EMRFD Message Archive 14904
Message Date From Subject 14904 2018-07-01 13:00:01 Andrea Baldoni Applying varactor tuning to a Vackar
I built a JFET Vackar oscillator, similar to the one in Fig. 4.14 B,
page 4.9, 500kHz.
I would like to add a varactor to tune it.
With 5V power, 850uA, I obtain about 8V RMS on the inductor (~ 22V pk-pk),
really too much for any varactor configuration.
Methods to reduce voltage at tuning diodes like capacitive divider,
using two back-to-back diodes, putting varactor in a less voltage node
(like at the source), aren't enough to keep the voltage reasonable and
still having the needed tuning range.
I can calculate if I can solve the problem using multiple diode couples, but,
since I don't need such amplitude in the oscillator, my real question is:
how to have a much much reduced amplitude in the inductor, in a reliable way
(without causing oscillator instability or prevent it to start)?
Tweaking feedback capacitors works but is very critical (I am already using
a feedback low enough not to cause clipping). Adding a resistor in
parallel with the inductor works, but will degrade Q. Using a higher source
resistor works to some minor extent and anyway is even more
critical than feedback capacitors.
Lowering supply voltage to 2.5V seems to cause a very linear reduction (8V RMS
become 3.9V), but is that the preferred way?
However it's not enough, and oscillator stop to be reliable with less
than 2V.
There is a simple general method, or I should insert sort of ALC loop (or a
"soft clamp", that anyway will degrade signal)?
Thank you.
Best regards,
Andrea15014 2018-08-16 17:47:39 Qrp Gaijin Re: Applying varactor tuning to a Vackar 15015 2018-08-16 18:13:18 Qrp Gaijin Re: Applying varactor tuning to a Vackar