Very simple fm radio variable capacitor substitute

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
I want to make a very simple fm radio, but have run into a bit of a problem. I have no variable capacitor. Any ideas on an alternative circuit for a variable capacitor? Thanks. :)
 

ian field

Joined Oct 27, 2012
6,539
You beat me to it - there are off the shelf varicaps dimensioned for band II tuning - its possible to get away with common or garden silicon diodes if you can be bothered fiddling about for ones that give the required range.

Years ago; one of the magazines published an AM/MW project that used a parallel pair of TO220 style Shottky barrier rectifiers as varicaps.
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
Also the page about leds as varicaps didnt really explain how to set up and use an LED as a varicap. Just that it worked. Is there a page that has a tutorial? It would be very convenient to use LEDs instead of veriable capacitors or varicaps because I have tons of LEDs.
 

bertus

Joined Apr 5, 2008
20,180
Hello,

When you would post a schematic of the reciever you want to make, we would likely be able to add the varicap in the circuit.

Bertus
 

ian field

Joined Oct 27, 2012
6,539
Also the page about leds as varicaps didnt really explain how to set up and use an LED as a varicap. Just that it worked. Is there a page that has a tutorial? It would be very convenient to use LEDs instead of veriable capacitors or varicaps because I have tons of LEDs.
Years ago I made an oscillator with a 10.7MHz ceramic resonator & an FM IFT - it didn't seem to be doing much, so I strapped an inverse parallel pair of red LEDs across the secondary winding. It blotted out FM reception in my flat and couldn't be tuned out because it was jamming up the IF amp.

I enquired on various forums, but no one could throw any light on what was going on.
 

ian field

Joined Oct 27, 2012
6,539
Here's the schematic.
It would be tricky to bias a varicap - you'd have to stabilise the decoupled collector rail.

If the decoupled rail is "tight" enough - you might be able to get away with grounding one end of the varicap.

The inductor will obviously shunt any DC for a varicap, you have to DC block it with a series capacitor big enough to have little effect on the total capacitance. The varicap is used reversed biased, so there's only a tiny leakage current - you can feed it with a high resistance that doesn't clobber Q very much.

Pedantic theorists will tell you that the RF which is an AC waveform, will sum with the varicap bias and degrade tuning accuracy - millions of TV tuners don't worry about it and work just fine. High end FM tuners sometimes have twin or double ended varicaps which cancel out the AC offset.
 

ian field

Joined Oct 27, 2012
6,539
What if i used a potentiometer to increse or decrease the current coming from a non variable capacitor?
Voltage variable capacitance in fixed capacitors is an undesirable property that manufacturers do anything they can to avoid.

Some types of ceramic capacitor are particularly bad in this respect - but unlikely to serve your purpose.

A seriously huge ceramic and plenty of variable bias might do - you'd have to put it in series with the highest capacitance that matches your inductor/tuning range, but I doubt the results would be worth having.
 
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