Patent GB-817633-A (GB)
Title: Improvements in or relating to apparatus for determining the frequency response of an electronic circuit
Filed: 1955-09-15
Abstract: 817,633. Frequency response measurement. SANGAMO WESTON Ltd. Sept. 15, 1955 [Sept. 20, 1954], No. 26466/55. Class 37. [Also in Group XL (c)] Apparatus for determining the overall frequency response curve of an electronic circuit (e.g..the signal channel of a television receiver) comprises a frequency modulated linear sweep signal generator energizing the input of the circuit to produce a demodulated output applied to the Y plates of a C.R. oscillograph, whose X plates are supplied with a timebase voltage synchronous with the frequency sweep, incorporating a calibrator wherein a known variable frequency signal is mixed with the sweep signal and the resultant signal is applied through a low-pass amplifier to intensity-modulate the oscillograph beam, so that the displayed frequency response curve is lightened (or darkened) at a point, corresponding to the calibration frequency, at which a beat signal appears at the output of the low-pass amplifier. In Fig. 5, an oscillator 15 feeding a gain control circuit 17 is linearly swept over a range of, e.g., 44 to 50 Mc/s. at, e.g., 60 c.p.s., by a precision mechanical modulator 16 tuning the oscillator tank circuit, and the output is applied to the input of a television receiver TVR to produce a demodulated video signal at the second detector applied to Y deflect a C.R. oscillograph OSCP, whose X deflection timebase is synchronized with the frequency sweep at 60 c.p.s. The sweep oscillator output is combined with the output of variable frequency oscillator' 18 set to, e.g., 47 Mc/s., in a mixer 19, whose output comprises principally a. frequency-modulated signal in the range 3-0-3 Mc/s. energizing a low-pass audio amplifier 20, whose output is zero except when the input signal coincides with the oscillator signal, and which then modulates the oscillograph beam to display on the frequency response curve a bright (or dark) spot corresponding to the 47 Mc/s. point (Fig. 6, not shown). A further sideband marker spot may be displayed by adding to the mixer input a crystal controlled signal of, e.g., 4.5 Mc/s. (Fig. 7, not shown), so that further modulation pulses appear at the output of amplifier 20 when the calibration frequency is, e.g., 5.5 Mc/s. (Fig. 8, not shown). In a modification (Figs. 9, 12) the output of the sweep frequency oscillator 15 is applied to the first control grid of hexode 31 of mixer 19, the output of the calibration oscillator 18 is applied to the cathode, the 4.5 Mc/s. output of crystal controlled oscillator 30 is applied to the second control grid, and a parallel resonant circuit TC sharply tuned to, e.g., 75 kc/s., is inserted in the anode load, to produce output signal pulses when the 3-0-3 Mc/s. frequencymodulated component passes through 75 kc/s., i.e. at 150 kc/s. separation along the frequency sweep, symmetrically disposed with respect to the calibration frequency. The pulses are demodulated at D.C. to double voltage pulses b repetitive at the sweep frequency by either rectifier diode 35, 36 selected by switch 37, which are amplified at 20 and clamped to zero voltage level at CC by either of diodes 39, 40 selected by switch 41 ganged with switch 37 to determine the pulse polarity. After further amplification the double pulses are applied to intensity-modulate the oscillograph trace and produce pairs of 150 kc/s. spaced light (or dark) spots on the frequency response display, symmetrically one on each side of the calibration frequency. Similar sideband double marker spots appear at, e.g.,  4.5 Mc/s. spacing from the calibration frequency (Figs. 10, 11, not shown), on switchable application of sideband oscillator 30 to the second control grid of the hexode mixer. The marker signal may alternatively be superimposed upon the Y-deflection signals of the oscillograph, and are then displayed as single or double peak markers extending from the trace in the Y direction. Resonant circuit TC is adjustable to vary the spacing of the double marker pulses and the voltage pulses producing the latter may operate trigger circuits for indication. Any non-linear frequency difference mixer is applicable to replace hexode 31.