Patent GB-907328-A (GB)
Title: Apparatus for measuring the mark-space ratio of a train of pulses
Inventor: HUTCHINSON TERENCE
IPC: G01R19/00
Filed: 1960-04-11
Abstract: 907,328. Code telegraphy. SHORT BROS. & HARLAND Ltd. March 29, 1961 [April 11, 1960], No. 12906/60. Class 40 (3). [Also in Groups XXXVI and XL (c)] Apparatus for measuring the mark-space ratio of a train of pulses comprises a current-measuring device for measuring the average current traversing a path in a switching circuit which is arranged to be switched by said pulses between two conditions, in one of which the magnitude of said current is of a first predetermined value, and in the other it is of a second predetermined value. Fig. 1 shows a mark-space comparator for use in calibrating the outputs of test equipment sub-modulators generating high frequency oscillations modulated by mark-space signals. The appropriate gating pulses determining the mark-space ratio of the output of any one of four sub-modulators are selected by a switch 11 and fed to a gating valve V1 which is normally situated in the equipment and connected via input terminals T1, T2 to the comparator. The signals appear at the anode of V1 as negativegoing portions corresponding to " mark "  signals and positive-going portions corresponding to " space " signals and after amplification by V2, V3, are supplied via capacitor C2 to appear in the same sense at the control grid of V4, which with a further valve V5 constitutes a switching circuit responding to the pulses. In operation, with a negative-going pulse on the grid of V4, V4 is cut off and V5 conducting, and current flows through the ammeter M, and with a positive-going pulse V4 conducts, V5 is cut off and no current flows through the ammeter. A resistor R14 to R18 is selected by switch SW1B so that a suitable peak current is supplied to the ammeter for the particular pulse train under test. To calibrate the meter M, a switch SW1A is closed causing V4 to cut off and V5 to conduct and the current through V5 is adjusted by resistor RV1. When the valve V5 is switched on and off rapidly at a constant on/off ratio, the current through the anode will be proportional to the " on " time divided by the repetitive interval between leading edges of the pulses in the train and by suitable choice of scale, the meter may be made to read the mark-space ratio directly. To compensate for temperature a thermistor may be included in series with the meter M. In a modification, Fig. 2, the circuit preceding capacitor C2 is the same as Fig. 1, and the pulse train as before switches valves V4, V5 on alternately. Hence, when a mark-space ratio of unity is supplied to the grid of V4 the meter M will read zero and for ratios other than unity the difference in average currents of the valves V4, V5 will give a direct measure of the mark-space ratio. A centre-zero or an end-zero meter may be used. To calibrate an end-zero meter, with no pulse input switch SW1A is opened and RV1 adjusted until the meter reads full scale deflection. SW1A is then closed and the meter terminals reversed by switch SW1B. Resistor RV2 is then adjusted until the meter again reads full scale. With a centre-zero meter the reversing switch is dispensed with. A diode clipper D1 prevents the circuit from being affected by amplitude variations by ensuring that the grid of V4 can only be held at a predetermined voltage level or at voltages negative to this level. A transistorized version is described, Fig. 3 (not shown).