Patent GB-608005-A (GB)
Title: Improvements in or relating to radio and like instruments for use in high frequency testing research and development
IPC: G01R27/30
Filed: 1946-02-13
Abstract: 608,005. Impedance measurements; cathoderay oscillographs. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and AGAR, W. O. Feb. 13, 1946, No. 4570. [Class 37] A cathode-ray oscillograph is arranged to produce a frequency-impedance curve for use in high-frequency examination of the behaviour of aerials, feeders, television apparatus, radio components, filters, &c. over a wide range of frequencies. The output from a fixed frequency oscillator 1 operating at 39 mc/s. is fed as one input to a mixer 2. The second input to the mixer is from a variable frequency oscillator 5 capable of producing any frequency between 40 and 60 mc/s., the timing condenser of which is continuously rotated by a motor 4. The output from the mixer 2, consisting of the sum and difference frequencies, is fed to a low pass filter 5 which passes only the difference frequency so that its output consists of a frequency varying continuously from 1 to 21 mc/s. and back again. Part of the output-is tapped off to control an A.V.C. network 6 which operates on the fixed frequency oscillator 1 to maintain the amplitude of the filter output constant. Output from the filter 5 is also taken to a frequency discriminator 7 at which the voltage developed across a condenser into which the varying frequency is fed with constant voltage is picked off. The picked off voltage, which is dependent on the frequency, is rectified and amplified at 8 and passed to the horizontal deflecting plates 9 of a cathode-ray oscillograph 10. Output from the filter 5 is also taken to a rectifier circuit including a pair of diodes 11, 12 having their cathodes 13, 14 connected to the output terminals of the filter. Across each diode is connected a low resistance 15, 16 in series with a condenser 17, 18, the junctions 21, 22 being connected to terminals 24, 25 to which the impedance to be tested is connected. The anodes 19, 20 are connected to earth through de-coupling resistances 26, 27 and a load resistance 28, a variable tap 29 on which is taken through a two-way switch 30 to an amplifier 31, the output of which is passed to the vertical deflection plates 32 of the oscillograph 10. Since the resistances 15, 16 are in series with the impedance under test and the voltage input to the rectifier network is constant, the voltage appearing across the resistance 15, 16 will depend on the ratio of the impedance under test to these resistances. The D.C. component of the rectified voltage across the resistances 15, 16 appears over the decoupling resistances 26, 27 and the load resistance 28 from which it is passed via the amplifier 31 to the vertical deflection plates 32. Output from the filter 5 is also taken to a further mixer circuit 33 receiving its second input from a 1 mc/s. oscillator 34 having a very high harmonic content, the mixer output being fed to the amplifier 31. Small vertical deflectors are thereby superimposed on the horizontal trace of the oscillograph at every megacycle and serve as calibration markings. Transfer impedance measurements may be made by connecting the oscillograph to the output of the apparatus under test, the input being connected to the terminals 24, 25, and the switch 30 being in the right-hand position. With the switch 30 in the left-hand position admittance tests may be made. The frequency of the oscillator 3 may be varied by a velocity modulated tube instead of by a mechanically rotated condenser. The horizontal sweep of the oscillograph may alternatively be produced by cyclically varying a potentiometer, across which a D.C. potential is applied, in synchronism with the means for cyclically varying the oscillator 3.