Suppression of parametric oscillations in IMPATT amplifiers and oscillators

#The present invention relates to IMPATT amplifiers and oscillators, and more particularly to a circuit for suppressing undesirable parametric oscillations capable of being generated in such amplifiers and oscillators.

#In parametric amplifiers and oscillators, the parametric effects, under controlled conditions, provide amplification and frequency conversion. IMPATT amplifiers and oscillators, however, employ semiconductors which exhibit nonlinear reactance and parametrically generated negative resistance under large-signal conditions; these effects are usually undesirable since they cause spurious oscillations leading to severe distortions of the output signal, a reduction of power output and efficiency, and an increase in noise output.

#In recent years, there have been significant improvements in avalanche diodes to realize the full high power capability of IMPATT devices operating at high frequency. Among the improvements has been the reduced series resistance of, for example, the gallium arsenide (GaAs) avalanche diodes having a Read Structure which has yielded more than 5 watts cw at greater than 30% efficiency when operated as an IMPATT oscillator. However, the limitation to the application of this diode to its operation in an IMPATT amplifier or oscillator circuit has been its inherent nonlinear reactance. When the reactance is varied at large-signal frequencies, identified as f.sub.s, unwanted negative-resistance is parametrically generated. Because of the reduced series resistance of the improved diode, the magnitude of the parametrically generated negative-resistance can readily exceed its series resistance; consequently, this nonlinear, negative-resistance diode is vulnerable to unwanted parametric oscillations induced when pumped by f.sub.s. This results in severe distortions in its output signal and a reduction of its power output and efficiency.

#FIG. 1 is a schematic diagram of an IMPATT amplifier in accordance with one embodiment of the invention;

#FIG. 2 is a fragmentary, sectional side view of a pictorial representation of one embodiment of an IMPATT amplifier constructed in accordance with one embodiment of the present invention;

#FIG. 3 is a fragmentary, sectional top view of the pictorial representation of FIG. 2;

#Although the present invention is disclosed in connection with an IMPATT amplifier, it is equally applicable to an IMPATT oscillator; the term IMPATT apparatus is meant to include either an IMPATT amplifier or an oscillator.

#Referring to FIG. 1, a high-frequency, large-signal input of frequency f.sub.s is propagated along a balanced transmission line 10 and 10A to its junction with the outer conductors 12 and 12A of the unbalanced transmission line and the cathode terminals 13 and 14 of at least one pair of matched nonlinear, negative-resistance diodes 15 and 16. Diodes 15 and 16 thus being connected in an anti-series arrangement, these diodes may also, alternatively, be connected so as to have a cathode-to-cathode common terminal and the respective anodes connected to the junction of the balanced and the unbalanced transmission lines. A common terminal 17 is interconnected with a central branch 18 of the unbalanced transmission line. A constant current d-c bias supply 19 is connected in parallel across the unbalanced transmission line conductors 12 and 18 and 12A to 18 to cause diodes 15 and 16, respectively, to operate in the avalanche condition. As is well known to those skilled in the art, the pumping action of the large-signal frequency f.sub.s on the nonlinear inductive reactance of the diodes 15 and 16, operating in the avalanche condition, will generate an unwanted parametrically-induced negative resistance in diodes 15 and 16 and unwanted parametric oscillation, commonly referred to as idler frequencies, in the balanced and unbalanced transmission line circuits. Resistor 20 is the resistance loading in the central branch 18 to overcome the unwanted parametrically-induced negative resistance in diodes 15 and 16. Shunt inductance 20A, having an impedance much greater than resistor 20 at the lowest anticipated idler frequency, and appearing as an open circuit at high frequencies, shunts the d-c power from bypass resistor 20, thus practically eliminating the d-c power dissipation through resistor 20.

#Referring to FIGS. 2 and 3 which show a cutaway side and top or plan view (not to scale) of a waveguide enbodiment of an IMPATT amplifier to illustrate the mounting and arrangement of the diodes 15 and 16 includes a larger area portion 21 having opposing walls 22 and 23 (FIG. 2) and walls 24 and 25 (FIG. 3). Connected to the walls 22, 23, 24 and 25 at the extreme lefthand end of the waveguide portion 21 is a conventional circulator (not shown) to separate the input and the output signals. The large area portion 21 is enclosed at its righthand end as viewed in FIG. 3 by a wall 26 which has an opening or adjustable iris 27 centrally thereof. A smaller area portion 28 which is formed by opposing wall portions 30 and 31 (FIG. 2) and opposing wall portions 32 and 33 (FIG. 3) is connected at one end to the wall or plate 26 with the iris 27 providing communication between the portions 21 and 28 of the waveguide for transfer of electromagnetic energy.