Hi there folks,
I've run into this circuit a number of times, most recently right here on ET Forums. It's interesting enough where it should be studied at least a little bit.
See the attachment for the circuit details, and i'll explain what this is all about.
In Fig. 1 of the attachment we see the parallel LC network, the Equations for that network, and the impedance vs frequency F. F0 is the center frequency and as shown the network has highest impedance at that frequency.
With the equations we can calculate either the center frequency F0, or L or C.
Because of the impedance characteristic, an LC network in series with the output will block a particular frequency, while an LC network in parallel with the output will allow a particular frequency to pass and shunt other frequencies.
Now it is known that with proper filtering a square wave can be filtered into a pure sine wave (or nearly pure). It is also known that the third harmonic is the strongest unwanted frequency and the fifth is fairly strong too, so we want to filter those two out with two LC networks. Fig. 2 shows two networks in series and they are in series because they are intended to block the third and fifth harmonics. These networks are shown as L1 and C1, and L2 and C2.
The third LC network shown is in parallel with the output because we want to tune that for high impedance so that it will pass 100kHz but shunt other frequencies like the third and fifth and higher.
Using the Equations shown, we start with C1=1nf and C2=1nf, and we tune L1 so that L1 and C1 block 300kHz, and we tune L2 and C2 to block 500kHz. These are the third and fifth harmonics of the fundamental which is 100kHz. With C1 being 1nf we calculate L1=0.0002814477 Henries. With C2=1nf we calculate L2=0.00010132118 Henries.
With the third parallel LC network made of L3 and C3, we start with
L3=0.00253302959 Henries, a known inductance rather than a known capacitance.
The fixed value of R1 is 5000 ohms.
The question is, what is the best value for C3 with this chosen value of L3?
Remember we are trying to pass 100kHz and block all others, with two of the networks made to specifically block the 3rd and 5th harmonics.