By Peter Traneus Anderson, KC1HR 990 Pine Street Burlington, VT 05401
This article describes an experimental 75-meter, 80-milliwatt single-sideband (SSB) exciter designed to make maximum use of modern integrated circuits (ICs). This exciter contains no crystal or mechanical bandpass filters, and no audio phase-shift network. Instead, IC low-pass filters are used to implement a variation of Weaver's method of SSB generation. I will discuss the theory first, and then specific circuit details.
D. K. Weaver's method of SSB1 is little known in Amateur Radio, having appeared only once in QST in an article by Howard F. Wright, W1PNB2. Wright explains well how the Weaver method works, and explores the method's strengths and weaknesses by reporting the results of building a working Weaver exciter. A comparison of Wright's exciter to the exciter described here shows the tremendous improvement in analog signal processing components since 1957.
The block diagram in Fig 1 shows the similarity between Weaver and phasing methods of SSB generation. Like the phasing method, the Weaver method relies on converting the input audio into two signals, la and Qa. After !a and Qa are generated, the remainder of the circuitry is identical for phasing and Weaver methods.
Ia and Qa are applied to a pair of balanced mixers whose radio frequency (RF) carrier inputs, Irf and Qrf, are phased 90 degrees apart. The mixer outputs are added together and the result is ideally the desired SSB signal.
The Weaver and phasing methods have different methods of generating la and Qa, and this results in different SSB signals being generated by the two methods. The spectra are shown in Fig 2. In the phasing method, the SSB passband is entirely on one side of the carrier frequency. In the Weaver method, the SSB passband is centered on the carrier frequency and extends to one-half of the passband width on either side of the carrier.
In the phasing method, the input audio is applied to a band-pass filter. As shown in the block diagram in Fig 3, the filter output drives an audio phase-shift network which generates la and Qa such that ia and Ga are the same amplitude and 90 degrees apart in phase for all frequencies in the audio passband. The band-pass filter must have steep skirts for this filter alone sets the passband width for the final SSB signal.
In the Weaver method, a crude band-pass filter, two identical balanced mixers, and two identical sharp-cutoff low-pass filters are used to create la and Qa. See the block diagram in Fig 4. The band-pass filter can be crude; its function is to block far-out-of-band signals from getting to the mixers. The filter output drives one input of each mixer.
1 Notes appear on page 9.
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