Softrock Lite II

Posted on Feb 7, 2014

The Local Oscillator stage implements a basic Colpitts Crystal Oscillator with a buffer stage to increase the signal level. The oscillator produces a signal that is at the crystal`s specified fundamental frequency. In reality, for each frequency the crystal circuit will oscillate at a slightly lower frequency (~ - 1 kHz), due to the capacitive div

Softrock Lite II
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ider (C10/C11) pulling the crystal down somewhat. The effect is more pronounced for the higher bands. Alan, G4ZFQ points out that on the 30m, 20m, 15m receivers, the Local Oscillator produces a signal that is 4/3 times the desired center frequency as opposed to the 4x the center frequency output for the lower band models. "Subharmonic" works like this:- The LO outputs a 13. 5MHz signl that goes to the dividers /4, resulting in 1 3. 375MHz square wave ( rich in odd harmonics) being fed to the mixer. At the mixer, a strong 3rd harmonic is present on the clock inputs, along with the fundamental of 3. 375 MHz. The 3. 375 fundamental multiplied by 3 yields the third harmonic of 10. 125MHZ. The Bandpass filter (BPF) performs the essential function of severely attenuating any signals centered around the 3. 375MHz fundamental frequency and first harmonmic, but allows 30m signals centering around the third harmonic of the 3. 375MHz LO output. The result is that the mixer is dealing with signals in the passband, centering on 10. 125MHz, as though the dividers were passing a fundamental frequency of 10. 125 to the mixer. BPFs are all that stop Softrocks from working on unwanted frequencies! Mount the HC49 crystal mounting in the upper left corner of the board, mounting it vertically to the board. A small plated-through hole in the lower left corner of the crystal mounting position provides a place for a grounding wire to be soldered...

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