I am learning about how VCO modules operate and I am rekindling an old interest in electronics in general. For one particular VCO, the manufacturer has provided a circuit diagram. Does anybody know the purpose of the capacitors in series between the tuning voltage source and the mains voltage source?

Circuit diagram


closed as unclear what you're asking by Andy aka, The Photon, RoyC, Sparky256, Finbarr Feb 23 '18 at 9:46

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    \$\begingroup\$ I realy can't see any series caps here... \$\endgroup\$ – Eugene Sh. Feb 22 '18 at 19:45
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    \$\begingroup\$ ...or mains voltage... \$\endgroup\$ – calcium3000 Feb 22 '18 at 19:50
  • \$\begingroup\$ There are capacitors from the 5V power supply (Vcc) to ground, and from the tuning voltage (VT) to ground. There are none in series to either voltage. \$\endgroup\$ – JRE Feb 22 '18 at 19:59
  • \$\begingroup\$ A link to the data sheet is important. Have you read it to see what it says about those capacitors? \$\endgroup\$ – Andy aka Feb 22 '18 at 20:03
  • \$\begingroup\$ OK you've had an hour to provide the data sheet and correct your teminology. I'm voting to close this question as unclear. You should stick around for an hour to service comments. Do not naively expect your question is perfect enough in detail to be answered. \$\endgroup\$ – Andy aka Feb 22 '18 at 20:42

Your terminology is incorrect but the shunt caps on the supply and tuning voltage is to provide low ESR to noise over many decades of frequency. Often the Self Resonant Freq. or SRF limits the useful RF spectrum and a smaller 1nF is needed to take over when the 10uF becomes inductive. Hence these parts are 5 & 4 decades apart in value to extend the low impedance to ripple for potentially 8 decades in bandwidth. Each source Zo(f) must be known to demonstrate precisely.

Superimposing the impedance charts may be more instructive. There are many similar answers on this site if you search; Shunt caps, SRF.

It often extremely important to remove all ripple noise V to reduce phase noise on the VCO. However if were used in a PLL where negative feedback requires more bandwidth to track a lower noise XO & phase detector, then the Vtune caps may be smaller.


They are decoupling capacitors to keep VCC power rails devoid of high frequency noise. Without it, the frequency of the output wave will fluctuate with the noise in power rails.

  • \$\begingroup\$ all noise can be measureable so never devoid in terms of measurement but devoid of problems perhaps. :) \$\endgroup\$ – Sunnyskyguy EE75 Feb 22 '18 at 21:03

The clearest thing to say about those capacitors is that they are there to remove noise on the power supply and the tuning voltage.

Noise on either of those will cause the generated frequency from the VCO to be noisy. That is, the frequency will wander up and down in response to the noise.

Your generated signal would be frequency modulated by whatever crap is on the power supply or the tuning voltage. You want a clean signal, so you need a clean power supply and a clean tuning voltage.

  • \$\begingroup\$ how would you define clean in sensitivity terms? This would help those to understand better in mV/dB @1kHz offset for a given MHz/V VCO \$\endgroup\$ – Sunnyskyguy EE75 Feb 22 '18 at 20:07
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    \$\begingroup\$ I wouldn't because I don't know enough about the subject. And honestly, I don't think it would help the guy asking the question either. The question is far too basic for that level of detail. \$\endgroup\$ – JRE Feb 22 '18 at 20:23

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