In-line predistorter for linearization of electronic and optical signals

What is claimed is:

1. An in-line predistorter circuit for generating predistortion that can be used to substantially cancel or reduce the distortion from a nonlinear device, the circuit comprising an RF signal path carrying an RF signal input to the nonlinear device, and a plurality of in-line distortion producing circuit elements placed in sequence along the RF signal path, wherein real and imaginary distortion, sufficient to substantially cancel or reduce the distortion from a nonlinear device, is synthesized by additively combining the distortion contributions from the combination of in-line distortion producing circuit elements, wherein no separation of the RF signal into a fundamental signal path and a separate and distinct distortion producing path occurs within the in-line distortion producing circuit elements.

2. The in-line predistorter of claim 1 wherein at least one of the in-line circuit elements comprises a Schottky diode and a resistor connected in parallel across the RF signal path, wherein voltage across the resistor and Schottky diode varies as the RF signal varies, and wherein the circuit element generates primarily constant, frequency independent, real second order distortion.

3. The in-line predistorter of claim 1 wherein at least one of the in-line circuit elements comprises first and second serially coupled Schottky diodes connected in parallel with a resistor across the RF signal path, wherein the voltage across the resistor and Schottky diodes varies as the RF signal varies, and wherein the circuit element generates primarily constant, frequency independent, real second order distortion.

4. The in-line predistorter of claim 1 wherein at least one of the in-line circuit elements comprises a Schottky diode, resistor and capacitor all connected in parallel in the RF signal path, wherein the circuit element produces both real and imaginary distortion.

5. The in-line predistorter of claim 1 wherein at least one of the in-line circuit elements comprises a first section for producing sublinear CSO and compressive CTB, and a second section for producing sublinear CSO and expansive CTB, wherein the additive contribution of CTB by each section of the circuit results in the substantial reduction of CTB.

6. The in-line predistorter of claim 1 wherein at least one of the in-line circuit elements comprises a resistor and a pair of antiparallel Schottky diodes connected in parallel in the RF signal path, wherein the circuit element produces primarily real constant third order distortion.

7. The in-line predistorter of claim 1 wherein at least one of the in-line circuit elements comprises a resistor, antiparallel diodes, and a capacitor connected in parallel in the RF signal path, wherein the circuit produces both real and imaginary components of third order distortion.

8. An in-line predistorter circuit for generating predistortion that can be used to substantially cancel or reduce the distortion from a nonlinear device, the circuit comprising:

an RF signal path carrying an RF signal input to the nonlinear device, and a plurality of in-line distortion producing circuit elements placed in sequence along the RF signal path,

wherein real and imaginary distortion, sufficient to substantially cancel or reduce the distortion from a nonlinear device, is synthesized by additively combining the distortion contributions from the combination of in-line distortion producing circuit elements, and

wherein at least one of the in-line circuit elements comprises a Schottky diode connected in parallel with a resistor across the RF signal path, with a time delay in series with the resistor, wherein the circuit element produces both real and imaginary distortion.

9. An in-line predistorter circuit for generating predistortion that can be used to substantially cancel or reduce the distortion from a nonlinear device, the circuit comprising:

an RF signal path carrying an RF signal input to the nonlinear device, and a Plurality of in-line distortion producing circuit elements placed in sequence along the RF signal path,

wherein real and imaginary distortion, sufficient to substantially cancel or reduce the distortion from a nonlinear device, is synthesized by additively combining the distortion contributions from the combination of in-line distortion producing circuit elements, and

wherein at least one of the in-line circuit elements comprises a varactor connected between the main signal path and ground, wherein the circuit element produces primarily imaginary second order distortion.

10. An in-line predistorter circuit for generating predistortion that can be used to substantially cancel or reduce the distortion from a nonlinear device, the circuit comprising:

an RF signal path carrying an RF signal input to the nonlinear device, and a plurality of in-line distortion producing circuit elements placed in sequence along the RF signal path,

wherein real and imaginary distortion, sufficient to substantially cancel or reduce the distortion from a nonlinear device, is synthesized by additively combining the distortion contributions from the combination of in-line distortion producing circuit elements, and

wherein at least one of the in-line circuit elements comprises a resistor and antiparallel diodes connected in parallel in the RF signal path with a time delay connected in series with the resistor, wherein the circuit produces both real and imaginary components of third order distortion.

