US3824482A

US3824482A - Pump generated bias for parametric amplifiers

Info

Publication number: US3824482A
Application number: US00383573A
Authority: US
Inventors: J Whelehan
Original assignee: Cutler Hammer Inc
Current assignee: Cutler Hammer Inc
Priority date: 1973-07-30
Filing date: 1973-07-30
Publication date: 1974-07-16
Anticipated expiration: 1991-07-16

Abstract

Parametric amplifier varactor bias, generated by the rectification of pump power, varies with pump power level to prevent detuning of the varactor circuit caused by changes in pump power level.

Description

United States Patent [191 [111 3,824,482 Whelehan, Jr. July 16, 1974 PUMP GENERATED BIAS FOR [56] References Cited PARAMETRIC AMPLIFIERS UNITED STATES PATENTS [75] Inventor: James J. Whelehan, Jr., Smithtown, 3,388,263 6/1968 Daniel 330/49 Primary Examiner-Herman Karl Saalbach [73] Asslgnee' g g Mlwaukee Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-Henry Huff [22] Filed: July 30, 1973 21 Appl. No.: 383,573 E571 ABSTRACT Parametric amplifier varactor bias, generated by the rectification of pump power, varies with pump power 8 330/ 1 33? level to prevent tletuning of the varactor circuit [58] Field ($53M f 556M313, 127, 199 Caused by changes pump 3 Claims, 6 Drawing Figures RF SIGNAL INPUT 52:22 3? 1 RF PUMP L PARAMETRIC SOURCE CIRCUIT AMPLIFIER C'RCULATOR gaQVIBLTAGE f 57 RIF P RTIONAL T0 PUMP POWER DIVIDER -VARACT0R SIGNAL I NETWORK BIAS OUTPUT PAIENIEIIIIIIIBN I 3.824.482

SHIiEI 1 0f 2 RF SIGNAL INPUT 27 PUMP? 7 I RF PUMP SIGNAL PARAMETRIC 3 IR ULAT SOURCE CIRCUIT AMPLIFIER C c 0R N do VOLTAGE 5 I l PROPORTIONAL RF TO PUMP POWER DIVIDER WARACTOR SIGNAL NETWORK BIAS OUTPUT F/GU/PE AIEIIIEIIIIIIIBAII 3.824.482

SHEET 8 [If 2 DIODE w 29 CAPACITANCE 30 m 32 2a (REVERSE BIAS FIGURE 4 SIGNAL II LEVEL-\- *FREQUENCY FIGURE 5 SIGNAL A LEVEL\ 33 36 qFREQUENCY FIGURE 6 PUMP GENERATED BIAS FOR PARAMETRIC AMPLIFIERS BACKGROUND 1. Field The invention pertains to improvements in the generation of parametric amplifier varactor bias.

2. Prior Art Varactor bias in parametric amplifiers is conventionally supplied by a separate stable power supply dedicated solely to this purpose. Variations in pump power are ordinarily minimized by a variety of methods ranging from feedback leveling loops .to temperature controlled environmental chambers. Although satisfactory compensation has been obtained with prior systems, it has often been achieved at'the expense of size, weight, primary power and cost.

SUMMARY The object of this invention is to provide a simple varactor bias supply for a parametric-amplifier and to automatically vary the bias voltage to prevent detuning of the parametric amplifier caused by pump power variations.

According to this invention, a dc voltage generated by rectification of pump power is used to provide the bias for the varactor. This voltage can beobtained from a diode connected to .the pumpsource. When the pump contains afrequency multipler, this dc voltage is readily obtained by sampling the dc voltage developed across a multipler diode. This dc voltage, whether obtained from a multipler diode or a separate detection diode, varies as a function of pump power, and when adjusted in level and applied to the varactor as bias, it varies to oppose the variation in varactor capacitance which would occur due to changes in pump power level. This bias thereby maintains a constant average varactor capacitance and prevents the detuning of the parametric amplifier.

DRAWINGS FIG. 4 is a graph of .a typical varactor capacitance characteristic as a function of reverse bias.

FIG. 5 is a graph of the passband characteristic of a typical double tuned parametric amplifier. FIG. 6 is a graph of the passband characteristic of a double tuned parametric amplifier showing the type of detuning which can occur with a variation in pump power level.

DESCRIPTION Referring to FIG. .1, a source 1 produces RF energy which is applied to a pump circuit 2. The pump circuit 2 provides a pump signal of desired frequency and power to a parametric amplifier 3. When source l produces a signal at the desired pump frequency,the pump circuit merely connects the source 1 to the parametric amplifier; however, when a pump frequency higher than the source frequency is desired, the pump circuit normally includes a'frequency multiplier. The combination of the source 1 and the pumpcircuit 2 ishereinafter referred to as the pump power source. The parametric amplifier, when supplied with pump power, develops a negative resistance which is coupledito acirculator 4 to amplify signals passing through the circulator in a known manner. v

A varying dc voltage, used in producingthe varactor bias for the parametric amplifier, is obtained .from a diode connected to pump circuit 2. This voltage is applied to the varactor by way of divider network 5. When a frequency multiplier is used in thepump circuit, the varying dc voltage may be obtained from a frequency multiplier diode.

The parametric amplifier 3 contains a varactor or varactors and resonators arranged in known manner, for example as shown in USE-Pat. No. 3,105,941. The varactor is normally aback biased diode exhibiting a nonlinear capacitance characteristic as a function of voltage. Atypical varactor characteristic "is illustrated in FIG. 4. The ordinate 27 represents diode capacitance while the abscissa 28 represents reverse diode bias. Curve 29 represents the varactor capacitance characteristic.

