Patent GB-725903-A (GB)
Title: Improvements in or relating to frequency-stabilizing circuits
IPC: H03J7/26
Filed: 1953-08-14
Abstract: 725,903. Automatic frequency control systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Aug. 14, 1953 [Aug. 19, 1952], No. 22464/53. Addition to 679,457. Class 38 (4). The frequency-stabilizing circuit for control of a lcoal oscillator by received pilot frequencies, as disclosed in the parent patent, is modified by providing a search-voltage source which sweeps the local oscillator frequency through a limited range and is shut off whenever one of the pilot frequency selectors gives an output differing from the correct value by an amount less than the cut-off frequency of a low-pass filter. The single sideband receiver, Fig. 1, comprises superhet stages 4, 6 by which the incoming waves are reduced in frequency to a band which may extend from 60 to 64 kc/s., with the extreme frequencies representing the pilot frequencies, and an intervening band the intelligence frequencies. One pilot frequency of about 60 kc/s. is selected by a filter 8 and passed through a limiter 18 to a transformer 31<SP>1</SP> feeding a rectifier 32<SP>1</SP>; and the pilot frequency is also passed to a mixer 46 for combining with a reference source 47 of the correct pilot frequency. The other pilot frequency of about 64 kc/s. is reduced to 60 kc/s. by a beating source 14 and is passed through a filter 9 (similar to the filter 8) and a limiter 19 to a transformer 31 feeding a rectifier 32. The filter 9 also feeds a relay 28 which when energized connects a switch arm 29 to the output of the filter 9 and when de-energized allows the switch to connect up to the filter 8. Thus when either filter is passing current of about 60 kc/s. it reaches an amplifier 30 and is fed to the grid of a pentode valve 24 which if not biased beyond cut-off feeds a mixer 40 through a tuned anode circuit 34 and secondary coil 36. The local reference source 14, by which the intelligence signal is demodulated, also feeds the mixer 40, whose output is conveyed through a LP filter 41 to adjust, and to maintain, the frequency of the local oscillator 7 to a value that gives the pilot frequencies their correct values. When the AFC system is thus locked-in, a secondary coil 37 on the output side of the pentode 24 feeds a rectifier which applies negative voltage to a lowfrequency search-voltage source 23 connected to the frequency controller 43 of the local oscillator 7. The search voltage sweeps the frequency of the oscillator 7 to and fro over a limited range whenever the AFC fails to lock-in. The pentode 24 has its cathode bias set to a positive value by a potentiometer 26, and its grid is positively biased by the series connection of the load resistors of the rectifiers 32, 32<SP>1</SP> and of a further rectifier 49 fed through a low-pass filter 48 from the beat output of the mixer 46. In Fig. 3, the rectified output E is plotted against the pilot frequency deviation F, curves a and b representing the output arising from the band-pass filters 18, 19 and c that arising from the low-pass filter 48. The curve d represents the series combination of a, b, c, and the dotted line E<SP>1</SP> the cut-off bias due to potentiometer 26. The pentode 24 is thus rendered operative by the presence of frequencies close to F 0 , and the selectivity of the system is determined by the narrow limit of the LP filter 48. This allows the selector filters 8, 9 to be desired with considerable bandwidth, thus avoiding instability of the AFC loop circuit and reducing the buildup time of the filters. In order to ensure that the AFC circuit retains control should one of the pilot frequencies temporarily disappear, a coil 35 fed from the pentode output is connected through a rectifier 38 to a network 39 comprising a condenser-shunted resistor arranged to add positive bias to the grid of the pentode 24. The mixer 46 may take the form of a rectifier, and the beat output of the lowpass filter 48 may be amplified before applying it to the rectifier 49 (Fig. 2, not shown). The low-pass filter 48 may comprise a series resistor and a shunt condenser, and its cut-off frequency may be lower than 20 c/s.