Single-sideband demodulator covers the HF band
Israel Schleicher, Bakersfield, CA -- EDN, October 26, 2000
The circuit in Figure 1 complements a previous Design Idea ("SSB modulator covers HF band," EDN, Sept 30, 1999, pg 122). The modulator employs a phasing network to split a low-frequency audio signal into in-phase and quadrature (orthogonal) components. This circuit delivers a phase error of only 0.15° and has a low sensitivity to component tolerances, which are advantages over other phasing circuits (Reference 1). By reversing the direction of the network, that is, feeding the output with two orthogonal signals and tapping the input, the network functions as a detector. Feeding the two signals one way may produce a signal at the input, but if you interchange the two signals, no signal will go through.
Because the network in the modulator circuit has two floating differential outputs, the demodulator network requires two floating sources. The simplest way to accomplish this requirement is by using transformers. T1 and T2 are 600? 1-to-1 telephone-coupling transformers with a center-tapped bifilar primary. It is important to minimize the capacitance between the primary and the secondary windings.
Q1 to Q4 and Q5 to Q8 function as balanced mixers. They provide a high dynamic range for the circuit, which is part of a direct-conversion receiver. IC1 provides two, quadrature LO signals, and this IC requires a drive of four times the carrier frequency. IC2 allows for upper or lower sideband selection. The prototype circuit measures 37 dB of unwanted side-band rejection for a 1-kHz modulated carrier and 32-dB rejection for a 3-kHz modulation. A sharp 3-kHz lowpass filter must follow the circuit.
References Zavrell, Robert J, "New low-power single sideband circuit," Philips Semiconductor, AN1981.
