Single-sideband demodulator covers the HF band

Israel Schleicher, Bakersfield, CA -- 7/20/2000

The circuit in Figure 1 complements a previous Design Idea (Reference 1). The modulator used a phasing network to split a low-frequency (audio) signal into in-phase and quadrature (orthogonal) components. The phasing network has an advantage over other phasing circuits in that it delivers a phase error of only 0.15° and has low sensitivity to component tolerances (Reference 2). By reversing the direction of the network—in other words, 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 goes through. Because the network in the modulator circuit has two floating differential inputs, for the demodulator you must feed them from two floating sources.

The simplest way to obtain the feed signals is to use transformers. T₁ and T₂ are 600W , 1-to-1 telephone-coupling transformers with center-tapped bifilar primary winding. It is important to minimize the capacitance between the primary and secondary. Q₁ through Q₄ and Q₅ through Q₈ function as balanced mixers. They provide a wide dynamic range to the circuit. The circuit forms part of a direct-conversion receiver. IC₁ provides two quadrature local-oscillator signals. IC₁ requires a drive signal with four times the carrier frequency. IC₂ allows upper or lower sideband selection. Measurements on the prototype circuit show 37 dB of unwanted-sideband rejection for a 1-kHz modulated carrier and 32 dB of rejection for a 3-kHz modulating signal. You must use a sharp-roll-off, 3-kHz lowpass filter with the circuit. (DI #2563)

REFERENCE

1.Schleicher, Israel, "SSB modulator covers the HF band," EDN, Sept 30, 1999, pg 122.

2. Zavrell, Robert Jr, "New low-power single-sideband circuits," Philips Semiconductor, Application Note AN1981.

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