# LC Circuit with Variable Filter - Unusual

#### Sir Kit

Joined Feb 29, 2012
57
The tank circuit below is designed to have a variable resonance.

Can someone please tell me, in general terms, what the function of the lower two caps and pot would be?

In other words, how they, and their selected values, affect the output of the entire circuit.

#### ci139

Joined Jul 11, 2016
1,696
Try simulating the fading oscillations of
just the parallel LC tank
and
the previous in series with the capacitor (with the same value than in the parallel tank)

#### Sir Kit

Joined Feb 29, 2012
57
You seem to have a general idea of what the parallel tank would do, i.e. "fading" effect. Would you be able to please elaborate on why this occurs so I know what I am looking for?

#### ci139

Joined Jul 11, 2016
1,696
Function = trivial = high pass + DC blocking (untying from the reference voltage point)
... further -- the RC has a 2πfRC time constant (only here the parallel capacitance gets to be the part of it . . . in ideal depending how/what the designer considered the effect of (the parallel to C) the series RC to be --e.g.-- a fine tuning --or-- the whole C||RC resonator "fully" contributing it's time constant)

#### Sir Kit

Joined Feb 29, 2012
57

As I understand, the upper tank circuit would (probably) be tuned to a specific frequency. The lower RC circuit then acts as a tunable high frequency cut-off for any such tuning.

Taking a specific example to clarify. Let's say the coil has an impedance of 10uH at 1120KHz, C1 and C2 are 1nF. R1 is 100K. That would provide a degree of "tuning" within the AM band.

Assuming R2 is also 100K, how would the values of C3 and C4 be calculated to perform the tunable high pass filter effect you describe? In other words, to impose an adjustable "fading" over the entire tuning range of the upper tank circuit.

#### ci139

Joined Jul 11, 2016
1,696
sorry i don't work with filters too frequently

for the given topology i guess it has the tuning as a whole for 1120k the C value should be a half $$\left({\frac12}\right)$$ the one in the identity $$2πfL=\frac1{2πf\mathbf{C}}$$ -- that in case both the parallel and the series C-s are equal