capacitance

[mstechca]

I was looking at the website for my counter:

https://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...

 

[chemelec]

No it is Not Possible.

 

[Someone Electro]

nope not posible.

but 5 pF is wery litle capacitance

 

[audioguru]

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.

 

[Roff]

<snip> any reverse-biased diode or transistor junction will work. <snip>

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

(108/88)^2 ≈ 1.5

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.

 

[audioguru]

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

 

[Roff]

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

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.