I've never had this problem before. But I suspect your signal amplitude is changing with frequency indicating lack of negative feedback on AC gain of the VCO. The transistor Vce MUST always be > 2V for low harmonics for peak swings. The lower Vce min is , the more harmonics. COntrolled loop gain mitigates this to ensure no saturation using feedback R to control gain for variable Q filter which changes with frequency which changes with C which changes with V on varicap.
It seems to me that if I drive the varicap at it's higher voltage ranges these harmonics occur.
Is this really from the varicap, or just a circuit behaviour from the change in capacitance?
Check if the same harmonics are seen when you set the same frequencies with fixed value caps.
1sv149 and mv209 have different capacitance. One is 30pF to 500pF, and one is 4pF to 30pF, so replacing one to each other is meaningless.
The main reason of oscillator harmonics is the distorted output waveform, so using a scope to view the waveform can give information what happen with different resonator values.
as you tune the varactor, the frequency changes. Most likely the DC operating point of the active device changes too. So it is not surprising that certain frequency outputs have higher or lower spurious outputs.
I replaced the varicap with capacitors, first a 470pf, then a 330pf then a 100pf then a 47pf. There were no harmonics. When I put a < 22pf cap harmonics start to appear again.
So do the harmoincs appear at the same frequency range with the fixed value caps?
Does it answer your question?
Not really. My simple question was: at a frequency where you get harmonics with the varicap, do you get harmonics if you use the required fixed value cap/caps (for same oscillator frequency)?
A fixed capacitor is > 47pf, I get no harmonics.
But, a varactor <= 47pf DOES give harmonics.
To summarize, I have three situations:
1. One, where the oscillator generates harmonics, no matter if I use a varicap or fixed cap (and that is for <22pf).
2. A second one, where the oscillator generates harmonics only with the varicap in place (22-300pF)
3. A third one, where no harmonics are generated either by the varicap or by a fixed capacitor (300-500pf)
Understood now.
I don't think the main problem is the diode. If the diode bias was the (core) issue, we would expect more trouble at low bias voltage where the (relative) effect of the RF voltage across the diode is larger. At high bias voltage, small +/- voltage changes have less effect, and that is where you see the problem. Also, you see it with fixed value caps as well if you go up in frequency even more. If you are able to fix that problem with fixed caps at high frequency, I expect that the varactor will also work fine.
Dear Biff 44, the thing is not so easy. Any non-linear device generates harmonics. Using Manley-Rowe theory, adding an idler resonator assists in generating harmonics with a high efficiency.
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