Qex

Circuit Details

The circuit diagrams for my specific 75-meter exciter are shown in Figs 5 through 10. All resistors are 1% tolerance to minimize gain-matching errors. 0.1% resistors would be better in critical areas.

Oscillator U1 provides a stable digital clock to drive the frequency dividers in the clock generator in Fig 5.

Complementary metal-oxide-siiicon (CMOS) digital integrated circuits U2 through U7 form a digital frequency divider, providing the 1477-Hz carriers Ic and Qc and 115.2 kHz squarewave LPCLK. LPCLK sets the passband corner frequency in the switched-capacitor low-pass filters. This clock is 100 times the passband corner frequency desired in the filters.

Ic and Qc are each actually two digital signals, as described below, as required by the particular audio mixers used. The lc and Qc generator includes an input, SB, which can be grounded to switch sidebands in the final output, by changing the phase of Qc by 180 degrees. Fig 6 shows the timing relations of these signals. For the particular RF circuitry used here, leaving SB open (hence pulled high by the resistor) gives lower-sideband SSB output, and grounding SB gives upper-sideband SSB output.

The input band-pass filter, shown in Fig 7, is built from a single-pole high-pass filter and a three-pole low-pass filter. This filter also provides a midband gain of about 200, to amplify a 10-millivolts peak input signal to 2-volts peak, the signal level needed by the audio mixers and low-pass filters.

The audio input from a high-impedance dynamic microphone is high-pass filtered and amplified by op amp U11 A. Op amps U11B and U12A form a three-pole low-pass filter using op amp U12A. Op amp U12B is used as a unity-gain inverting amplifier.

The audio balanced mixers, shown in Fig 8, are built with CMOS analog multiplexers rather than with diodes or transistors. The 1477-Hz signals, lc and Qc are CMOS-level digital signals controlling the multiplexers. The switches in each mixer select whether to multiply the audio signal by +1, 0, or -1, by connecting to the output of U12A, ground, or the output of U12B to the switch output.

One mixer is built from analog multiplexer U8, and the other mixer is built from analog multiplexer U9. The term "analog multiplexer" means that U8 (or U9) connects one of its analog inputs to its analog output, depending on the

U4 UNUSED INPUTS=GND

115.2 kHz

> LPCLK

Fig 5—Clock generator.

U4 UNUSED INPUTS=GND

115.2 kHz

> LPCLK

Fig 5—Clock generator.

SIX CYCLES OF 8862 Hz ONE CYCLE OF 1477 Hz -------- 677 uS

Fig 6—1477-Hz clock waveforms.

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