11. An in-line predistorter circuit for generating predistortion that can be used to substantially cancel or reduce the distortion from a nonlinear device, the circuit comprising:

an RF signal path carrying an RF signal input to the nonlinear device, and a plurality of in-line distortion producing circuit elements placed in sequence along the RF signal path,

wherein real and imaginary distortion, sufficient to substantially cancel or reduce the distortion from a nonlinear device, is synthesized by additively combining the distortion contributions from the combination of in-line distortion producing circuit elements, and

wherein at least one of the in-line circuit elements comprises a pair of oppositely directed varactor diodes connected between the RF signal path and ground, wherein the circuit produces imaginary third order distortion.

12. An in-line predistorter circuit for generating predistortion that can be used to substantially cancel or reduce the distortion from a nonlinear device, the circuit comprising:

an RF signal path carrying an RF signal input to the nonlinear device, and a plurality of in-line distortion producing circuit elements placed in sequence along the RF signal path,

wherein real and imaginary distortion, sufficient to substantially cancel or reduce the distortion from a nonlinear device, is synthesized by additively combining the distortion contributions from the combination of in-line distortion producing circuit elements, and

wherein at least one of the in-line circuit elements comprises a resistor connected to the main RF signal path and two oppositely directed varactor diodes connected to ground, wherein the circuit produces third order distortion that is out of phase with the signal on the main signal path.

13. An in-line predistorter circuit for generating predistortion that can be used to substantially cancel or reduce the distortion from a nonlinear device, the circuit comprising:

an RF signal path carrying an RF signal input to the nonlinear device, and a plurality of in-line distortion producing circuit elements placed in sequence along the RF signal path,

wherein real and imaginary distortion, sufficient to substantially cancel or reduce the distortion from a nonlinear device, is synthesized by additively combining the distortion contributions from the combination of in-line distortion producing circuit elements, and

wherein at least one of the in-line circuit elements comprises a resistor connected in series with the varactor 252 between the RF signal path and ground, wherein the circuit element produces distortion that includes both real and imaginary parts.

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FIELD OF THE INVENTION

This invention relates to an electronic circuit for providing a linear output from an amplitude modulated transmission device such as a semiconductor laser which has an output distorted from its input due to inherent nonlinearity. The distortion of the nonlinear device is compensated by applying a predistorted signal to the input of the nonlinear device. The predistortion is chosen such that the distortion of the nonlinear device restores the undistorted signal.

BACKGROUND

Directly modulating the analog intensity 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 amplitude modulation techniques have the advantage of significantly smaller bandwidth requirements than baseband digital modulation or frequency modulation, amplitude modulation may suffer from noise and nonlinearity introduced by 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 one another. A highly linearized analog optical system has wide application in CATV, interactive TV, and video telephone transmission, for example.

Linearization of optical and other nonlinear transmitters has been studied for some time, but proposed solutions suffer from practical disadvantages or cost penalties that limit usefulness to high value devices. Feedforward techniques, for example, require complex system components such as optical power combiners and multiple optical sources.

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 is modulated by the combined signal, the device's inherent distortion is canceled by the distortion signal generated by the predistortion, and only the linear part of the source signal is transmitted. The intermodulation products in the predistortion signal are at frequencies that are additive and subtractive combinations of integer multiples of the input frequencies. In the distribution of AM signals for cable television, for example, there are often as many as 80 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 before application to the nonlinear device.

Advanced multi-path predistortion circuits are flexible and highly effective for linearizing output of a wide range of nonlinear devices. One such multi-path predistortion circuit is disclosed in U.S. Pat. No. 4,992,754, issued to Blauvelt et al. The circuit is capable of generating frequency specific distortion products for compensating frequency-dependent nonlinearities, and is useful for applications requiring an exceptionally high degree of linearity, such as, for example, CATV applications.

Although multi-path predistortion circuits can be used in a broad variety of applications, the design of these circuits is relatively complex. This complexity manifests itself in circuits that are often too expensive for applications needing only a modest degree of linearization. One skilled in the art would appreciate a low-cost circuit of relatively simple design for limited application, particularly if such a circuit were fabricated from existing low-cost components commonly used in signal transmission applications.

