I was looking at the website for my counter:

http://www.fairchildsemi.com/pf/CD/CD4024BC.html

The datasheet states that each input pin has a capacitance of 5 to 7.5pF.
Is it possible that I can some how makee each input pin have no capacitance?

I didn't realize this when I was making my digital station changer.

I knew something was going on...

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No it is Not Possible.

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nope not posible.

but 5 pF is wery litle capacitance

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MStechca,
The only input pin you were using was its Clock input which was connected to a switch, not the frequency-determining parts.
Your frequency-selection capacitors were connected to its outputs, which are shorted to either the positive supply for a logic high (which is decoupled to ground) or to ground for a logic low, so the capacitors didn't do anything. The counter wasn't switching the capacitors in and out!
Since your counter was at a distance from the tuning coil, the connecting wires had capacitance between them and to ground and served as an antenna.

Why don't you tune your transmitter with a reverse-biased varactor (vari-cap) diode? As I showed, you don't have to buy a special one, any reverse-biased diode or transistor junction will work. It can be connected directly to the tuning coil with a coupling capacitor with very short wires and its capacitance is changed by changing its voltage, with a resistive ladder connected to the outputs of your counter if you want. The counter and resistive ladder perform as a digital-to-analog converter. The varactor diode is reverse-biased so draws no current and its capacitance can be isolated from the resistive ladder with a high-value series resistor.
You can even modulate the varactor diode's voltage with the output of a preamp for the transmitter to have FM.

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Uncle $crooge

 

With the restriction that, if you want to tune the entire FM band, the absolute minimum capacitance tuning ratio must be

This goes up when you consider device and stray (wiring) capacitance.

Also, we need to consider that the tank voltage modulates the capacitance. For high-level oscillations like we see on Mstecha's transmitter, this creates even harmonic distortion, and possibly forward-biases the diode on a portion of each cycle, both of which are, I think, undesirable. That's why I pointed him (in another thread) to Harry's Homebrew Pages, which has a transmitter that overcomes these problems.

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Ron

 

Good points, Ron. :lol:
I remember that a dual varactor cancels 2nd harmonic distortion somehow.

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Uncle $crooge

 

Yeah, I remember playing with that years ago. I think you still have the problem of potentially forward biasing the diode on signal peaks. In order to get good tuning range from a varactor, you need to operate from a bias voltage as low as 1 or 2 volts. If the AC swing on your tank is 4V p-p or more, that can be a problem. You can use a small-valued series cap and a large varactor, which tends to attenuate the AC, but then you need a varactor with a bigger tuning ratio. I believe varactors are easier to use in receivers, where amplitudes can be low, or in transmitters which have a low-level oscillator followed by a power amplifier stage, such as Harry Lythall's.

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Ron