GB474387A - Improvements in or relating to piezo-electric crystal circuit arrangements - Google Patents

Abstract

474,387. Impedance networks. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and RUST, N. M. April 29, 1936, No. 12217. [Class 40 (iii)] A piezo-electric crystal PE, Fig. 10, coupling two tuned circuits, is shunted by a magnetic coupling M in order that the carrythrough effect of the shunt capacity of the crystal may be neutralized or modified for the purpose of causing the overall response characteristic to be symmetrical or of some other desired form. In the mesh shown in Fig. 1, which is equivalent to a piezo-electric crystal unit, the characteristic curve of the total conductance is symmetrical as shown at A, Fig. 2, while that of the susceptance due to the series condenser C1 and inductance L is antisymmetrical as shown at B. That of the susceptance due to the shunt-condenser C2 is the horizontal line C, so that the curve of the total susceptance is displaced upwards and is consequently asymmetrical as shown at D. In order that this carry-through effect of the shunt-condenser C2 may be exactly compensated so that the susceptance curve of the may be rendered anti-symmetrical, the mutual inductance M, with appropriate sign and dimensions, is added, and the Specification shows that this arrangement is equivalent to shunting a positive or negative inductance upon the crystal PE and modifying the tuning- inductances L1, L2. Alternatively, the carrythrough effect may be enhanced, undercompensated, over-compensated or reversed. The arrangement is shown as applied to highimpedance. pentode valves operating at a frequency of 450 kc./sec., the coils L1, L2 being tapped as shown to enable the impedance of the crystal to be matched. The effect of the resistance of the coils may be neutralized by earthing the lower ends of the coils M through a variable resistance, Fig. 13 (not shown). For measuring the characteristics given by various couplings, the output terminals may be connected to a valve voltmeter and the tuned circuits and the coupling M may be separately screened, Fig. 13 (not shown).