In particular, this would be a fairly low frequency VCO with a frequency range of 10 Hz to 20 kHz. I'm thinking of the configuration with the T-junction that has 2 opposite facing, reversed biased varicap diodes and a resistor perpendicular to the intersection point.

  • \$\begingroup\$ Certainly and the bigger the diode the more capacitance but not necessarily C1/C2 tuning ratio but there may be easier ways if you define specs \$\endgroup\$ Mar 22, 2019 at 4:33
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    \$\begingroup\$ reversed biased varicap diodes and a resistor perpendicular to the intersection point. Uhm, why not simply draw a schematic of that? \$\endgroup\$ Mar 22, 2019 at 7:14
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    \$\begingroup\$ The low end of that range, 0 Hz, is out of reach with high-ratio varicap diodes, let alone the lower-ratio you'd get with a Zener. \$\endgroup\$
    – Whit3rd
    Mar 22, 2019 at 7:23
  • \$\begingroup\$ In general, making an oscillator at 20 kHz using varicaps will be a huge challenge. Generally varicaps are only used in oscillators starting in the MHz range. And as Whit3rd writes: 0 Hz is impossible. Even if you'd accept 1 Hz to 20 kHz that's far too much for any analog solution unless you switch the components to get the range. Only a DDS can do "almost 0 Hz" to 20 kHz. \$\endgroup\$ Mar 22, 2019 at 7:32
  • \$\begingroup\$ After all that I did, don't you have any more questions? \$\endgroup\$ Mar 24, 2019 at 2:49

2 Answers 2


Can Zener or Schottky diodes be used as a substitute for varicap diodes in tuning a VCO?

Yes they can, depending on the circumstances.

A varicap diode is similar to a "normal" silicon diode, however the diode is designed such that the junction capacitance under reverse biased conditions is more predictable, has a more linear relation to the reverse voltage and sometimes also has a larger value.

A Zener diode is similar to a "normal" silicon diode, however the reverse breakdown voltage has a more predictable value. Besides that, it also has a reverse biased junction capacitance just like any other diode.

A Schottky diode also has a reverse biased junction capacitance which is also dependent on the reverse voltage.

So all three diodes (normal, Zener, Schottky) have "varicap" behavior so can in principle be used.

However, the varicap is better optimized for the task, see my paragraph about the varicap above.

Also, the Zener and Schottky diodes have some disadvantages which may render then useless in some applications.

They often have a higher leakage current. Especially the Schottky diodes have high leakage at high temperatures.

They might be more noisy. Especially the Zener diodes are known to be noisy (inject noise into your circuit).

They might have a capacitance with a lower quality factor. This is due to the series resistance of the diode's capacitance. Such a series resistance might lower the Quality factor of your tuning circuit or oscillator tank. This might lead to lack of loopgain in your oscillator so the oscillator will stop oscillating. The series resistance might also inject additional noise.

So yes, possible but there are some "gotchas" you have to be aware of.

  • \$\begingroup\$ I thought Zener noise resulted from the avalanche process...operated below avalanche voltage as a varactor, would it be so noisy? \$\endgroup\$
    – glen_geek
    Mar 22, 2019 at 8:52
  • \$\begingroup\$ @glen_geek You're right, it can be that the noise of a Zener diode is less when it is operated below it's Zener voltage. As this is not a normal mode of operation for a Zener diode so I doubt if anything will be mentioned in the datasheet about this. \$\endgroup\$ Mar 22, 2019 at 9:34
  • \$\begingroup\$ @Bimpelrekkie Here is a schematic that shows the configuration I'm talking about: \$\endgroup\$
    – Mr X
    Mar 23, 2019 at 18:33
  • \$\begingroup\$ i.stack.imgur.com/FxEyE.gif \$\endgroup\$
    – Mr X
    Mar 23, 2019 at 18:33
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    \$\begingroup\$ The only difference between a Varicap and a diode is that a Varicap would have tighter tolerance specs for C ratio and so process controls are made to meet these specs in mind rather than the forward VI characteristics. \$\endgroup\$ Mar 24, 2019 at 2:51

It is impossible to design a VCO with a frequency range of 0 Hz to 20 kHz using a DC controlled Varicap bridge due to the limitations of C1/C2 at V1/V2 is limited to <<100:1 best case and more often much less. e.g. 30:1V might achieve 60:1 C ratio in a half bridge voltage-biased diode-capacitance regardless of size being 1mA or 1kA diode.

Consider this. How many decades down is DC from 20kHz??

3 dec.(20Hz) or more? Thus 2 decades is even exceeding this requirement.

This why Spectrum Analyzer designs Up-convert the signal or use the VCO above the fmax to sweep then tune the VCO over a max of 50% range, then mix to down-convert at some IF to span a wide range of many decades.

Thus, another approach is needed, which exists. Any questions?

I just whipped up this DC to 20kHz (>6 decade) VCO triangle square output only. Effectively the 0 to 0.1Hz is only mV levels on the fine VCO input with coarse on 0V. Using Rail to Rail +/-10V OPA with any small signal logic level NMOS FET.

enter image description here

Each trace has different time scales to demonstrate 0 to 20kHz. You can change the 5V bias parts to simplify using +/-10V or single supply.

  • \$\begingroup\$ Yes. What is that approach? \$\endgroup\$
    – Mr X
    Mar 23, 2019 at 18:30
  • \$\begingroup\$ sine , square or triangle? \$\endgroup\$ Mar 23, 2019 at 18:31
  • \$\begingroup\$ All 3. It would be a multi-waveform VCO. \$\endgroup\$
    – Mr X
    Mar 23, 2019 at 18:33
  • \$\begingroup\$ How much THD? or resolution? \$\endgroup\$ Mar 23, 2019 at 18:34
  • \$\begingroup\$ The quality of a sine wave is defined by its SNR or THD and amplitude range \$\endgroup\$ Mar 23, 2019 at 18:44

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