UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Haigh Wobbulator is there sufficient interest for a PCB?

orbanp1 · 5th Feb 2022, 11:54 pm

No surprise there.
On the cathode of that varicap there is about 15Vpp signal (+/- 7.5V from the ground), and the varicap is after all a diode.
That is from a simulation, with a coil of Q 50, at about 1.2MHz.
The KV1236 varicap has a capacitance of 500pF to 30pF from 1V to 8V.
Even if you add the offset DC voltage on the cathode (the tuning voltage) the diode will still clip the signal, as well as change the capacitance as the sinusoidal signal changes.
To limit such effects they usually use several diodes in parallel, and two sets of such parallel diodes connected in series, with opposing polarity.
You would also want a "regular" cap parallel with the varicaps, so the varicaps only provide a fraction of the tuning capacitance.

Peter

5th Feb 2022, 11:54 pm	#103
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Haigh Wobbulator is there sufficient interest for a PCB? No surprise there. On the cathode of that varicap there is about 15Vpp signal (+/- 7.5V from the ground), and the varicap is after all a diode. That is from a simulation, with a coil of Q 50, at about 1.2MHz. The KV1236 varicap has a capacitance of 500pF to 30pF from 1V to 8V. Even if you add the offset DC voltage on the cathode (the tuning voltage) the diode will still clip the signal, as well as change the capacitance as the sinusoidal signal changes. To limit such effects they usually use several diodes in parallel, and two sets of such parallel diodes connected in series, with opposing polarity. You would also want a "regular" cap parallel with the varicaps, so the varicaps only provide a fraction of the tuning capacitance. Peter Attached Thumbnails Last edited by orbanp1; 6th Feb 2022 at 12:09 am.