can I Replace Variable inductor in Fm Transmitter with varicap diode + pot?

Allex

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I've build this fm transmitter that uses a var coil to tune the freq. I would like to be able to tune via a pot.
Is this possible?

This is the transmitter:
http://www.eleccircuit.com/ba1404-hi-fi-stereo-fm-transmitter/

and this is a replacement i found for a variable capacitor, would this work?:
http://electronics-diy.com/varicaps.php

I'm fairly new to oscillators as well as electronics in general.

Thnx!

 

abbey road d enfer

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It should work, but there are two potential issues.
First, you need a higher voltage (8-20V) than the normal operating voltage of the transmitter (1.5-3V).
Second, the frequency stability; varicaps vary with voltage, so you have to make sure the tuning voltage is stabilized, but vary also with temperature and the alignment of the Moon with Pluto  ;D.
You may need to a fine-tuning adjustment for drift compensation.
 

Allex

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Thanks for the reply.

I'm planning on using batteries, 2x 1,5v for the transmitter and another 9v for a small antenna amp so i could power the varicap diodes with 9v.
What capacitance would I need for a frequency range between 85 and 108 MHz?

 

abbey road d enfer

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Allex said:
Thanks for the reply.

I'm planning on using batteries, 2x 1,5v for the transmitter and another 9v for a small antenna amp so i could power the varicap diodes with 9v.
What capacitance would I need for a frequency range between 85 and 108 MHz?
The real question is: how much variation is needed to cover the range. Not much; the effective capacitance variation range needed is about 1:1.6. The varicap is never directly across the inductor; there's always a cap in series with the varicap and one in parallels with the inductor. They reduce the actual variation range of the varicap.
With 9V, you get a usable range of 1:2.5 (10-12 pF to 25-30pF) for a standard BB105.
You may use the existing inductor for experimentation but it appears it will need to be changed anyway. It looks like it wants to see about 10-20pF. The varicap + fixed cap would be more like 20-40pF. You may unwind a few turns to tune it.
 

Allex

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I think i'm missing something here, so the varicap will not replace the vari coil?
and where do i need the 10 to 20 pF? at pin 9 and 10 or at both sides of the vari coil?

I've got a few BB409's so i could place 100pF in series with the varicap output to give me a range of +-12 to +-20 pF (9v to 3v)
If i don't add the 100pF I would have a variation of 1:1,6 across pin 9 and 10 is that what I'm looking for?

If you could tell me how this IC establishes the frequency I would really appreciate it.
 

abbey road d enfer

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Allex said:
I think i'm missing something here, so the varicap will not replace the vari coil?
No, the varicap replaces or rather complements the fixed cap that's across the inductor.
and where do i need the 10 to 20 pF? at pin 9 and 10 or at both sides of the vari coil?
In principle it's across the inductor; but since one side of the inductor is AC-wise grounded (DC-wise it's at V+), the varicap should be connected one side to pin 10 via a suitable cap) and the other to ground.
I've got a few BB409's so i could place 100pF in series with the varicap output to give me a range of +-12 to +-20 pF (9v to 3v) If i don't add the 100pF I would have a variation of 1:1,6 across pin 9 and 10 is that what I'm looking for?

If you could tell me how this IC establishes the frequency I would really appreciate it.
the frequency is governed by the formula LC (omega)² = 1 where omega = 2pi.F.
Now I don't know about the internal structure of the IC, so I don't know the value of the stray capacitances and the influence of the additional caps (between 10 & 9 and between 9 & 8) but the actual value of C is larger than the cap that's across the inductor.
 

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