understanding radio tuner circuit and plotting frequency response

[Nirlup]

Hello everyone,

I have been trying to understand how the LC tank circuit in an FM radio works (for example in this circuit). I assumed it works as a band-pass filter, but when I googled around for band-pass filters I mostly found a combination of high-pass and low-pass filters that didn't look at all like the LC combination in radio circuits.

Assuming the LC tuner circuit does work as a band-pass filter, is it possible to pull out the LC tuner circuit and what would the circuit look like? I did find this "AM tuner circuit" but I don't see how that accomplishes anything. Won't V_in = V_out because everything is parallel?

So my question is: assuming that the LC tank circuit works as a band-pass filter, can I create a schematic with just the band-pass filter in it so that I can analyze it? My current goal is plotting the frequency response, which is why I want a simpler (passive & linear) circuit to analyze.

[danadak]

To analyze a tank circuit you either stimulate with a current source or a V
source with its non ideal internal impedance. The latter is more exact for
"normal" looking solutions. Also takes care of the problem Vin = Vout
because the internal V source Z presents a  non absolute V driver to the
parallel network.

In fact when you spice the circuit put in non ideal L's and C's to get real
results.

Essentially the tank eliminates unwanted signal into the amp stage helping
eliminate overload and distortion. Crucial if you have a lot of adjacent channel
interference due to a high power station overwhelming the input stage in
receiver.

Regards, Dana.

[albert22]

I agree with you that vin=vout as shown in that circuit. I dont know what they wanted to point out in that page.
An intuitively and very simplistic way to see the parallel LC (tank circuit) is that it is a short circuit for every frequency that is not the resonant frequency. At the resonant frequency you get an open circuit.
The series LC is the inverse short circuit at resonance and open for every other frequency.

[orolo]

The LC filter is the combination of a low-pass filter (the inductor) and a high-pass (the capacitor). It is the simplest (least number of poles) band-pass filter you can find, and the response is correspondingly simple. The more complex band-pass filters contain more elements but have a better response, like a wider passband and/or steeper roll at the stopband.

Consider the FM tuner, and imagine the two extreme casses:

1) Well away from the resonant frequency, you can assume the LC is a short. Therefore, the two transistors act like parallel diodes, and give no amplification. Only if you have a very close AM station you will catch some signal.

2) Right into resonance, the LC acts like an open circuit. The two transistors become a Darlington pair, giving a lot of gain. Also, the base of T2 acts as a rectifier. You can expect a strong signal at the input of the 386. The frequency modulation will take the signal somewhat away from resonance, modultaing the amplitude of the sound generated.

About the AM example, they are only concerned about the resonant frequency. I think it's assumed, but not stated, that Vin and Vout have input and output impedances (in series), and the filter is placed in between. The input and output impedances, as long as they are purely real (resistances) do not affect the frequency of resonance, but the quality factor of the oscillator filter. So if you are only concerned about computing the resonant frequency, these resistances can be added to the parallel resistance R. So for a simple computation of the resonant frequency, the circuit provided is enough.

[Nirlup]

Thanks for the responses everyone. I think I now have a better idea of how the tank circuit works and with orolo's explanation of the fm tuner circuit I have also gotten pointers on what to google to further understand it.

I've created some

for the following circuit:


If I would attach just a whip antenna to this circuit and measure the output with an oscope, could I expect to read a carrier wave (assuming it's tuned to a used fm radio frequency) or would the signal be far too weak without any kind of amplification?

My scope channels mention that the load impedance of the scope is 13 pF and 1M?. Am I correct to assume that the capacitance would have quite an effect on the tank circuit's resonance frequency given that the capacitor in the tank circuit will be the same order of magnitude?