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# Must the L>>C in the parallel resonant circuit??

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#### ZengLei

##### Full Member level 1
I want to design a radio myself,
so it need a LC parallel resonant circuit to filter the wave

Must the L>>C in the parallel resonant circuit??

Thanks~~~~~

The usual values for L and C are chosen to give acceptable quality factor (low loss) at the working frequency ( series resistance << reactance << isolation resistance). This means reactance between tens of Ω and hundreds of Ω.

If you want to design a radio for brodcasting, you will use a standard variable capacitor, maybe taken from another radio. This will have some sections in tthe range:
- tens of pF - 200...500pF for LW, MW, low HF;
- few pf - 20pF for FM band;
- 10pf-100pF for HF, rarely.

After you choose a variable capacitor value according to band, you can determine the L value.
L>>C in all cases (L~µH,nH , C~pF).

### ZengLei

Points: 2
the L and C should be in a series resonant circuit in series from the antenna or should be in an parallel resonant circuit in parallel between the signal path and ground...

### ZengLei

Points: 2
Often L is way greater than C because L has Henry as Unit and C has Farrad as a unit.
e.g. for about 400MHz L will be in nH and C will be in pF.

The ratio between L and C defines the impedance of LC circuit, which affects the matching of the filter. So , it depends on the rest of the circuit, which value you need. The impedance of this filter is defined by: Z = sqrt( L / C ). So the bigger the inductance, and smaller the capacitance, the bigger the impedance of the filter.

Talking about L>>C has no sense. In resonance the reactances are equal.
ωL=1/(ωC)
Quality factor or bandwidth of tank determines values of these reactances.

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