Those skilled in the art would also appreciate a circuit that could produce frequency dependent third-order distortion. Simple third-order distortion, such as that produced by an ideal diode, has the property that the distortion is real and independent of frequency. Many non-linear transmitters or amplifiers, however, contain reactive elements such as inductances, capacitances or delays, which cause the device to produce distortion depending on the input and output frequencies and the distortion frequencies. Nazarathy, U.S. Pat. No. 5,161,044, discloses a circuit in FIG. 15 of that patent which produces essentially real, frequency-independent predistortion. The capacitors and inductors in Nazarathy are added for biasing purposes and to block the DC and AC currents. However, the circuit disclosed by Nazarathy may not have the right phase or frequency dependence for each set of input frequencies, to be substantially the same in magnitude and opposite in sign to the distortion produced by the non-linear device.

The present invention accordingly is addressed to a low-cost predistortion circuit reducing second and higher order distortion products produced by a nonlinear device and to a circuit for generating frequency dependent third-order distortion.

SUMMARY

Thus, in practice of this invention according to one embodiment, an in-line predistortion circuit is provided for reducing distortion in the transmission of analog signals. The distortion so generated, or predistortion, is adjusted to be substantially equal in magnitude and opposite in sign to the second or higher order intermodulation product distortion inherent in a nonlinear modulation device to which the signal is applied. The real component of the predistortion signal is produced by a first device such as an amplifier, and is adjusted in amplitude to match the amplitude of the distortion by the nonlinear device. The imaginary component of the predistortion signal is adjusted through introduction of a distortion signal out of phase with the real component of the predistortion signal on the in-line electrical path. The real and imaginary components are combined to produce a single modulation signal including intermodulation product distortion for application to the nonlinear device. The in-line predistortion circuit largely linearizes the transmission of modulating signals by canceling distortion inherent in nonlinear transmitting devices and can be formed with commonly-used, low-cost components.

In an alternate embodiment, the real component of the predistortion signal is produced by a FET configured as a voltage-controlled resistor, connected from the RF signal path to ground. In another embodiment, the real component of predistortion is produced by the parallel combination of a diode and a resistor connected in series with the RF signal path. The magnitude of the predistortion produced by these devices is adjustable by changing the DC bias current supplied to the device.

In another alternate embodiment, a separate predistortion circuit is provided for generating frequency-dependent third-order distortion. Frequency dependent third-order distortion is generated by the combination of a pair of antiparallel diodes with reactive circuit elements and delays. The magnitude of the predistortion produced by this circuit is adjustable by changing the DC bias current supplied to the diodes.

In the in-line predistorter of the present invention, the desired real and imaginary distortion terms may be synthesized by summing the distortion contributions from several different distorter elements. In the simplest case, one distorter produces a constant real distortion, another produces distortion proportional to frequency, f, and so on. However, it is not essential to have the simplest set of distorters. Distorters with more complex distortion characteristics can be used so long as they provide an independent set. Accordingly, there are disclosed additional "building block" circuits for obtaining desired distortion characteristics.

By way of example, a Schottky diode connected in parallel with a resistance element in the main RF signal path ideally produces distortion that is generally in phase with the voltage across it. As another example, a varactor connected between the RF signal path and ground, ideally produces complementary distortion that varies as a function of frequency and is 90 degrees out of phase with the voltage applied to it. By configuring different in-line circuits that include the basic components, it is possible to generate distortion that is sufficient to compensate for the non-linear characteristics of most devices used for the transmission of broadband signals.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of this 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 the general features of an in-line predistortion circuit according to the present invention;

FIG. 2 is a block diagram showing an embodiment of an inline predistortion circuit;

FIG. 3 is a block diagram of an alternate embodiment of the in-line predistortion circuit of FIG. 2;

FIG. 4 is a schematic diagram of an intrastage filter employed in a predistortion circuit;

FIG. 5 is a schematic of an interstage filter employed in a predistortion circuit;

FIG. 6 is an illustration of the effect of predistortion on the waveforms of a modulated signal;

FIG. 7 is an illustration of the real and imaginary vector components of distortion;

FIG.8 is a schematic diagram of a hybrid predistortion circuit;

FIG. 9A is a schematic diagram of a circuit including a FET configured as a voltage controlled resistor for producing primarily a real component of predistortion;

FIG. 9B is an alternate embodiment of the circuit of FIG. 9A;

FIG. 10A is a schematic diagram of a circuit including a parallel combination of a diode and a resistor for producing primarily a real component of predistortion;

FIG. 10B is an alternate embodiment of the circuit of FIG. 10A;

FIG. 11A is a schematic diagram of a circuit for generating frequency dependent third-order distortion;

FIG. 11B is an alternate embodiment of the third-order distortion circuit of FIG. 11A;

FIGS. 12, 12A, 13-21 illustrate exemplary in-line predistorters circuits that can be used as building blocks, either by themselves, or in combination with any of the circuits disclosed herein, for generating the desired predistortion using a combination of in-line elements:

FIGS. 12-16 are diagrams of circuits for generating primarily second order distortion;

FIGS. 17-21 are diagrams of circuits for generating primarily third order distortion; and

FIG. 22 is a diagram of a circuit for substantially canceling undesired third order distortion generated by second order distortion circuit.

