Patent GB-719814-A (GB)
Title: Improvements in or relating to electronic test apparatus for an automatic frequency control system
Filed: 1952-04-09
Abstract: 719,814. Pulse radar. GENERAL ELECTRIC CO., Ltd, and ROBINS, W. P. April 1, 1953 [April 9, 1952], No. 9133/52. Class 40 (7). [Also in Groups XXXV and XXXVI] Electronic apparatus for testing the response of an automatic frequency control system regulating the frequency of an oscillator to a value fixed relatively to' the steady frequency of an associated signal source whose frequency is alternately shifted up and down through equal intervals at a controllable repetition frequency comprises a radio frequency spectrum analyser wherein a recorded trace is cyclically traversed synchronically with a linear frequency sweep oscillator whose output supplies a mixer together with the output of the signal source and the A.F.C. oscillator, thus producing a beat signal which is rectified in a narrow band amplifier-detector tuned to half the frequency difference between the signal source and the A.F.C. oscillator to deflect the said trace perpendicularly to the sweep axis, whereby there are displayed envelopes representing the frequency spectra of the emitted signals corresponding to source frequency shifts compensated by the A.F.C., juxtaposed against envelopes representing the corresponding frequency changes of the A.F.C. oscillator.  Fig. 1 shows test apparatus for an automatic frequency controlled system A comprising a pulse modulated signal source 1 (e.g. a radar transmitter) operating in conjunction with a receiver having a local oscillator 2 supplying a mixer 3 followed by an intermediate frequency oscillator in the usual manner; outputs from the local oscillator and the signal source being applied to an A.F.C. mixer 4 and A.F.C. controls circuits 5 which regulate the oscillator frequency to maintain the required heterodyne intermediate frequency For testing, a sawtooth oscillation generator 8 controls a double beam C.R.O. 10 and an analyser oscillator 9 to produce two parallel linear traces of the C.R.O. and synchronously therewith a linear frequency sweep of the oscillator output over a predetermined band. The analyser mixer 11 receives the outputs of the signal source, the A.F.C. oscillator, and the analyser oscillator to produce a beat signal which is applied to a narrow band amplifier detector 12 tuned to half the intermediate frequency, whose rectified output is applied through gating circuits 13, 14 to deflect the respective C.R.O. traces perpendicularly to the sweep axis. A multivibrator 15 synchronized to operate at adjustable submultiples of the modulating pulses of the signal source and to change over. immediately prior to the transmission of a signal pulse, generates a square waveform which is applied through a cathode follower buffer 16 to shift the signal frequency alternately up and down by equal amounts, and applied direct to the gate circuits 13, 14 to open each in turn and transmit alternately signals deflecting the C.R.O. traces (Fig. 2), which respectively display envelopes 22, 23 representing the amplitude frequency spectra of the transmissions of the signal source when its frequency is shifted in the corresponding direction, but subject to A.F.C. correction ; superimposed on envelopes 24, 25 representing the respective outputs of the A.F.C. oscillator 2, whose edges 26, 27 should approach the ideal positions 28, 29 for correct operation. Thus the accuracy of the A.F.C. control may be assessed for different multivibrator frequencies which are inversely proportional to the length of time allowed for the correcting response of the A.F.C.  The signal source may be replaced by a simulator, and a single beam C.R.O. displaying all the envelopes on a single trace may be used. Where the frequency shifts imposed on the signal source are variable according to the amplitude of the controlling square wave (as when it employs a klystron oscillator), variations of A.F.C. response with variations of magnitude of frequency error may be examined.