment block 19. The tilt adjustment is a variable filter or other similar means which increases the amplitude of the distortion at high frequencies for an "up-tilt" and decreases it at high frequencies for a "down-tilt." This adjustment, like the amplitude adjustment, may be done either manually or automatically. By passing more or less of the high-frequency distortion products than the low-frequency distortion products, the tilt adjustment enables the predistortion signal to be tailored more precisely to the inherent distortion characteristics of the nonlinear device. Typically, the amplitude adjustment is made to compensate for the distortion occurring at the low frequency end of the band. The frequency adjustment is then made as an up-tilt to compensate for distortion at the high frequency end of the band. It may be noted that this same effect can be achieved by amplitude adjustment at the high frequency end, and an up-tilt or down-tilt on the low-frequency end as an appropriate attenuation or amplification of the signal. An additional fine phase adjustment block 21 on the secondary electrical path provides for more accurate setting of the relative phase between the distortion generated in the secondary path and the distortion inherent in the nonlinear device. This adjustment, like the amplitude adjustment, may also be made manually and may be frequency dependent. It is found that manual adjustment of amplitude, frequency and phase is usually completed in less than a minute. What one does is make the appropriate adjustment while observing the distortion in the output of the nonlinear device. The adjustment seeks to minimize the final distortion. The optimum adjustment is when the predistortion signal is of the same magnitude as the distortion inherent in the nonlinear device, and the predistortion is exactly 180° out of phase with the distortion. It is significant that the phase adjustment is made relative to the distortion of the device. Previously time delays have been introduced so that the predistortion is exactly in phase (or 180° out of phase) with the primary signal. This may be sufficient for some purposes, but is not suitable for others, such as TV bandwidth modulation of a laser, for example. Once the relative phases of the signals on the primary and secondary electrical paths have been set, they are recombined by the output directional coupler 11. The combined signal 25, including the predistortion component from the secondary path, is output to a nonlinear transmission device for modulation of the signal. An example of a predistorter or distortion amplifier block 15 is shown in detail in FIG. 2. A portion of the input signal 14 on the secondary electrical path is fed into a 180° splitter 30 which divides the signal into a first electrical path 38 and a second electrical path 39 of equal magnitude and opposite sign. If desired, the signals so divided need not be of equal magnitude if subsequently amplified or attenuated. The first electrical path feeds into a first amplifier 32 generating second order and higher order intermodulation products of the fundamental frequencies in the input signal 14. The second electrical path, carrying a signal opposite in sign to the first electrical path, feeds into a second amplifier 33 generating even order intermodulation products which are of the same sign as those output by the first amplifier 32, but generating odd order intermodulation products opposite in sign to those output by the first amplifier. The signals are combined additively by a 0-degree combiner 34 which substantially reduces the fundamental frequencies and odd order intermodulation products, leaving even order intermodulation product components in an output signal 37. Ideally, this process produces pure second order and higher order even components of intermodulation distortion. The first and second amplifiers 32 and 33 are adjustable. This adjustment can be accomplished by varying the bias currents to the amplifiers which has little effect on gain of the fundamental frequencies. An increase in the bias current of the first amplifier 32 with a corresponding decrease in the bias current of the second amplifier 33 will unbalance the two amplifiers in the sense that the magnitude of intermodulation products produced will no longer be identical between the two amplifiers. Thus, the odd order intermodulation products would not cancel one another. The unbalancing of this distortion circuit, which is referred to as a push-push amplifier, would generate intermodulation distortion of all orders. The fundamental frequencies may be suppressed by particular amplifier design or by filtering means (not shown) either in series with, after or integral to each amplifier. Preferably, the bias currents of both amplifiers 32 and 33 are adjusted in equal and opposite directions or senses, so that the unbalance affects only the odd order intermodulation products and the even order intermodulation products remain balanced and substantially unchanged in magnitude. One embodiment of the predistortion circuit is shown in FIG. 3. The signal 14 in the secondary path from the signal splitting coupler 10 is first attenuated by way of an adjustable attenuator R 1 , R 3 to assure a constant signal level. If the signal is too small there may not be sufficient distortion to compensate for the distortion of the transmission device. Conversely, if the signal is too large, the distorter could be overloaded and itself produce unacceptable distortion. The attenuated signal is split in the 180° splitter 30, and capacitively coupled to the first and second amplifiers 32 and 33. The bias of the amplifiers is adjusted to obtain the desired second order and higher even order intermodulation products, and the recombined signal is attenuated by way of the amplitude adjustment 17 to obtain the desired amount of distortion at relatively low frequencies, such as 50 MHz. Next one checks the higher frequency end of the band and adjusts the frequency filter 19 until the distortion matches the inherent distortion of the transmission device at this higher frequency. This has little effect on the predistortion at the lower frequency end of the band. It, in effect, tilts the amplitude as a function of frequency around a pivot near the low end of the band. The time delay 23 is adjusted at the high frequency end of the band to adjust the phase of the signal in the primary signal path. Again, this has little effect at the low frequency end of the band. Finally, the phase adjustment 21 is used to more precisely adjust the phase of the predistortion generated in the secondary path to compensate for the phase distortion by the nonlinear device. If need be, the adjustment sequence can be repeated to more closely match the inherent distortion of the transmission device. Ordinarily, the initial attenuator and the bias of the predistortion amplifiers need not be adjusted, but may remain in a preset state. The three adjustments of the amplitude, tilt, and phase are sufficient. The principal delay in the primary path may also be fixed for a given secondary path. The signal in the secondary path is recombined with the signal in the primary path by way of the directional coupler 11, and the signal 25 thereby predistorted is applied to a laser 40 or the like for modulation. Many variations and modifications will be apparent to those skilled in the art. For example, although described and illustrated in the context of a TV signal modulating a laser or light emitting diode, other nonlinear devices such as amplifiers may have inherent distortion largely cancelled by this technique. The fine adjustment of the relative phase of the signals in the primary and secondary paths is in the secondary path in the illustrated embodiment, but this could also be in the primary path with the coarse adjustment. The secondary path is preferred since such a delay in the primary path may have an inappropriate impedance for this path. In this embodiment, there is a single secondary signal path with its distortion generator. If desired, a third "secondary" path could be employed with one path generating second order cancellations signals and another path generating third order cancellation signals. Each of these paths may have its own adjustment for frequency dependence of amplitude and phase. In such an embodiment it is preferred to have fine adjustment of phase in each of the secondary paths. In the event two or more secondary paths are used for higher order distortion, the amplitude, tilt and phase in either path may be adjusted first since there is no interaction between them.

