Patent GB-585688-A (GB)
Title: Improvements in or relating to frequency selective circuits
IPC: H04B1/16
Filed: 1944-05-22
Abstract: 585,688. Carrier telegraphy and telephony. HADFIELD, B. M. May 22, 1944, Nos. 9813 and 21044. [Class 40 (iv)] [Also in Group XL] In a frequency-selective circuit applicable in carrier signalling, high attenuation at frequencies remote from the natural frequency of the arms is secured by deriving the output as the difference between two voltages or currents obtained from two tuned circuits which have the same natural frequency but different magnification factors, the magnification factor Q being defined as WoL/r for a resonant circuit and R/WoL for an anti-resonant circuit, where Wo is the natural pulsatance and L the inductance, and R, r are series or shunt resistances. The Specification shows that the voltage V1 tapped off the resistance r1 in a series circuit, Fig. 3, is so related to the input voltage e1 that e1/V1= {1-jQ(W0/W-W/W0)}, where W is the applied pulsatance ; hence at frequencies remote from the natural frequency the ratio e1/V1 is +jQ. Thus, the difference V between the output voltages V1, V2 of the two circuits, Fig. 3, is zero at such frequencies if e1, e2 are made inversely proportional to the magnification factors Q1, Q2 where both circuits are tuned to the same frequency, but Q 1 is different from Q2. The voltages V1, V2 may be rectified before being subtracted from one another, Fig. 5 (not shown), and they may be tapped off anti-resonant circuits in series with resistances, Figs. 4, 6 (not shown). Alternatively, the voltages may be taken from the inductances L1, L2, Fig. 3, or from an inductance shunted by a resistance, a capacitance being in series and the input voltages E1, E2 equal. The output may be reduced exactly to zero at any desired divergence from the natural frequency by tapping a fraction k off the larger voltagedrop VI, and for wider divergences will be zero as a result of rectification. In the practical embodiment shown in Fig. 8, the anti-resonant pairs L1C1, L2C2 are shunted by resistances R1, R2 which form a potentiometer and are so proportioned as to give the requisite output when L1=L2 and C1=C2. The received signals are applied through a high-impedance valve V1 to the balanced circuit shown, and after double-wave rectification at MR1, MR2, and smoothing at C3, C4, R3 ... R6, are tapped off a potentiometer P, across which the sum of the output voltages is connected, so that the p.d. between the neutral point N and the tap p is half the difference between voltages which are proportional to the rectified voltages and are equal to them when the tap is central. When the load L is a relay, marking, signals applied to the final valve or V2 are negative, so that negative bias for the spacing condition is not required, but when the signals carry speech modulation the polarity of the connections N, p may be reversed. In a radiotelephony receiver the circuit can be tuned by means of the condensers C1, C2 and relectivity can be controlled by varying the tap p. In a carrier telegraph system, the resistances such as V1, V2, Fig. 3, or R1, R2, Fig. 8, may have the same values in all the channels, the inductances L1, L2 being the same for all channels in resonant circuits, Fig. 3, and the capacitances C1, C2 in anti-resonant circuits, Fig. 4 (not shown). As a result, the time constant is the same in each channel and the attenuation of adjacent channel interfering signals is consequently uniform. The effects of transients originating from desired and from adjacentchannel signals are discussed in the Specification, and it is stated that a closer spacing of channel frequencies is made feasible by the invention. Telegraph impulses may be shaped to an exponential envelope at the sending end to reduce the effects of transients due to adjacent channels. Greater freedom of design is obtained and the sizes of the components and the power requirements are reduced in the carrier telegraph receiver shown as 2-channel in Fig. 9, by transferring the resistances R1, R2 to the output side, ripple being' smoothed at R3, C3, R4, C4, and tuned out of the relay Ry1 by condenser C5. The potentiometer P1 may be omitted and the output voltages fed to the grids of a twin valve. Distortion of the envelopes of telegraph signals may be reduced, by making the time constants R3.C3, R4.C4, Fig. 9, twice as great as R1.C1 and R2.C2, the rectification being full-wave. An alternative circuit for smoothing and correcting the envelopes of the rectified signals is shown in Fig. 11, where the output of the potentiometer R1, R2, Fig. 9, is applied at the points E1, D, E2 to the input electrodes of two valves which energize opposed windings of the relay R, smoothing circuits C3, R3 and C4, R4 being provided as in Fig. 9. Alternatively, smoothing may be affected by means of inductances connected in series with resistances and with the relay windings in the cathode circuits of the valves, the inductive time constants being twice as great as R1.C1 and R2.C2, Fig. 9. The resonant or anti-resonant circuits may each comprise more than two reactances so that a band-pass effect is obtained.