Driving varicap with high voltage generates harmonics.

[volker@muehlhaus]

Volker, but that does not explain the second point above, where the oscillator generates harmonics only with the varicap in place , when the varicap is biased for 22-300pF.

Based on this, the varicap biasing MUST affect the oscillator harmonics somehow.

I don't see that as a proof that it's the biasing/voltage dependency. It could be other parasitics (series resistance) of the varicap that change the oscillator behaviour.

If it was the voltage dependency, I would expect the largest effect for small tuning voltage where the have largest relative RF swing and largest dC/dV. But you see an effect at large tuning voltages only, not small.

 

[neazoi]

In your circuit, the oscillator impedance is probably > Xcvar, so you get higher voltage modulation at low capacitance, respectively high bias.

It might be the case. This would explain it.
I know I showed you another circuit, because I had the varicap schematic drawn on it, but the actual oscillator I am using is this **broken link removed** (Fet version of the above) and I connect the combination of the varicap and capacitors just before the crystal and after the 220k shunt.
I suppose the Fet version causes even higher oscillator impedance and so the effect you describe could be even more more true?

 

[FvM]

I connect the combination of the varicap and capacitors just before the crystal and after the 220k shunt.

I would prefer a sketched schematic.

 

[neazoi]

I would prefer a sketched schematic.

Ok here it is

 

[D.A.(Tony)Stewart]

Output impedance is too high, Have you tried your crystal this design?
http://mobiledevdesign.com/site-fil...devdesign.com/images/archive/0401Koster58.pdf

What is the load capacitance and tolerance and temperature range of your crystal ? pn?


YOu ought to be familiar with all these characteristics too.
http://www.euroquartz.co.uk/portals/0/pdf/tech-notes.pdf

 

[FvM]

Ok here it is

Yeah, it's similar to the previously linked design by connecting the varicap in series with the crystal. So you get the same effect regarding varicap behaviour.

At low bias voltage and high capacitance, the varicap is effectively shorting the sgnal voltage. Though it has high dC/dV in this range, it can't generate much harmonics.

 

[neazoi]

Yeah, it's similar to the previously linked design by connecting the varicap in series with the crystal. So you get the same effect regarding varicap behaviour.

At low bias voltage and high capacitance, the varicap is effectively shorting the sgnal voltage. Though it has high dC/dV in this range, it can't generate much harmonics.

Thank you!
So my observations should be correct, then, I guess.

I have also tried to replace the varicap with a dual type bb212 and again the same behaviour.
I have also tried to place the single 1sv149 varicap circuit after the crystal and again the same vehaviour.

- - - Updated - - -

Oh, just to mention, I have noticed that at 160m and 80m crystals, the varicap can be set to the whole range (20-500pf) without generating harmonics.
It is only with higher frequency crystals, that harmonics are generated if varicap capacitance is more less than 300pf or so (0-2.2v)

Have you got any Idea why this is happening in my circuit?

 

[FvM]

As stated by all contributors, harmonic generation with varicaps is normal operation. The amount depends on the signal magnitude across the diode, the dC/dV gradient and the harmonic supression achieved by the oscillator circuit.

Using controlled low oscillator voltage, high Q tank circuits and additional filters are the obvious means to reduce harmonics.

 

[neazoi]

Varicap driving on oscillator, how to?

Hello,
I have an oscillator with a series varicap out of it.
Is this a good way to drive it?

More specifically:
1. Are the chokes needed?
2. If so, what values of chokes should I use for HF, 1-30MHz?
3. Will I need to place the chokes as they are, or on the left, after the resistors, closer to the varactor?
4. Is it proper to add some 0.1uF shunt capacitors and at which points?

 

[neazoi]

Re: Varicap driving on oscillator, how to?

Ah, I am sorry, I opened it in another thread, thinking that it might relate to any other oscillator, not just this one.
Thanks for moving it.

I would appreciate for your comments on this circuit, based on my points.