Patent GB-790807-A (GB)
Title: Improvements in or relating to stabilized high-frequency multi-channel generators
IPC: H04B1/40
Filed: 1955-12-12
Abstract: 790,807. Automatic frequency control systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Dec. 12, 1955 [Dec. 14, 1954], No. 35596/55. Class 38 (4). [Also in Group XL (c)] A decade signal generator produces a signal frequency determined by coarse and fine frequency spectrum generators and an interpolation oscillator and comprises an auxiliary A.F.C. system in which an auxiliary oscillator is retroactively stabilized at a frequency determined by the coarse frequency and interpolation frequency generators, and a main A.F.C. system in which a main oscillator producing the final. output is retroactively stabilized at a frequency determined by the auxiliary oscillator and fine frequency generator. As shown, in the auxiliary A.F.C. loop 30 the output of the auxiliary oscillator 32 is beat with that of the coarse frequency generator 34 in a mixer 33 and the difference frequency selected by a filter 37 of 1 Mc/s. bandwidth is applied to a further mixer 38 to which is also applied the output of the interpolation oscillator 39 variable over a similar 1 Mc/s. bandwidth so that the D.C. component of the output of the mixer 38, after amplifying and smoothing and being applied to a frequency control element 42, stabilizes the auxiliary oscillator 32 at selected frequencies variable over 1 Mc/s. range and spaced by 10 Mc/s., the frequency interval of the coarse frequency generator. In the main A.F.C. loop 31. the selected frequency output of the auxiliary oscillator 32 is beat in a mixer 46 with that of the main oscillator 44 and a particular 1 Mc/s. bandwidth of the difference frequency, selected by a filter 48 adjustable in steps of 10 Mc/s., applied to a mixer 49 to beat with harmonics of a pulse generator of recurrence frequency 1 Mc/s. derived from a crystal oscillator 50. The output of the mixer 49, after amplifying and smoothing and application to a frequency control element 54 then locks the main oscillator frequency at 1 Mc/s. intervals as selected by the oscillator 50 acting as the fine frequency step generator in conjunction with the adjustable filter 48. Low-frequency search oscillators 43, 55 of. R-C type may sweep the frequencies of the auxiliary and main oscillators over a search range of 1 Mc/s. until they lock on to the frequency selected, the search oscillators then being rendered inoperative due to the oscillators and amplifiers 40, 52 having a common cathode load resistor and the inputs to the amplifiers opposing the feedback of the oscillators. Alternatively, frequency discriminators may bridge the mixers and phase detectors 38, 49, instead of the use of search oscillators. Also a multi-crystal fine step oscillator may replace the pulse generator and harmonics producer 50 of the main A.F.C. loop. In a modification (Fig. 4, not shown) the coarse, fine and interpolation frequencies, spaced by steps of 10 Mc/s., 1 Mc/s. and 0.1 Mc/s., are derived from a single 1 Mc/s. crystal oscillator by multiplication and division, and applied to pulse producing, harmonic selecting and mixing valves of beam gating and deflection type corresponding to the mixers 33, 49 and 38 of Fig. 2 and of type described in Specifictaion 679,767 acting also as a phase detector. The harmonic selected is dependent on the frequency band selected for the filters or oscillators preceding or following the gated beam valves. Frequency correcting means.-A bridge circuit comprising two rectifiers and two inductances may be connected in series with one or a pair of capacitors across the tuned anode circuits of the main and auxiliary oscillators 44, 32, and the A.F.C. voltage applied across one of the bridge diagonals to vary the resistance of the rectifiers and therefore control of the series capacitor(s) on the tuning.