Claims: A predistortion circuit comprising: means (10) for splitting an input modulation signal for a non-linear device into a primary electrical path (13) and a secondary electrical path (14); means (11) for additively recombining the primary and secondary paths (13,14) into a single path to form a signal for application to a non-linear device (40); means (15) for generating intermodulation products in the secondary path (14) having a relative amplitude corresponding to the amplitude of distortion in the non-linear device (40) and for suppressing fundamental frequencies of the modulation signal; means (17) in series with the means (15) for generating intermodulation products for adjusting the amplitude of the signal on the secondary electrical path (14); and means (23) in the primary electrical path (13) and/or the secondary electrical path (14) for adjusting phase of primary path signal with respect to phase of secondary path signal; characterized by means (19) for providing frequency dependent amplitude adjustment of substantially only the second order and higher even order intermodulation products in the secondary path. A predistortion circuit as claimed in claim 1, wherein the means (23) in the primary electrical path (13) for adjusting the phase of the primary path signal with respect to the phase of the secondary path signal comprises a time delay means. A predistortion circuit as claimed in claim 1 or 2 further comprising means (21) in series with the other means (15,17,19) in the secondary electrical path (14) for compensating relative phase differences between the secondary path (14) and the non-linear device (40). A predistortion circuit as claimed in at least one of claims 1 to 3 wherein the primary electrical path (13) carries a major portion of the input signal power. A predistortion circuit as claimed in at least one of claims 1 to 4 wherein the means (15) for generating intermodulation products comprises a push-push amplifier (32,33). A predistortion circuit as claimed in claim 5 wherein the push-push amplifier (32,33) comprises: means (30) for splitting the input signal into first and second electrical paths (38,39) said signals being equal in magnitude and opposite in sign; first amplification means (32) in the first electrical path (38) for generating positive intermodulation products of the signal carried on the path; second amplification means (33) in the second electrical path (39) for generating positive even order intermodulation products and negative odd order intermodulation products of the signal carried on the path; and signal combining means (34) for additively recombining the first and second electrical paths (38,39) after the intermodulation products have been generated, thereby substantially cancelling the odd order intermodulation product components. A predistortion circuit as claimed in claim 5 or 6 wherein the push-push amplifier (32,33) further includes biasing means for balancing the first and second amplification means (32,33), the biasing means including first sense of adjustment to the bias current of the first amplification means (32) and an opposite sense of adjustment to the second amplification means (33), for maintaining substantially constant magnitude of even order intermodulation products. A predistortion circuit as claimed in claims 6 or 7 further comprising filtering means in series with each amplification means (32,33) for suppressing the fundamental frequencies. A predistortion circuit as claimed in claim 8 wherein the filtering means is integral with the first and second amplification means (32,33). A predistortion circuit as claimed in at least one of claims 1 to 7 comprising means (34) in the secondary electrical path (14) for suppressing the fundamental frequencies of the modulation signal. A predistortion circuit as claimed in claim 10 wherein the means for suppressing comprises the first and second amplification means (32,33). A predistortion circuit as claimed in at least one of claims 1 to 11 wherein said splitting means (10) is a first directional coupler, and said recombining means (11) is a second directional coupler. A predistortion circuit as claimed in claim 12 wherein the input signal is split into a plurality of secondary paths, each path comprising means for generating one or more intermodulation products of the input modulation signal. A method of reducing distortion in an amplitude modulating device (40) comprising the steps of: splitting an input signal for said modulating device (40) into primary and secondary electrical paths (13,14); generating intermodulation products in the secondary path (14) having a relative amplitude corresponding to the amplitude of distortion in the non-linear device (40); suppressing fundamental frequencies of the modulation signal; adjusting the amplitude of the signal on the secondary electrical path (14), and recombining the primary and secondary electrical signals for providing an output signal and applying same to the non-linear modulating device; characterized by providing frequency dependent amplitude adjustment of substantially only the second order and higher even order intermodulation products in the secondary path. A method as recited in claim 14, wherein the intermodulation distortion is made substantially equal in magnitude to the distortion of the non-linear modulating device (40) at a relatively lower frequency, and tilt is adjusted to make the intermodulation distortion generated at a relatively higher frequency substantially equal in magnitude to the distortion of the non-linear modulating device (40) at the relatively higher frequency without substantially changing the magnitude of the intermodulation distortion generated at the relative lower frequency. A method as recited in claim 14 or 15 wherein the time delay of the signal in one of the paths (13,14) is adjusted at a relatively higher frequency to be about 180°C out of phase with the distortion of the non-linear device (40) at the higher frequency.

Inventors: BLAUVELT HENRY A, LOBODA HOWARD L, BLAUVELT, HENRY A., LOBODA, HOWARD L.
Applicants: ORTEL CORP, ORTEL CORPORATION
Application year: 1990
Main classification: H04B10
IPC codes: H04B10, H04N7, H01L33, H01S5, H03C1, H04B1, H03F1