GB892542A - Improvements in or relating to electric high frequency generators - Google Patents

Abstract

892,542. Non-linear networks. STANDARD TELEPHONES & CABLES Ltd. May 19, 1960, No. 17678/60. Class 40(8). A circuit for stabilizing the energy level in a load excited by a generator at a fundamental frequency, comprises a non-linear reactance coupled to the load and means for absorbing energy of sub-harmonic frequency generated by the non-linear reactance. The reactance may vary with input voltage or current and may consist, e.g. of a backbiased silicon diode rectifier or a ferriteloaded coil. For such a reactance a critical voltage or current is required for generation of oscillations at a sub-harmonic frequency determined by the parameters of the circuit. Moreover, the threshold value is also the maximum value which the voltage or current can attain, for if this value is exceeded oscillations build up until the voltage or current stabilizes at the critical value. The stabilizing network of Fig. 2A is connected through an isolator to a high-frequency generator of frequency f and to a load through a further isolator (isolators not shown). The non-linear element is in the form of a diode represented by a combination of a non-linear capacitance 10 and a linear capacitance 11. The input is shunted by a filter 12 which passes all frequencies except a narrow band around f, and a further filter 14 which passes all frequencies except a band around f/2 is arranged in series with the non-linear element 10, 11. The filters are shunted by inductances 13, 15 whose values are adjusted to compensate the effect of the linear capacitance component 11 of the diode at the frequencies f and f/2. Energy of sub-harmonic frequency is absorbed in the resistance 16. The Specification discusses the variation in stabilization over a band of frequencies determined by the quality factor of the sub-harmonic loading circuit. Minimum variation is achieved when the input loading is equal to the subharmonic loading. A current-stabilizing circuit may be realized by constructing the dual of the circuit shown. This comprises a non-linear inductance in series between the source and the load shunted by a band-pass filter centred on f/2 and a resistive load (Fig. 2B, not shown). At high frequencies, the circuit of Fig. 2A may be realized by mounting a semi-conductor diode 27 in a waveguide 24. One side of the diode is chokecoupled to the lower face of the waveguide and the other to the upper face through a co-axial cavity 28 the length of which is one half wave at the signal frequency f and one quarter wave at the sub-harmonic frequency f/2. The cavity thus performs the function of the filter 14. Inductances 29 are realized by posts 29 and inductor 15 by the stub 30. This has a length of one quarter-wave at the signal frequency and provides an inductive reactance at the sub-harmonic frequency. A resistive card 31 projecting into the cavity 28 provides the load 16. In order to eliminate second and higher harmonics, two empirically adjusted waveguides stubs 32, 33 are arranged to short-circuit the diode at these frequencies. Tuning screws 34, 35 are used to flatten the frequency-voltage response of the output. They are adjusted so that the diode is situated at a V.S.W. minimum at mid-band, thus increasing the power output at this frequency. This counteracts the increase in power output which occurs as the frequency deviates from its mid-band value.