Patent GB-535388-A (GB)
Title: Improvements in or relating to radio telegraph systems
IPC: H04L27/06
Filed: 1939-10-06
Abstract: 535,388 Wireless receiving systems. CREED. & CO., Ltd., SALMON, R. D., and MASON, F. P. Oct. 6, 1939, No. 27374. [Class 40 (v)] In a wireless telegraph system in which the signals are received as modulations of a low frequency impressed on a radio frequency, the receiving apparatus is arranged so that, in the absence of incoming energy there is produced the same effect as is produced by interference or by the receipt of the low frequency carrier. An incoming radio frequency is passed through an amplifying stage RFA, Fig. 1, following which part is passed through a detector HFDET and amplifier AMP so as to obtain a direct current proportional to the amplitude of the incoming radio frequency. This is applied to the first valve of a final amplifier PPA so as to produce a bias which reduces this valve to its cut-off point. The remaining part of the incoming energy is demodulated in the usual way and the low frequency output passes through a rectifying stage AFR so as to produce direct current pulses representing the signal as transmitted. These pulses are supplied to the first valve of the amplifier PPA in such a way as to overcome the negative bias produced by the rectified high frequency and to pass the signals to the recording apparatus. When complete fading occurs and there is no incoming energy the negative bias on the final stage is removed and current is therefore passed as if low frequency modulation had been received. Since interference passes through the low frequency and output stages it also produces the effect of received marking impulses. In one modification, Fig. 2 (not shown), a local generator of low frequency oscillations has its output normally suppressed by a stage similar to the stage HFDET above. In the absence of incoming energy this oscillation is no longer suppressed and is allowed to pass through low frequency and rectifying stages to the output. In another modification an electromagnetic relay normally biased by rectified radio frequency oscillations is employed, Fig. 3 (not shown), and in a third modification an automatic volume control circuit is arranged to pass current to the main circuit when fading occurs, this current producing the effect of a marking impulse. In a valve circuit, Fig. 5, embodying the invention rectified radio frequency is applied to the grid of a double-diode-triode VI so as to drive it more negative as the radio frequency amplitude increases. The grid of a following triode V2, being connected to the anode of the triode of the first valve, accordingly becomes more positive at the same time. The output of the low frequency stages of a preceding receiving apparatus is applied to a transformer Tl and is thence passed to the double-diode of the first valve V1 so as to impress a negative potential on the grid of the second valve V2. Connected to the output of the valve V2 are two triodes V3, V4 interconnected so as to have two states of stable equilibrium, as described in Specification 526,680, such that on receipt of a marking impulse the valve V3 passes current while on receipt of a spacing impulse the valve V4 passes current. When a marking impulse is received, the negative bias on the grid of the valve V2 obtained through the transformer T1 overcomes the positive potential produced by the radio frequency oscillations and drives the grid of the valve V2 sufficiently negative for the grid of valve V3 to allow current to flow. On receipt of a spacing impulse the grid of the valve V2 becomes less negative so that the bias of the valve V3 becomes more negative and its output ceases while as a result of the connection with the valve V4 this latter valve passes current. During absence of incoming energy no bias is applied to the grid of the valve V3 from the valve V2 but the grid becomes sufficiently positive to allow current to flow as if a marking impulse had been received. Specification 514,853 also is referred to.