Patent GB-629532-A (GB)
Title: Apparatus for the reception of phase-modulated alternating current signals
IPC: H04L27/227
Filed: 1946-05-02
Abstract: 629,532. Code telegraphy. GARDERE, H. May 2, 1946, No. 13282. Convention date, May 7, 1945. [Class 40 (iii)] A receiver for phase-modulated signalelements of two kinds differentiated by a phase difference of less than # radians produces a constant-phase demodulating current from both kinds of received signal currents by phasechanging means which are switched into or out of circuit as required by blocking means controlled by direct currents produced by demodulation of the received signal currents. Normal D.C. telegraph signals of the mark and space variety are converted at the transmitter, by suitable means, into signals of a common frequency, differentiated by a phase difference of #/2 radians. In the embodiments described the marking phase leads the spacing phase. Suppose in the first case that the first signal received at the receiver input e is a space, then part of the signal leaks through a blocking device br, after being selected by a channel filter fs. This signal is again filtered by a band-pass filter fp centred on the channel frequency, amplified at ar and applied with the same phase as when received to the control terminals of a balanced demodulator mr and also given a phase lag of #/2 at sp, amplified at at and applied with that phase lag to the demodulator mt. The incoming signal is also amplified at as and applied to the input terminals of both demodulators mr and mt. The output of mr is therefore a positive D.C. current imr which unblocks the blocking device br via a rectifier rr, while the output of mt is zero. The blocking device br therefore passes an increased signal current which merely confirms the existing state of the circuits and, after passing a low-pass filter fc to remove unwanted A.C. components, the positive D.C. reaches the output terminals s as a positive spacing signal which persists so long as a spacing condition is received. When eventually a mark is received, the output of the filter fp remains unchanged owing to its time constant and the phase at the control terminals of each demodulator is unaltered, while the signal input is advanced by #/2. The inputs to mr are therefore now in phase quadrature, producing zero output, while those to mt are in opposition and produce a negative D.C. output imt which unblocks bt via its rectifier rt. The signal now passes through bt and receives a lag of #/2 at st so that the control current of demodulator mr is in phase, that of mt lags by #/2, while the signal current leads by #/2. The output of mr is therefore zero and that of mt a negative marking current. The device will thus correctly translate the received signals. If, however, the first signal received is a mark it leaks through br, arriving in phase with the main signal at mr and producing therefore an incorrect positive or spacing output at s. At the start of a following space signal, the output of fp is unaltered but the signal input lags by #/2 with respect to the first signal. The output of mr is therefore now zero while the output of mt is a positive current which keeps br open so that the second, spacing, signal is applied to fp with its correct phase, but owing to the time lag of this filter its output only gradually changes from that due to a mark to that due to the following space. At the end of the second signalelement the control currents for the demodulators mr, mt are thus the same as when in the first case above a space was received and the apparatus will now correctly translate further received signals. The received signals are thus correctly translated after, at the latest, two kinds of signal element have been received. A second embodiment, Fig. 4 (not shown) employs only one demodulator.