Patent GB-847374-A (GB)
Title: Improvements relating to the measurement of radio-frequency voltages
Inventor: BIRNSTINGL DAVID WILDON
IPC: G01R31/26
Filed: 1956-02-07
Abstract: 847,374. Voltage measurement; transistor testing. BRITISH THOMSON-HOUSTON CO. Ltd. Jan. 16, 1957 [Feb. 7, 1956], No. 3820/56. Class 37 Apparatus for comparing the relative magnitudes of r.f. voltages derived from different points in a circuit, comprises an a.f. modulator for amplitude modulating an r.f. source, connection means for applying the modulated r.f. voltage to a circuit, switch means for alternatively connecting selected points in the circuit to a rectifier to extract the a.f. component from the modulated r.f. voltage, and indicating means to which the output of the rectifier is applied to indicate the relative magnitudes of the a.f. components to provide a comparison of the r.f. voltage levels at the said points. In Fig. 1, not shown, the modulated voltage is fed to a diode rectifier in series with a capacitor shunted by resistance. The voltage across the capacitor is applied to an a.f. amplifier via a condenser, blocking the D.C. components of this voltage, the output of the amplifier giving an indication on a meter of the magnitude of the a.f. voltage which is also a measure of the original r.f. voltage, the degree of modulation being known. In Fig. 2, the modulated voltage is applied to a diode detector P1 charging condenser C1 to the envelope voltage of the r.f. input, a blocking condenser C2 permits only the a.f. modulating component to be applied to a transistor amplifier, transistor P2 being connected as a collector amplifier, and P3 as an emitter amplifier, the output of P3 appears in the emitter circuit where it is applied via transformer T to a milliammeter. In Figs. 3 and 4, not shown, the rectifier is biased to a point where an increase in voltage results in a large increase in current whereas a decrease in voltage produces a small change in current, this biasing being blocked by a capacitor from the modulated voltage input terminal. The D.C. and r.f. components are filtered out as before and the a.f. voltage is similarly amplified and indicated on a meter. In a circuit for measuring the alpha cut-off frequency of a transistor T (Fig. 5) a switch S1 can connect either an r.f. modulated with a.f. voltage source V1 or an a.f. voltage source V2. A constant current generator G connected between the emitter and base electrodes excites the transistor and may be replaced by a high voltage battery in series with a high resistance, the collector electrode supply being tapped therefrom. To determine the current in the emitter and collector circuits the voltages across resistors RI and R2 are measured by a valve voltmeter V through a 3 pole two position selector switch S2. In operation, to find the current gain, with switch S2 as shown, the amplitude of source V2 is varied until voltmeter V gives a reference reading of say 100 units. S2 is then moved to its other position and a potentiometer R3 adjusted until voltmeter V again reads 100 units; the current gain is then determined by the potentiometer setting. Potentiometer R3 may be set and kept at unity and the ratio of the respective voltmeter readings gives the ratio of the collector and emitter currents. To measure r.f. current gain, V1 is connected and S3 is opened to remove the normal short circuit on rectifier P with the potentiometer R3 at its appropriate setting and the ratios of the collector and emitter currents are measured for varying frequencies of r.f. source V1. When the collector current is by definition 3db or 29 % less than the emitter current (indicated as 100 units) the r.f. frequency is the cut off frequency of transistor T.