I want to dynamically fine-tune the capacitor (about 100pF) of an RLC series resonant circuit, so as to keep it tuned despite variations of the coil's environment. Operating frequency is f = 13.56MHz, Q ≈ 30, about 0.1W power dissipation in R (mostly hidden in the coil). I only need like ±10% adjustment of C.

I see that I could use a motorized capacitor (approriately scaled with fixed capacitors), but what I find is way overkill and over-expensive.

What are my options ?

I vaguely see that a varicap may help (although polarization might be tricky), but I have no experience with these, and I even wonder if a varicap is at all compatible with my high-Q requirement.

I'm pointed that the voltage across the capacitor is critical in the varicap scenario. It can be estimated from the givens; I get:
(Q R)2 C = L  and (2π f) 2 L C = 1,
thus R = 1/(2π f C Q) ≈ 3.9Ω
thus IEFF = (P/R)1/2 ≈ 160mA,
thus UEFF = IEFF/(2π f C) ≈ 19VEFF across the capacitor.

With some margin, I believe 80VPP will never be exceeded.

Currently my capacitor has no grounded side; but that polarization issue seems fixable.

  • \$\begingroup\$ Depending on the steps you need something similar to a decade could work, or maybe a golomb ruler like setup \$\endgroup\$
    – PlasmaHH
    Apr 18, 2016 at 11:27
  • 1
    \$\begingroup\$ A proper varicap may help. Modern diodes have very high Q value (low losses), it is not a problem for your Q=30 requirement. The problem is the operating voltage. What RF voltage do you have on this ~100 pF capacitor? \$\endgroup\$
    – Master
    Apr 18, 2016 at 11:54
  • \$\begingroup\$ Motorized inductor? \$\endgroup\$
    – Andy aka
    Apr 18, 2016 at 12:24
  • \$\begingroup\$ @Master: I guess operating voltage is <80Vpp (see addition in question). \$\endgroup\$
    – fgrieu
    Apr 18, 2016 at 16:41

1 Answer 1


You have a large number of options.

a) Your link to motorised cap is indeed way over the top. However, if you can buy, or rescue from a junked radio, a vane-type variable tuning capacitor, you can buy a geared stepper motor for a few $£euro from fleaBay, and couple them yourself.

b) Although you say you only need 10% variation in C, it is usually unwise to use a variable capacitor as the entire tuning C. Far better to used a fixed capacitor, and connect a variable capacitor with a relatively large capacitance swing to it with a small capacitor. This has a number of advantages.

The stability and resolution of the resulting combination is far better than that of the variable capacitor alone.

The RF voltage across the variable element is lower than that across the combination. This is not so important if the variable is a vane-type, but could be very significant if the variable is a varactor.

If the variable is a varactor with a low Q, the Q of the combination will tend to the Q of the fixed capacitor, which you could choose to be higher.

c) You can adjust the inductor, perhaps by moving a ferrite core in and out of a solenoid, perhaps by making the inductor from two solenoids in series, and rotating one to change their mutual inductance, so changing the total inductance.

  • \$\begingroup\$ Perfect suggestions! 80 V pp is far too much for normal varactors. I would put forward one more solution, but it is really rather complicated. You can use PIN diodes for stepping through the required range by connecting / disconnecting fixed ceramic capacitors. And the remaining small adjustment range can be covered by varactor. This is stable and the most compact solution. However it is not a simple one. \$\endgroup\$
    – Master
    Apr 18, 2016 at 22:08

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