Driving varicap with high voltage generates harmonics.

neazoi · Jun 11, 2015

Hello I have an HF oscillator (vco) that uses a varicap. The varicap is the 1sv149
I have noticed that if I drive the varicap with 0-2.5v there are no harmonics or spurious generated, whereas when I drive it harder (2.5v-9v) small amount of harmonics start to appear.

It seems that the higher the voltage applied to the varicap the more the harmonics and spurious out of it. Maybe it is operating more in it's non-linear region?

Has anyone encountered this issue before, is it true?

D.A.(Tony)Stewart · Jun 12, 2015

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.

volker@muehlhaus · Jun 12, 2015

neazoi said:
It seems that the higher the voltage applied to the varicap the more the harmonics and spurious out of it. Maybe it is operating more in it's non-linear region?

Have you checked if the oscillator gives the same change in harmonics with fixed value caps (over the same capacitance range)?

neazoi · Jun 12, 2015

SunnySkyguy said:
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.

My oscillator topology is similar to my other oscillator **broken link removed**
I have put the varicap in series with the crystal. One end of the varicap is connected to the base through a capacitor and to the varicap vcc through a resistor. It's other end is connected to the crystal through a capacitor and shunted with a resistor to the GND to complete the DC path.

The situation is as described in post #1. Also if I change varicap to mv209, harmonics occur no matter of the control voltage. It is only the mv209 that allows low harmonics when driven as in post #1.
I have not tested other varicaps yet.
As you have guessed, the frequency does not vary too much since a crystal is used, to it does not have to to with it, but only with the effect of the varicap.
It seems to me that if I drive the varicap at it's higher voltage ranges these harmonics occur.

- - - Updated - - -

volker@muehlhaus said:
Have you checked if the oscillator gives the same change in harmonics with fixed value caps (over the same capacitance range)?

Not yet, I will try to replace the diode with a 200-500pf fixed capacitor and later on with a <100pf one, to see if what you mention is true.
This will be helpful in understanding what is going on, I think this is a good starting point, thank you very much!

I will let you know later today

volker@muehlhaus · Jun 12, 2015

neazoi said:
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.

neazoi · Jun 12, 2015

volker@muehlhaus said:
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.

Yes volker, that is what I am going to try as mentioned in the previous post.
Thanks a lot! I will let you know about the results later on.

vfone · Jun 12, 2015

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.

neazoi · Jun 13, 2015

vfone said:
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.

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.

However the 0-2.5v of the 1sv149 I have applied, makes it to have a capacitance of about 300-500pf and there are no harmonics.

The varicap is placed in series with a 100pf capacitor at it's anode and another 100pf cap at it's cathode (my osc topology cannot shunt the varicap to gnd, it must be connected in series). So the total capacitance is about 40-45pf?

Isn't that weird?

jiripolivka · Jun 13, 2015

Frequency multipliers with VARACTORS were popular in 20th century. Step-recovery diodes were specifically designed to generate harmonics with a high efficiency.
There are problems with such devices: due to resonance, RF voltage may exceed the rated maximum diode voltage (reverse) and current (forward). Frequency multipliers use idler circuits for efficiency, and generally are narrow-band and temperature sensitive.
I have developed many varactor multipliers but today wideband amplifiers-multipliers are preferred. These new ones, however, generate more noise, and are more expensive.

biff44 · Jun 15, 2015

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.

jiripolivka · Jun 15, 2015

biff44 said:
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.

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.

volker@muehlhaus · Jun 15, 2015

neazoi said:
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?

neazoi · Jun 17, 2015

volker@muehlhaus said:
So do the harmoincs appear at the same frequency range with the fixed value caps?

The total capacitance, is a 100pF in series with the varicap (300-500pf) and also in series with another 100pF.
This setup generates no harmonics.
Replacing the varicap with a capacitor of the same value (300-500pF) generates no harmonics as well.

Now, if I apply even more voltage at the varicap, the total capacitance is 100pF in series with the varicap new capacitance and in series with another 100pF. This setup generates harmonics and the varicap capacitance is below 300pf (more voltage applied to it).
If I replace the varicap with a small capacitor (<22pf) harmonics are still generated. However, if I replace the varicap with a 47pf capacitor, harmonics are not generated.
When the varicap is set to 47pf, harmonics are generated.

Does it answer your question?

It seems to me that if I bias the varicap harder, this is the cause for harmonics....?

volker@muehlhaus · Jun 17, 2015

neazoi said:
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)?

neazoi · Jun 17, 2015

volker@muehlhaus said:
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)?

I was trying to answer that. Ok here it goes, to answer this one:
At the frequency where I get harmonics with the varicap, I do not get harmonics if I use the required fixed value cap, but only, if this fixed capacitor is larger than 22pf.

To put it the other way, If I use a capacitor smaller than 22pf, I get harmonics no matter if this is a fixed capacitor or a varicap.

However, if this fixed capacitor is > 47pf, I get no harmonics.
But, a varactor set at 47pf DOES give harmonics.
That is the difference.

Apart from all above, I get no harmonics, if my varicap is set for 300-500pf or if I replace it with a 300-500pf fixed capacitor.

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)

My initial question relates to points 2 and 3

D.A.(Tony)Stewart · Jun 17, 2015

neazoi said:
A fixed capacitor is > 47pf, I get no harmonics.
But, a varactor <= 47pf DOES give harmonics.

In general...
Harmonics come when the signal voltage rises nearing saturation of the amplifier, or;
In the case of the diode, if the waveform voltage + DC bias across the diode results in near forward bias from higher impedance from lower capacitance in series.. ( this might apply to an effective current source in series )

I can't say that I understand what regulates the voltage on your oscillator but the higher series impedance of both fixed and varicap causes distortion below 47pF with a bit more from the varicap at 47pF and none on the fixed suggesting an combination of sources. What happens if you attenuate the AC signal such that it still oscillates but reduces possibility of saturation.

Have you considered a different design?

I prefer parallel mode for XO's rather than series mode to get +/200ppm tuning range on 10MHz range for FM modulation or say 50 ppm range for adjusting to 0.01 ppm error.

volker@muehlhaus · Jun 18, 2015

neazoi said:
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.

neazoi · Jun 18, 2015

volker@muehlhaus said:
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.

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.

The connection is as shown in the image.

FvM · Jun 18, 2015

A varicap will always generate harmonics by it's nonlinear C/V characteristic. The only point in need of explanation is why you get higher harmonics at low varicap bias.

We normally observe the opposite behaviour because the relative voltage modulation by the RF is higher at low bias voltage. But this only happens if the connected oscillator node has low impedance compared to the varicap reactance.

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

biff44 · Jun 18, 2015

jiripolivka said:
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.

yes but in an oscillator, being fully driven, 3rd and 5th harmonics are MUCH more likely to come from the active device than the varactor resonator. Only if I saw nonharmonically related outputs would I suspect some parametric conversion effect in the non-linear varactor.

ALSO the OP states he sees MORE harmonics when the varactor is the highest reverse bias--the point where the C vs V slope is almost flat...i.e. the most linear region to run the varactor in.

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