Patent GB-752072-A (GB)
Title: Improvements in or relating to apparatus for making high frequency tests by measuring the amplitude of an electric signal passed by an electric circuit under test over arange of frequencies
IPC: G01R27/32
Filed: 1953-04-01
Abstract: 752,072. Attenuation/frequency measurements. GENERAL ELECTRIC CO., Ltd., and EASTER, B. April 1, 1954 [April 1, 1953], No. 9073/53. Class 37. Apparatus for determining the insertion loss or gain of a test circuit at variable high frequencies comprises an oscillator supplying thereto an input high-frequency signal periodically scanned over a range of frequencies and amplitude modulated at a low frequency, a circuit for obtaining therefrom a first derived signal corresponding in amplitude to the instantaneous amplitude of the input signal, and a circuit for obtaining from the output of the test circuit a second derived signal corresponding in amplitude to the output amplitude, there being provided a frequency changer operative on either the first or the second derived signal whereby the derived signals are distinguished in frequency and an amplifier passing the distinct signals to respective rectifying circuits whereby a display oscillograph sweeping synchronously with the frequency scan of the input is deflected in response to the second signal, and the amplifier gain is controlled in response to the first signal to reduce errors due to variations thereof. An oscillator 1 operating at a mean frequency of, e.g., 2000 me/s. supplies signal to a circuit 2 under test, and is frequency modulated over a range of 100 mc/s. at a frequency of 1 c/s. by, e.g., a synchronous motor 5 varying the position of a shorting bridge across the oscillator tuning stub, while also synchronizing the timebase generator 6 of a cathode-ray oscillograph (not shown). The oscillator signal is also amplitude-modulated by a 5 kc/s. signal of, e.g., square waveform from generator 7, while the test circuit output is supplied to detector 8 feeding amplifier 9, whose output is rectified at 10 and applied to the deflection plates of the C.R. oscillograph. A monitor 11 (e.g. a directional coupler) taps off a small portion of the oscillator signal to detector 12 of the same law (e.g. linear or square) as detector 8, and the 5 kc/s. output is converted by mixer 13 and oscillator 14 to a 65 kc/s. signal which is separated by band-pass filter 15 and supplied to amplifier 9. The corresponding output is separated by band-pass filter 16 and the unidirectional signal from rectifier 17 is employed to control the gain of amplifier 9 whereby any amplitude variations of the test signal from oscillator 1 are compensated, and the C.R. oscillograph deflection is a measure of the insertion loss (or gain) of the test circuit. The output of the detector 12 may be applied direct to amplifier 9 and extracted direct from the output for rectification and control of the amplifier gain, while the signal from detector 8 is correspondingly frequency changed at the input and filtered from the output for rectification and display by the oscillograph. Alternatively the signals from the detectors may be combined and separated before and after amplifier 9 by time division multiplex. The Provisional Specification states that the detectors may be of the silicon crystal type.