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Old 26th Feb 2009, 12:04 am   #1
PJL
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Default IF wobbulator design

I've started off with an oscillator coil from a transistor and have now got a reinartz oscillator producing a pretty clean sinewave. The next step was to add a varicap to be driven from the sweep from the tek scope.

This looks easy I thought but it isn't. The varicap is coupled to the tuned circuit so it has AC on it. Too much AC and the varicap becomes forward biased. I am also guessing that if I don't keep all of the AC swing in the linear operating area of the varicap then frequency will change with amplitude.

I have AC coupled the sweep via a pot to set the sweep width and DC bias for centre frequency via a pot but the next problem I encountered is the oscillator amplitude drops badly when damped by even a tiny load (50K).

Do I have any hope at all using the reinartz oscillator design or am I chasing the wrong design?
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Old 26th Feb 2009, 3:30 am   #2
MichaelR
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Default Re: IF wobbulator design

Have you got a circuit diagram of what you have already

Mike
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Old 26th Feb 2009, 4:58 pm   #3
jim_jobe
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Default Re: IF wobbulator design

I have a simple PW design if you want it,
Jim
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Old 1st Mar 2009, 11:08 am   #4
jim_jobe
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Default Re: IF wobbulator design

Here's that RC design and the associated timebase generator if anyone's interested,
Jim
Attached Files
File Type: pdf WOBBULATOR.PDF (566.7 KB, 372 views)
File Type: pdf TIME BASE GENERATOR.PDF (325.2 KB, 1292 views)
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Old 1st Mar 2009, 4:07 pm   #5
PJL
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Default Re: IF wobbulator design

Thanks Jim, I am still playing but having some success. The design above could do with a buffer.

What is actually happening is quite complex. It uses the AC signal to rectify a DC offset for the varicap diodes across C3. The time constant C3/R2 must also be long enough to keep the sawtooth straight.

My experience was this approach led to the amplitude dropping off at the low frequenciy end of the sweep presumably due to the high capacitance during part of the AC cycle when there is little/no reverse voltage across the varicap . I have been using a resistor to provide a DC offset.
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