Patent GB-1147154-A (GB)
Title: Method and apparatus for testing electronic equipment
IPC: G01R27/32
Filed: 1966-03-22
Abstract: 1,147,154. Determining group delay. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. March 22, 1966 [March 22, 1965], No. 12566/66. Heading G1U. [Also in Division H3] The frequency phase-shift characteristic of a circuit, such as that in a television receiver, is tested over a given bandwidth, by applying the sweep output of a wobbulator 10 to buffer amplifiers 14a, 14b in first and second input signal circuits 16a, 16b. The output of wobbulator 10 sweeps from 64À25 to 74À25 Mc/s in a time-linear fashion Fig. 2 (not shown) and is controlled by a saw-tooth sweep signal at e. g. 50 c/s supplied by a sweep generator 12. A signal of 64À25 Mc/s from a signal generator 18a is linearly combined with the signal from amplifier 14a and applied via a buffer and variable amplifier 20a and switch SW1 to a television receiver TVR the combined R. F., I. F. and video networks of which are under test. The output of buffer amplifier 14b is similarly linearly combined with a 74À25 Mc/s signal from generator 18b and applied through a buffer and variable amplifier 20b to an envelope detector 22 the output of which sweeps in time-linear manner from 10 Mc/s to zero cycles per second in step either with the opposite sweep from zero to 10 Mc/s of a signal derived from a similar circuit 22a and 24a or the input 64À25 to 74À25 Mc/s to the test circuit. The output from detector 22 is fed to one input of balanced modulator 28 through a low pass filter 24 and calibrated delay line 26, a high impedance probe 30 feeding the signal from the video stage of receiver TVR to the other input. The two oppositely sweeping inputs to the modulator 28 produce a beat frequency which will vary slightly from 10 Mc/s in dependence on the phase shift of the first signal in the test circuit T. V. R. which is passed by a narrow band filter 32, a tuned amplifier and limiter circuit 34 and switch 36 to a narrow-band frequency detector 38. Switch 36 is driven by sweep generator 12 so that it alternately selects the signal from modulator 28 and a comparison 10 Mc/s calibrating signal obtained by combining the 64À25 Mc/s and 74À25 Mc/s signals from generators 18a, 18b and passing them through a detecting and low-pass filter circuit 42. The sweep waveform from generator 12 and the output from frequency detector 38 are respectively applied to the X and Y terminals of a C. R. O. The phase shift of the first signal passing through receiver T. V. R. comprises a constant "phase delay" portion at the modulating frequency which may be offset by the calibrated delay line 26 which can be switched to add a predetermined constant phase shift to the second signal in which case the C. R. O. shows the "group delay" at the carrier frequency. The I. F. circuit, the R. F. circuit and the combined I. F. and rider circuits may be tested individually, using an appropriate local oscillator (72) and mixer (70), where necessary, Figs. 7, 8 and 9 (not shown). The 10 Mc/s reference signal is generated by a variable frequency oscillator 40 controlled by the output of a phase comparitor, Fig. 4 (See Division H3) forming part of the frequency detector 38.