DETAILED DESCRIPTION

The concept of predistortion is shown abstractly in FIG. 6. An input signal Y0 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 Y1 from the predistortion network is a combination of the input source signal Y0 and predistortion resulting from the nonlinear transfer function in the predistortion network 40. Signal Y1 is fed into the nonlinear transmitter and, after modulation by the transmitter, appears as a substantially linear signal Y2 as a result of the inherent distortion of the transmitter inversely related to and canceled by the predistortion of signal Y1.

The distortion generated by an exemplary nonlinear device is shown graphically in FIG. 7. The graph shows a polar display of the real component 50 and imaginary component 60 of distortion which are combined to form a distortion vector 70. Using conventional notation, input signals

e.sup.iw.sub.1.sup.t (1)

and

e.sup.iw.sub.2.sup.t (2)

cause second order distortion products defined by the equation:

Ae.sup.i[w.sub.1.sup.+w.sub.2.sup.)t+.theta. (3)

The real component of the distortion, i.e., the vector component of the distortion signal along the real axis (0.degree. phase angle), is Acos.theta.; the imaginary component of the distortion, i.e., the vector component of the distortion signal along the imaginary axis (90.degree. phase angle), is Asin.theta.. To provide linearized output from a nonlinear device, both the real and imaginary components of distortion in the nonlinear device should be canceled. This is done by applying real and imaginary predistortion components varying inversely and oppositely from those produced by the nonlinear device.

Referring now to FIG. 1, in an exemplary in-line predistorter circuit according to the present invention, an input source signal 12 feeds into an in-line electrical path 14. By in-line electrical path, it is meant that the input source signal is passed through a single distortion producing path as opposed to being split between two or more separate paths connected in parallel. The in-line electrical path comprises in series a real distortion component generator 16 for generating primarily real distortion components and an imaginary distortion component generator 18 for generating primarily imaginary distortion components. Ideally, the combined distortion from the generators, applied to the nonlinear device, is equal and opposite to the distortion produced by the nonlinear device 20 to which the predistorted input source signal 22 is applied. The real distortion generator may include some imaginary component, and the imaginary distortion generator may have some real component. These are included when forming the vector sum of the real and imaginary components to match the distortion of the nonlinear device.

FIG. 2 illustrates an exemplary embodiment of a practical in-line predistorter circuit including, serially, a monolithic microwave integrated circuit (MMIC) amplifier 30, a CATV hybrid amplifier 32, an RF inverter 34, and a varactor 36 preceding a nonlinear device such as a laser. The signal on the in-line path feeds first into a single-ended amplifier, e.g., the MMIC amplifier, for generating primarily real components of predistortion. The MMIC amplifier is a low-cost component commonly used in RF circuit designs. The MMIC has the advantage of low cost, but similar performance is obtained from amplifiers built as hybrid integrated circuits or built from discrete components. The output of the MMIC amplifier comprises the amplified input fundamental frequencies and intermodulation distortion of the input signal frequencies. Primarily second order intermodulation products are produced by the MMIC amplifier.

The amplitude of the real component of the distorted output from the MMIC amplifier is preferably matched in amplitude to the amplitude of the real component of the inherent distortion predicted in the nonlinear transmission device. However, the MMIC amplifier has been found to exhibit distortion characteristics only proportional to those of a nonlinear laser and may need adjustment. The distortion from the MMIC amplifier is generally of larger magnitude than that produced by the nonlinear laser for equal input signal levels. To match distortion amplitudes, the output signal level from the MMIC must be lower than the input signal level to the laser. This requires using a gain block between the MMIC and the laser. It may also be required to introduce attenuation through the attenuator 38 before the MMIC to have each component operating at the current signal level.

Due to its low cost, wide use in coaxial distribution networks, and linear output over input frequencies of interest, the CATV hybrid amplifier 32 is suitable for boosting the output signal of the MMIC amplifier. The CATV amplifier produces negligible distortion over most low to moderate signal levels. At high signal levels, the CATV amplifier may exhibit distortion. However, this is not a problem because the signal levels at which distortion occurs are generally higher than those of interest for modulation of a typical nonlinear laser device.