Points

30, 31, 32represent various bias points at which 'the varactor may be operated. In normal operation, the 'varactor is biased at one of these points so that the pump swing about this "bias pointdoes not drive the varactor appreciably into the forward bias direction.

The varactor capacitance is instantaneously varied by the voltage swingof the pump signal; however, this varying capacitance appears as a'fixed average capacitance to a resonant circuit when the :pump and bias levels .are constant. When the pump level is not constant, the :a'veragecapacitance of the varactor varies in accorproduce an increase in average varactor capacitance.

The reason for the increase in capacitance with an increase in pump signal level can be seen by examining FIG. 4. An increase in swing about a bias point will increase the peak voltage excursion both to the left and rightof the bias point. To 'the left, the capacitance increases rapidly while to the right it drops slowly. The resultantover acomplete cycle is an overall'increase in average capacitance. The detuning effect caused by such an increase in capacitance can be compensated for by an increase in reverse biasplacing the quiescent "bias point at a lowercapacitance level. Such a change in bias would be exhibited in FIG. 4 by moving from Bias point 31 to bias :point 32. g

In FIG. '5 and FIG. -6, the ordinate 33 represents amplitude, while the abscissa 34 represents frequency. A typical double tuned parametric amplifier bandpass characteristic 35 is shown in FIG. '5. An uncompensated variation in pump power level will detune the resenators, distorting the passband characteristic as shown by bandpass characteristic 36 in FIG. 6. This figure shows peaking at the high end of the band and corresponds to a reduction in varactor capacitance and a reduction in pump power level. Compensation .for this change requires a reduction in reverse bias level, which plied to the input terminals 6 and 7 of the frequency multiplier is passed through filter 8 to diode 10. A portion of the fundamental energy is converted in diode 10 to the desired harmonic frequency. The harmonic frequency is passed through filter 9 and matching trans former 13 to the output terminals 14 and 15. Filter 8 represents a block to the harmonic frequency preventing it from flowing to the'input and filter 9 represents a block to the fundamental frequency preventing it from flowing to the output. A portion of the RF pump power is normally converted to a dc voltage which, in accordance with this invention, is used to produce the varactor bias voltage. The dc voltage is obtained from terminals'll and 12 which are connected across multiplier diode 10. This dc voltage is isolated from the remainder of the multiplier circuit by capacitors l6 and 17 connected to diode 10. Since parametric amplifiers normally operate at microwave frequencies, the symbols used'in FIG. 2 should be considered, when appropriate, as representing the equivalent microwave circuit element.

The diode contained in the frequency multiplier illustrated in FIG. 2 not only functions as a frequency multiplying element, but also functions as a shunt rectifier with the rectified output varying in accordance with the level of pump power. As'previously described, the dc voltage generated'in the multiplier diode may not be proper in amplitude to set the correct varactor bias. Therefore, this amplitude level is adjusted as necessary in a divider network. I

A typical-divider network is shown in FIG. 3. This network reduces the bias level through a resistive divider formed by resistors'20, 21 and 22. If necessary, temperature compensation may be provided by a temperature compensation element such as diode 26, which is shunted across resistors 21 and 22. The temperature compensating diode may be used for the purpose of overall system compensation, adjusting for the effects of temperature on a number of components including the divider network, the multiplier diode and the varactor- The dc voltage from the detection or multiplierdiode is accepted by the divider network at terminals 18 and 19 and filtered in a low-pass filter 25 connected across these terminals to remove RF components. Filter 25 is a symbolic representation of more complete filtering circuitry, which normally includes, in addition to the shunt capacitor, other components such as a high impedance series input element. After filtering and division, the bias voltage is delivered at

output terminals

23 and 24 connected across resistor 22. The voltage developed across these terminals is a varactor bias voltage of the proper amplitude and sense to prevent detuning of the parametric amplifier.

I claim:

1. A system for providing dc bias for the varactor o a parametric amplifier which includes a pump power source, wherein the improvement comprises:

a. an asymmetrically conductive diode connected to said pump power source for producing a dc voltage which variesas a function of pump power level, and i b. means for utilizing a proportion of said dc voltage to vary said bias, said proportion being chosen to substantially equalize the opposing effects of variation in dc bias level and variation in pump power level upon the tuning of the parametric amplifier.

2. A system for providing bias as recited in claim 1, wherein the pump power source includes a frequency multiplier and said diode is a multiplier diode in said frequency multiplier.

3. A system for providing bias as recited in claim 1, wherein said means for utilizing a proportion of said dc voltage includes a voltage divider for applying to the varactor a constant proportion of said dc voltage, which approximately equalizes the opposing effects of variationsin dc bias level and variations in pump power level upon the tuning of the parametric amplifier over a range of pump power levels about a nominal design level.

Claims

1. A system for providing dc bias for the varactor of a parametric amplifier which includes a pump power source, wherein the improvement comprises: a. an asymmetrically conductive diode connected to said pump power source for producing a dc voltage which varies as a function of pump power level, and b. means for utilizing a proportion of said dc voltage to vary said bias, said proportion being chosen to substantially equalize the opposing effects of variation in dc bias level and variation in pump power level upon the tuning of the parametric amplifier.

3. A system for providing bias as recited in claim 1, wherein said means for utilizing a proportion of said dc voltage includes a voltage divider for applying to the varactor a constant proportion of said dc voltage, which approximately equalizes the opposing effects of variations in dc bias level and variations in pump power level upon the tuning of the parametric amplifier over a range of pump power levels about a nominal design level.