Capacitance of variable capacitor for FM radio?

[VacuumTube194]

I would like to know the capcitance range of the variable capacitor in a typical FM radio, in Farads. Also, is there anywhere to buy those variable capacitors (the square white ones you see in the radios).

Finally, is there an electronic meter that can measure capacitance?

Thanks.

[Externet]

Hello.

If you call a "typical FM radio" a 3 metre broadcast FM receiver, I believe the typical tuning capacitor varies from 0.00000000001 Farad to 0.00000000016 Farad.

They usually contain a second variable capacitor in the package for receivers on the 300 metre AM band

The best place that I know of to get one of those variable capacitors is canibalizing an unwanted, discarded radio.

My plain Wavetek 16XL multimeter measures capacitance.

Miguel

[VacuumTube194]

Thanks for the response. I can cannabalize one, but then I don't know the exact capacitance. My project is that I am trying to turn an FM radio into a weather band radio ( in the 160 or so MHz range ). I was hoping to use a second tuning capacitor in either series or parallel to bump up the receiving frequency to that range.

Is that feasible at all? Any suggestions would be welcome.

[Externet]

Done that lots of years ago.
You will find selectivity problems, a tiny movement of the tuning may go trough several channels, making it hard to tune.
Stability suffers too, reception can fade. Sensitivity is poor, extra antenna is convenient.
But can be a good learning experiment.

Check if there is a tiny capacitor in parallel to the variables and remove it or diminish its value. If you do not have a VHF signal generator, it will be trial and error to get to the band you want. Remember any changes you do, to be able to go back to original condition to confirm all is still working.
Miguel

[Robert Reed]

Vac Tube
The major problem you will encounter here is that commercial FM is a very wideband system, Modulation and bandwidth limits of +/- 75 KHz. A fair amount of drift and instability will go unoticed here due to that wide bandwidth. However the weather band is strictly narrow band ( +/- 5KHZ). Now this normal drifting can become a real problem and also audio sensitivity at the discriminator will be quite low as the normal weather band mod here is on the order of 2-3 KHz. Aside from the new local oscillator tuning change, you will also need to change the RF stage tuning to match this new frequency. You really need crystal stability for your local osc. to overcome all the other deficiencies here. Also depending on the distance you are from the WX tower, you may have an overall Rx sensitivity problem with weaker signals. Most of these commecial FM receievers were not designed with real hi sensitivity as Commercial FM puts out a much higher Tx wattage.

[VacuumTube194]

Interesting, I'm thinking the solution might be to design a capacitor circuit that gets me into the 160 range, and design it so one of the variable capacitors will only very slightly change the capacitance, keeping the frequency between 160-161 or so (since the NOAA channels are slightly different per city). It sounds like a long shot as the circuit will have to be very exact.

What is the formula for the frequency? I vaguely remember from an EE college course (I was not an EE major) it was something like
w = 1/Sqrt(RLC).
So I believe I need to reduce the capacitance to increase the frequency?

[Bob Scott]

I vaguely remember from an EE college course (I was not an EE major) it was something like
w = 1/Sqrt(RLC).

w=1/Sqrt(LC) in radians per second

f=1/(2*PI*Sqrt(LC)) in cycles per second

So I believe I need to reduce the capacitance to increase the frequency?

Yes, or add a capacitance in series with the existing one.

Bob

[Chris Smith]

There are soo many radios with different circuits.

I remember one Variable cap at 3 to 43 pf, ....the problem here is I don’t remember what radio it was.

I have stripped about a thousand radios but in one case I remembered a fantastically built precision step geared air gap type capacitor of these values.

It could have been a dual frequency type [AM/FM] or a Greasy 10 [GRC-10] AM military radio.

When experimenting, use multiple band types meaning dont restrict your self to just one band of caps, [PF range] grab several of them and mix them.

[rshayes]

You will probably need to change both the shunt capacitors on the tuned circuits and the coils.

The ratio between the maximum and minimum frequencies of the tuned circuit will be the square root of the capacitance ratio. Your starting point is the FM band. The RF tuned circuit has to tune from 88 MHz to 108 MHz. The frequency ratio is 108/88 = 1.23. The capacitance ratio is 1.23^2 = 1.51. The ratio between the variable part of the capacitance and the fixed capacitance is .51 to 1. The fixed capacitance is the sum of the minimum capacitance of the variable capacitor, any additional shunt capacitors, and the stray capacitance of any active circuit elements. Most of these capacitances will be in the 5 to 20 picofarad range.

The oscillator will be either above or below the signal frequency by 10.7 MHz (the standard IF frequency). If is above, the oscillator will tune from 98.7 MHz to 118.7 MHz. In this case, the frequency ratio is 118.7/98.7 = 1.20. The capacitance ratio will be 1.45.

If you have a means of measuring the frequency of the oscillator, you can add capacitance (possibly a small trimmer) until the frequency ratio is the correct value for the new range of oscillator frequencies. The actual oscillator frequency will be too low. Now, replace the coil with a new coil of lower inductance. This can be adjusted until the frequency of the oscillator covers the correct range.

Once the oscillator is covering the proper frequency range, the RF tuned circuit can be similarly adjusted to the proper range. This can be done by using a parallel trimmer capacitor instead of a fixed capacitor and observing which direction it has to be adjusted to peak an incoming signal. Starting with a coil similar to the new oscillator coil should get you close. Be prepared to do a fair amount of experimenting.

The coils in an FM band receiver are often just 4 or 5 turns of wire in the #20 to #22 range wound about 1/4 inch diameter.These can be adjusted by changing the number of turns, or for fine adjustment, squeezing the turns together or stretching them farther apart.