Patent GB-848088-A (GB)
Title: Improvements in or relating to circuit arrangements for indicating the characteristic curves of filters and like networks
IPC: G01R27/32
Filed: 1957-04-08
Abstract: 848,088. Frequency measurement, impedance measurement. PHILIPS ELECTRICAL INDUSTRIES Ltd. April 8, 1957 [April 11, 1956], No. 11384/57. Class 37 In a circuit for displaying on an oscilloscope the frequency response curve 12 of an electrical network 7, a main oscillator 1 applies to the network a high-frequency voltage the frequency of which is periodically varied over the band for which the response curve is required, the output of the network being applied to the oscilloscope in conjunction with a signal indicative of the instantaneous frequency of the main oscillator; and an auxiliary oscillator 2 applies to tuned circuits 10 of a frequency marking system a lowfrequency voltage the frequency of which is peripdically varied in synchronism with that of the main oscillator over a band which includes the tuned frequencies of the tuned circuits thereby generating output pulses at instants when the high-frequency voltage passes through preselected frequencies, such pulses being applied to the oscilloscope to produce frequency marks. The main sweep oscillator comprises valves 20, 21, capacitor 24, and coil 3; the output thereof, having a mid-frequency of 40 mc/s, is fed to the test network 33, e.g. an intermediate frequency band-pass filter, and the resultant voltage to be examined, e.g. the primary voltage, is supplied, through rectifier 34, to the Y plates of the oscilloscope, the horizontal deflection of which is A.C. mains controlled. The auxiliary sweep oscillator comprises valves 22, 23, capacitor 25, and coil 4; the output thereof, having a midfrequency of 0À5 mc/s, is fed to the grid of a triode 40 in each of three or more similar circuits 37, 38, 39 which together constitute a frequency marking system; the anode circuit of each valve 40 includes an inductance-capacitance resonant circuit, 41, 42, 43, tuned so that its resonant frequency corresponds to a required frequency mark, and the frequency marker pulses respectively developed across these resonant circuits are supplied, after rectification and amplification, to the brightness control of the oscilloscope to produce bright spots on the trace at points corresponding to preselected frequencies of the main oscillator. Alternatively, the marker pulses may be fed to the Y-plates of the oscilloscope to vertically deflect the trace at these same frequencies. Both the main and auxiliary oscillator frequencies are swept in synchronism by mounting their respective coils 3, 4 on a ferrite rod 5 carrying a magnetizing coil 6 energized by the A.C. mains which also supplies the oscilloscope X-plates, the main oscillator being varied through Œ 4 mc/s per half period. The magnetizing coil 16 is supplied with a sinusoidal voltage from a mains excited phase shifter 28, and a direct voltage from a mains energized rectifier 27. The network frequency response curve is recorded during one half of each sweep cycle, and the frequency axis is recorded during the other half by supplying the output pulses of a square-wave generator, comprising a valve 48 overdriven by a 50 c/s voltage, to the grids of oscillator valves 20, 21, 22, 23, thus preventing oscillation during half of each period of the 50 c/s mains sweep voltage.