GB459563A - Improved modulation meter for modulated carrier waves - Google Patents

Abstract

459,563. Wireless signalling. WILLIAMS, F. C., Magdalen College, Oxford. Oct. 17, 1935, No. 28752. [Class 40 (v)] A modulation meter for modulated carrier waves comprises a rectifier with a potentiometer load circuit of appropriate time constant, the voltage at the slider or tapping of which is fed to a second rectifier and to an amplifier, the modulation depth being indicated by the position of the slider or tap which gives a predetermined reading on a current-indicator in the anode circuit of the amplifier. As shown in Fig. 2, the modulated wave in the circuit LC is fed to a diode d1 with its. load resistance r shunted by a condenser x1 which byepasses the carrier frequency and causes voltages to appear across the resistance rl which vary between the peak value and the trough value of the carrier envelope. A fraction of this voltage is tapped off at a slider t and is shunted bv a resistance r3 and condenser c3 of such values that the voltage across the condenser c3 is the mean value of the voltage difference between the slider t and the bus-bar b, the modulation variations being smoothed out by the condenser c3. When the fraction a is equal to the percentage modulation, the voltage at the junction j is just sufficient to start current flowing through a resistance r3 and through the second diode d2. An indication of this current is given by a micro-ammeter m in the anode circuit of the valve A the grid-cathode circuit of which is connected across the resistance r2. Percentage modulation can therefore be read off at the slider t, if this slider is adjusted so that the meter m is just departing from its normal no-input reading. The diode d2 instead of being associated with the diode d1 may form a diode-triode with the amplifier A. In Fig. 5, an arrangement is shown by which modulation may be measured either as a troughto-mean ratio as described above, or as a peak-to-mean ratio. A centre-tap on the resistance r1 is connected to the diode-anode d2, and the condenser c3 associated with the movable tap t is earthed and connected by a switch s to one or other end of the resistance r1. In Fig. 6, the first diode d, is associated with the amplifier A1 in a diode-triode, and the second diode is replaced by grid rectification in the amplifier.