Patent GB-679457-A (GB)
Title: Improvements in or relating to frequency-stabilizing arrangements
IPC: H03J7/26
Filed: 1951-06-22
Abstract: 679,457. Radio receiving systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. June 22, 1951 [June 27, 1950], No. 14894/51. Class 40 (v). [Also in Group XXXV] A frequency-stabilizing arrangement for automatic frequency correction of a local oscillator (e.g. in a single side-band radio receiver) of the kind using pilot frequencies and in which the oscillator frequency is temporarily modulated by a sweep generator when unstabilized, is characterized by two selectors for separating two pilot frequencies of determined frequency separation, the output voltages of the selectors when occurring simultaneously being arranged to suppress the sweep voltage, and one or both of the said outputs being supplied as a stabilizing frequency to a device for frequency correction of the local oscillator. The Figure shows the A.F.C. system applied to a single sideband receiver. The incoming signal comprises a side-band (e.g. 3.2 kc. in width) flanked on each side by pilot signals which may be 4 kc. apart. After a preliminary frequency-changing stage 3, 4 reducing the band to 1000-96 kc., the controlled local oscillator 7 of frequency about 1060 kc. further reduces the band to 64-60 kc. From this band the intelligence signal 60.3-63.5 kc. is selected by a filter 12 and is de-modulated at 13 by a 60 kc. source 14, yielding the intelligence signal in the phones 17. Of the two pilot frequencies, reduced to 60 and 64 kc. respectively, the 60 kc. signal is selected by a filter 8 and passed through a limiter 18 to a device 20. The 64 kc. signal is heterodyned by a source 11 of 4 kc. and from the resultant the 60 kc. signal is extracted by a filter 9 and also passed through a limiter 19 to the device 20. The latter includes two pentode amplifiers 21, 21<SP>1</SP> which receive the respective 60 kc. inputs on their inner grids, and transfer them through tuned anode circuits 43, 431 to coupled circuits which are alternatively connected by a switch 40 to a device 46 which compares their frequency with that of the 60 kc. source 14. The output from the comparer 46 is fed through a low-pass filter 47 to a frequency regulator 48 associated with the 1060 kc. source 7, thus ensuring that the frequency band fed from the mixer 6 is accurately related to that of the demodulator source 14 and therefore produces a correct rendering of the intelligence signal at the phones 17. The device 20 is arranged to provide continuous control of the oscillator 7 when one only of the two pilot frequencies is not received owing to selective fading of the incoming signal, and is also arranged to give a temporary frequency sweep to the local oscillator 7 during periods when both pilot frequencies are missing, thus allowing locking of the oscillator 7 to be re-established after a complete fade. With this object, each pilot frequency is fed to a rectifier 26, 26<SP>1</SP> and the resulting positive bias is fed in each case to the suppressor grid of the valve 21<SP>1</SP>, 21 associated with the other pilot frequency. The suppressor grids of the valves 21, 21<SP>1</SP> are normally negatively biased by a source 29, but the valves are unblocked when positive voltage is supplied either by the crossconnected rectifiers 26, 26<SP>1</SP> or by other rectifiers 32, 32<SP>1</SP> fed from the anodes of the valves. When both pilots are received, both valves 21, 21<SP>1</SP> are operative, and the output from valve 21 is fed through contact 45 of switch 40 to the frequency comparer 46. Should the first pilot fail, no output is provided by the valve 21, and a positive voltage supplied by rectifier 32<SP>1</SP> to the grid of a relay valve 38 overcomes the negative voltage previously provided by a rectifier 35 fed from the valve 21. The relay valve 38 energizes a relay 39 throwing the switch 40 to its other contact 45<SP>1</SP> connected to the output of the valve 211, which then maintains control. Should both pilots fail, positive voltages, previously derived from the rectifiers 32, 32<SP>1</SP> are removed from the amplifier 51 and a relay 52 is de-energized, thus allowing a switch 50 to close and bringing into operation a low-frequency of the local oscillator 7 slowly through a limited range. This enables frequency locking to be re-established when the pilot frequencies re-appear, whereupon the source 49 is disconnected. In a simpler form of the invention, the valve 21<SP>1</SP>, its output rectifier 32<SP>1</SP>, and the relay valve 38 are omitted, and the second pilot frequency is connected only to the rectifier 26<SP>1</SP> which controls the blocking of the valve 21.