EMRFD Message Archive 3782

Message Date From Subject
3782 2009-11-18 17:16:16 joop_l Rock bending
Looking at the circuit "crystal characterization oscillator.gif" in the files upload by Wes on June 7th, 2009, I started wondering if this would be a good basis for a ceramic resonator VFO.

After a bit of analysis in SPICE I noticed the upper range of the oscillator was primarily limited by the large parallel capacitance (relative to a crystal Cp). With crystal overtone oscillator circuits this is often compensated by adding a parallel inductance.
Adding an inductor did indeed allow tuning to higher frequencies with a smaller series capacitor.

The first uploaded result can found here:

I modified Wes' circuit a little. The transformer does not have bifilar turns for the primary and feedback. It has 7 turns for the collector coil and 2 turns for both feedback and the output. I found this feedback to be enough for oscillation. Also to reduce the energy in the resonator (self-heating) some more I added two anti-parallel 1N4148s over the collector coil as a limiter. The circuit operates at 8V. The emitter resistor has been reduced to 390 Ohm. The resonator is paralleled with an inductor of about 44uH.

The result is an oscillator that allows a 4MHz resonator to be tuned from 3.893 to 4.194 MHz with a 250pF - 22pF trimmer. A whopping 300KHz! The output is a soft-clipped square wave of pretty stable amplitude.

One problem is that at the bottom end of the range it might start in a spurious oscillation at power-up. This does not occur at the middle to high end, or when tuning all the way down from there.

The upload shows some stability measurements during the first 5 minutes after a frequency change. It gets less stable at the high end. Strangely enough the least stable is one down from the high end. This could be due to the less than ideal connection the the external trimming capacitor. It currently is a pcb trimmer with a socket for a second fixed capacitor. I will try again when I find a mechanically proper variable capacitor that fits the required range.

Joop - pe1cqp
3783 2009-11-18 18:45:15 Tayloe Dan-P26412 Re: Rock bending
I also tried using a ceramic resonator for an 80m DC cw transceiver that
I was working on for the local club. I could stretch it 80+ KHz, but
the stability was poor. I ended up using a 1.18 MHz VFO and tripling it
up to 80m. Tripling it kept the transmitter from pulling the VFO, which
is what tends to happen with a 1x oscillator.

- Dan, N7VE