A Practical Tantalum Thin-Film Single-Sideband Demodulator Using RC Time-Varying and Active Networks

The thin-film circuit described in this paper is equivalent to a demodulator circuit containing an extremely selective filter ("sideband" filter). Such a filter would, in conventional form, require very high Q inductors or, as the Q requirement increases, the use of crystal or mechanical filters. At the frequency of operation of this circuit (1 MHz), demodulation with conventional filters would have to be accomplished in two or more stages so that the selectivity requirement can be decreased for each filter. Thin-film techniques restrict us to circuits using only resistors (R), capacitors (C), and added semiconductor devices (thin-film inductors are not considered here since their inductance values are too small). The current trend is to realize frequency selective networks (conventionally in LC form) as active RC networks. However, although the circuit described here incorporates such a network, the main selectivity requirement cannot be met by present-day active network techniques. The solution is found in the use of time-varying RC networks, i.e., by combining passive RC thin-film phase-shift networks with miniature transistors, used as electronic switches, in the form of so-called quadrature modulation circuits. The phase-shift networks, which in principle can be passive thin-film RC circuits, are in practice more easily realized as combinations of much simpler RC circuits with buffer amplifiers.