# Tank circuit question (solved)

#### metermannd

Joined Oct 25, 2020
200
So after I finished my "what if" redesign of a power supply board, I decided to try and do the same with the receiver panel in this board set.

Most all the parts should be fairly straightforward to change to SMT (I may want to see if there is a suitable substitute for the CA3140 op-amp, tho').

Anyway, I ran into a problem with the two tank circuits on this board.

Both tank circuits are the same: a 6800pF film capacitor paralleled with a 22mH variable inductor to peak the signal response at 12.5kHz.

I have options as regards the 6800pF cap, no problem, but it unfortunately seems that, AFAIK, the 22mH variable inductor is obsolete.

Assuming I want to try selecting different values (giving priority to inductor value selection), how would I go about finding other values that would work just as well to give me a tank circuit with f = 12.5kHz? (no calculus, please)

#### metermannd

Joined Oct 25, 2020
200
So I plugged and cranked numbers and for a 2.5mH inductor, I need .065 uF to get a frequency of 12.490kHz, closest I can get is .064, which gives me 12.58kHz.

...and looks like I get the inductor from DK, the cap from Mouser. Ok.

#### dl324

Joined Mar 30, 2015
12,917
So I plugged and cranked numbers and for a 2.5mH inductor, I need .065 uF to get a frequency of 12.490kHz, closest I can get is .064, which gives me 12.58kHz.

...and looks like I get the inductor from DK, the cap from Mouser. Ok.
The problem is going to be tolerances. You can get variable caps, but they're going to be in the 10's of pF range and a 10% 65nF cap could be 57.6-70.4nF.

#### metermannd

Joined Oct 25, 2020
200
I'd forgotten that inductances in series add like resistances.

I'll probably take that approach and work out what combination of fixed and variable inductance works best in my case. Thanks!