Last update: 8/6/96
I just recently added another, smaller tuning diode for fine tuning. (I wanted the fine tuning to affect both the offset and non-offset frequencies, to keep things spotted properly for CW). This was an old Radio Shack varactor, rated at 20 to 50 pF, with a circuit like the other offset varicap, coupled to the oscillator with a 0.1 uF cap. This gives about 1 kHz range at 1.8 MHz, multiplying to a nice useable vernier range of 8 kHz on the higher bands, and spreading out more on the high end of the PTO.
Stability is still quite remarkable -- seems to move less than 20 Hz from a cold start in the evening to the next morning, when zero-beated to WWV at 15 MHz. In the last 3 hours, it moved only about 10 Hz from that same standard. I haven't done detailed temperature drift observations -- there are lots of diodes to affect this, so I wouldn't expect it to be superlative here -- but the typical creep from internal circuit heating seems to be absolutely minimal. Even my musical ear can't detect this miniscule drift during a typical half-hour CW qso.
John Seboldt K0JD