GB837480A - Improvements in and relating to electric apparatus for determining the response of an electric device - Google Patents

Abstract

837,480. Frequency response measurement. REGENTONE PRODUCTS Ltd. July 6, 1956 [April 6, 1955], No. 10062/55. Class 37. [Also in Groups XXXV and XL (c)] An apparatus for determining the frequency response of an electric device comprises an electric oscillation generator having a resonant circuit determining the frequency of the oscillations and including an inductor having a core of magnetic material, which is provided with means for biasing the core magnetically to a selected point on its B-H curve and further means for varying the magnetic flux in the core thereby to vary the frequency of the oscillations. As shown, in an apparatus for testing the high-frequency response of a radio or television receiver, providing oscillations at about 470 kc/s. and 10.7 Mc/s. with frequency sweeps of 15 to 100 kc/s. and 300 to 2000 kc/s. respectively, the oscillator comprises either section of a double triode valve V8 and a tuned circuit including L3 or L4, switch S2a, S2b determining which of the two sections is operative. Inductors L3, L4 are wound on a ferrite core secured to a sheet of insulating material 15, Fig. 3, a core 18 of magnetic material having a winding L5 being mounted on the other side. A permanent magnet 21 is carried by a threaded rod 22 and is used to bias the ferrite cores to a desired point on the B-H curve whereby the resonant frequency of the tuned circuits including the inductors L3, L4 may be adjusted. The frequency sweep is controlled by varying the value of current, from the 50 c.p.s. mains supply, in the inductor L5 by adjustment of potentiometer R58. The cathode-ray tube V3 receives its X-axis deflection voltage from the potentiometer R29 which is supplied from a winding on the mains transformer T1, and to ensure that the horizontal beam deflection is linear with respect to the frequency variation, a phase-shift circuit R57, C40 is included in the circuit of L5. In order to blank the return trace of the cathode-ray tube a voltage is derived from diode V5 and phase-shifting circuit R27, C13 and applied via potentiometer R28 to the control grid, and via capacitor C12 to the cathode of the cathoderay tube. A marker generator comprising double triode V6 applies marker pulses to the trace of the tube V3, oscillator V6a marking the centre frequency and V6b providing a plurality of marks on the trace by which the band width of the tuned circuits under test can be determined. Switches S1a, S1b, S1c select the appropriate marker frequencies for the particular range in use. The output from oscillator V8 may be amplitude modulated by the triode section of V7 controlled by switch S1f, before being fed via the attenuator R34- R45 to the device under test. The output from the device under test is amplified by valves V1, V2, rectifiers MR1, MR2 controlled by switch S4, suppressing the negative-going portions of the output signal.