GB645436A - Improvements in radio telegraph transmitting systems - Google Patents

Abstract

645,436. Frequency modulation. WESTERN ELECTRIC CO., Inc. Feb. 20, 1948, No. 5100. Convention date, July 21, 1944. [Class 40 (v)] The two radio frequencies required in a frequency-shift telegraph system are derived as the sum and difference frequencies of a high carrier frequency and a low frequency, these frequencies being applied in predetermined phase relation to four double-grid valves arranged as two pairs of balanced modulators. Keying is effected by reversing the phase of the low frequency applied to one balanced modulator pair, which has the effect of changing the output frequency from the sum to the difference or vice versa. The high frequency from source 24 is applied in phase opposition to the inner grids of the valves 22, 23 forming the first balanced modulator, and is also applied through a quadrate phasing network 26 in phase opposition to the inner grids of the second balanced modulator valves 16, 17. The low frequency from the source 10 is similarly applied to the respective outer control grids of the valves 16, 17, 22, 23. Owing to the quadrature relationship of the inputs, the output frequency taken in parallel from the four anodes comprises either the sum or the difference frequency. A telegraph key 1 applying either positive or negative voltage to the lattice network of rectifiers 8, 9 reverses the phase in the transformer 19 feeding the low frequency to the valves 22, 23 and causes the output frequency into the aerial 35 to change over from the sum to the difference or vice versa. Balance of the modulators is produced by adjustment of the screen-grid potential dividers 38, 40.