The amount of attenuation and the CATV amplifier gain may be varied as necessary to produce distortion products in the input modulation signal. The magnitude of distortion in the MMIC is determined by the strength of the input signal. The distortion is greater at high signal strengths. Thus, if a greater distortion is desired, the input signal may be attenuated less and the gain of the CATV hybrid amplifier reduced. Likewise, the bias on the MMIC amplifier and CATV amplifier may be adjusted for varying the relative amplitude of distortion. By driving the MMIC amplifier harder, a larger distortion is obtained (relative to the signal strength) than if the input signal is smaller.

The predistorted signal adjusted to the proper level by the CATV amplifier is inverted, if necessary, in the RF invertor 34 to provide a signal that can be used to cancel the real component of the distortion in the nonlinear device.

The imaginary component of the predistortion signal is primarily generated in the exemplary embodiment by the varactor 36, formed in a typical embodiment by a resistor 68 and diode 72 connected to ground. The varactor, which has a capacitance which varies with voltage, produces harmonic distortion products that increase with the square of frequency of the input signal and are 90.degree. out of phase with the fundamental signals. When the varactor is used without a resistor, the distortion generated is purely imaginary, increasing in amplitude in proportion to the frequency of the distortion signal. Including the resistor introduces a small real component that can be varied by varying the value of the resistor.

The imaginary component of the predistortion signal created by the varactor is controlled by varying the voltage to the varactor input 74 from an external source. As the voltage is increased, lower distortion is produced due to the smaller variation of capacitance with voltage at higher reverse bias. At lower voltages, the diode exhibits greater distortion. This adjustment in the varactor, like the amplitude adjustment, may also be made manually br automatically. Assuming a simple sine wave input on the in-line path through the varactor, the peaks of the sine wave would shift forward in time, and the valleys backward.

Although it is primarily used to generate real distortion components, the MMIC amplifier may have various mechanisms for distortion, some of which may be frequency dependent and some of which shift the phase of the distortion. Different mechanisms may predominate at different bias voltages. By varying the bias voltage to the amplifiers through bias inputs 76, 78 and the input voltage to the varactor at input 74, the distortion can be adjusted as necessary in most cases to compensate the nonlinear device.

It is found that manual adjustment of amplitude, frequency and phase is usually completed in less than a few minutes. What one does is make an appropriate adjustment while observing the distortion in the output of the nonlinear device. The adjustment seeks to minimize the final distortion of the nonlinear device. 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.degree. out of phase with the distortion. Such an adjustment may also be made automatically through the use of a feedback control circuit, for example. If the nonlinear characteristics of a particular device are known in advance or measurable, the bias voltages of the MMIC amplifier, CATV amplifier and the varactor can be electronically tuned even more rapidly.

Once the real and imaginary components of the predistortion signals on the in-line electrical path have been set, the signal is output to a nonlinear transmission device for modulation of the signal.

The exemplary embodiment described is useful for a variety of nonlinear device applications, but in some cases requiring greater linearity, different or additional components may be necessary on the in-line path. For example, in some cases, the circuit illustrated in FIG. 2 may not produce distortion products with phases that exactly match that of the nonlinear device across the band. Simply varying the bias of the components of a circuit such as illustrated in FIG. 2 may be insufficient to achieve the desired linearization. A reactive circuit such as illustrated in FIG. 3 may be used to correct such a phase difference. The circuit of FIG. 3 has a relatively flat amplitude response and a nonlinear phase response caused by the LRC combination 80 including a resistor 82, an inductor 84, and a capacitor 86. Other all-pass filter circuits with substantially more nonlinear phase responses can also be used, but at somewhat greater cost.

In the simple approach to in-line predistortion, the predistorter comprises individual building blocks with their signals summed to give the desired linearization signal. Among the basic building blocks are series and shunt forward biased diodes, transistors, single-ended amplifiers, CATV amplifiers, and varactor diodes. These blocks can be used as single distorter elements or in combination with passive components such as resistors, capacitors, and inductors. In many cases, however, it is not possible to synthesize the correct predistortion signal from individual blocks. For example, an exemplary transmission device may be a semiconductor laser or LED modulated by the output signal. The inherent distortion of such a device often is not independent of frequency. Generally speaking, the distortion is