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Silly problem with grid dipper

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dr pepper

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Ok so I want to make another dipper, I have done a few now.
Cant get this one to work, instead of using an old airspaced or polyvaricon cap I wanted to use varicap diodes.
I built this circuit:
http://www.qsl.net/iz7ath/web/02_brew/15_lab/02_dipper/english/pag09_eng.htm
I realised that this had an error, the lower part of the tank isnt grounded, doing so made it work, then I found the same schem but corrected.
http://www.tradeofic.com/uploadfile/ic-circuit/20096263543515.gif
I can only get the circuit to work if I use a variable capacitor in place of the varicap diodes, if I put the varicaps in circuit the system wont tune it just runs at the lowest freq and if the tuning voltage gets to 0.6v and turns on the lower varicap it stops oscillating.
I'm using a J310 fet and bb510 varicaps.
 

crutschow

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How close are the bb510 varicap parameters to the MVAM-2 shown in the schematic?
 

Les Jones

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I think the polarity of the lower diode (CR2) should reversed so that both diodes are reverse biased. The tuning voltage then should swing from zero to about + 15 volts.

Les.
 

dr pepper

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There is a big difference between the mvam 2 and the bb510, the mvam 2 is 20 to 40 pf, the bb510 is 20 to 440pf.
That said the circuit works well with a 20 - 500pf airspaced variable cap, and a 10 - 365pf polyvaricon.
Do you think the o/p waveform is tuning the diodes and killing oscillation?
I dont have any mvam 2's, but I tried a couple of leds reversed biased as varicaps (the ground side one lights up), and I had the airspace cap in parallel too, it worked, but obviously there is very little tuning range from the 'varicap leds' like this.

Yes I tried reversing the lower diode, after all dual varicaps are often common cathode, but that didnt work either.

I was thinking of putting a largeish cap (0.01u) between the lower diode and ground, but if I did that there would be no dc path to ground for the diode bias voltage.

I'm thinking the Q of the tank is probably on the low side with the high - pf varicaps (I wonder if the 100k on the tuning voltage line is effectivly shunting the tank), and better just to the point where it works with the airspaced caps.
 
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