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Old 1st Feb 2022, 2:38 pm   #101
bc312
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Default Re: Haigh Wobbulator is there sufficient interest for a PCB?

Thanks Peter - no problem.

Mike
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Old 5th Feb 2022, 1:59 pm   #102
ian_rodger
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Default Re: Haigh Wobbulator is there sufficient interest for a PCB?

I have just completed building my wobbulator (thanks to Peter Electronpusher0 efforts). I just wanted to check with others who have built this if the output waveform I get is correct. In summary, with the varactor diode shorted I get a reasonable sine wave. With the varactor in circuit, however, I get significant distorion. Is this to be expected due to the voltage swing across the varactor modifying the frequency curve or have I another problem to investigate? Scope photos below illustrate output waveform with and without the varactor in circuit.
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Old 5th Feb 2022, 10:54 pm   #103
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Default 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
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Old 6th Feb 2022, 12:25 pm   #104
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Default Re: Haigh Wobbulator is there sufficient interest for a PCB?

Just a couple of queries:

1. The Output Level (R13) is referenced as 4k7 by Haigh, 10k by Matthews. Is there a preferred value?

2. The Front Panel drawing has a scale for "Timebase (Hz x 10)" but the circuit diagram has two controls, "Sweep Rate" and "Sweep Amplitude". Which is referred to as "Timebase"?

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Old 6th Feb 2022, 2:36 pm   #105
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Default Re: Haigh Wobbulator is there sufficient interest for a PCB?

The output level pot is not critical, either value works.

Sorry about the confusion over the "timebase", that is the sweep generator The "Sweep Rate" is labelled as "Timebase (Hz x 10)" on the front panel since it performs a similar function to the timebase on a scope.

I laid out the front panel for the prototype I built some time ago. In that version the wobbulator pcb was the same as that published in the original article and I made the sweep generator / timebase on vero board.

When I laid out the pcb recently I forgot that I had labelled the sweep rate pot as Timebase on the front panel.

Hope thats clear.

Peter
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Old 6th Feb 2022, 3:13 pm   #106
ian_rodger
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Default Re: Haigh Wobbulator is there sufficient interest for a PCB?

Quote:
Originally Posted by orbanp1 View Post
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
Thanks for confirming my thoughts. I measure about 6Vpp a.c. on the varicap in my circuit, with DC offset determined by the tuning pot. So looks like its working as intended.

Ian
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