Diode Quest

[Nigel Goodwin]

How do I calculate the capacitor's inductance?

You don't, there's no reason to want to?.

wait a sec... you say that a few uF's make too much inductance. Why do you suggest adding 100uF to 1000uF?

As Audioguru said, you want two capacitors across the supply, a decent size electrolytic (which will decouple low frequencies, and greatly improve battery life), and a much smaller capacitor (like a 1000pF) to decouple at high frequencies.

 

[DigiTan]

Would the placement of the larger audio bypass caps be as critical as the placement for the RF caps?

 

[mstechca]

Me: How do I calculate the capacitor's inductance?
Nigel: You don't, there's no reason to want to?

This is what audioguru said:

A few uF's cap is useless at these high RF frequencies due to its inductance.

I want to avoid RF shorts across the supply.

and if I use two capacitors across the supply, don't they add up to a bigger value and create a lower reactance, so low that a short eventually occurs?

Just think about it: 1/(2*pi*F*(C1+C2)), where C1 and C2 are the capacitors across the supply, F is the frequency (I'm going over 100Mhz), and pi = 3.14.... (whatever the actual value for PI is).

you know what, just tell me the best capacitor value to use and why.

and one more thing... I'm gonna cheat :twisted:
I'm gonna take the coupling capacitor connected to the new varicap circuit and make it a variable one. Also, I'm going to add another variable one with my 0.1uH inductor connected to the NPN's emitter.

now I shall see that my troubleshooting is limited to fiddling with two trimmers.

 

[audioguru]

I want to avoid RF shorts across the supply.

Why? Then transistors' collector load resistors would also include the resistance of the battery and there might also be feedback through the supply. All circuits must have a low impedance supply bypass.

and if I use two capacitors across the supply, don't they add up to a bigger value and create a lower reactance, so low that a short eventually occurs?

The small capacitor is a low impedance at RF frequencies and the large one at audio frequencies. The large cap isn't even a cap at RF frequencies, it is a high impedance inductor, look at the way it is made: around and around just like a coil.
The supply bypass caps keep the supply voltage constant so the voltage doesn't flop around with the signal.

you know what, just tell me the best capacitor value to use and why.

Again and again: use a 1000pF ceramic disc cap with very short wires for 80MHz to 300MHz circuits and 100uF to 1000uF for audio circuits. A radio operates at both frequencies so needs both caps.

I'm gonna take the coupling capacitor connected to the new varicap circuit and make it a variable one. Also, I'm going to add another variable one with my 0.1uH inductor connected to the NPN's emitter.

now I shall see that my troubleshooting is limited to fiddling with two trimmers.

A trimmer cap across the coil is a good idea to get the total capacitance near what is required. A series trimmer cap will allow you to adjust the tuning effect of each step of your D to A. They affect each other, have fun fiddling.

Earlier you commented that you need a really fast counter. Why? Its speed is the frequency of its clock, which is just a pushbutton pushed at only about 4Hz max. I don't think you can push the button a few million times per second but its bouncing contacts probably will.

 

[mstechca]

I guess then I'll have to use those caps.

but I will eventually need high-speed IC's if later on, I decide to interface my radio to the PC, and have PC rapidly switch stations.

maybe this can start a packet radio modem. :wink:

 

[Nigel Goodwin]

but I will eventually need high-speed IC's if later on, I decide to interface my radio to the PC, and have PC rapidly switch stations.

No matter how fast you switch stations, it's still going to be extremely slowly (for it to work) - you won't EVER need high speed chips, clockwork would be more than fast enough!.

 

[audioguru]

but I will eventually need high-speed IC's if later on, I decide to interface my radio to the PC, and have PC rapidly switch stations.

Spread spectrum technology?

 

[mstechca]

No matter how fast you switch stations, it's still going to be extremely slowly (for it to work) - you won't EVER need high speed chips, clockwork would be more than fast enough!.

I'm not going to get too carried away with speed.

Spread spectrum technology?

why not?

If I make a modem, I could hook it up to a microcontroller, maybe a 555 timer too, and have them detect what station is a valid packet radio station. and if I have to have the circuit automatically start at the lowest legal transmitting frequency, I will. :wink:

Its a heck of a lot better than continuously using trim capacitors, because as soon as I touch the trim capacitor with a screw driver, the frequency changes. It must be that my screwdriver is an antenna with a fake 1pF capacitor in series with it.

 

[audioguru]

Its a heck of a lot better than continuously using trim capacitors, because as soon as I touch the trim capacitor with a screw driver, the frequency changes. It must be that my screwdriver is an antenna with a fake 1pF capacitor in series with it.

You must use a plastic scewdriver to adjust low-value trimmer capacitors. Metal screwdivers have the same extra stray capacitance problem as wiring on a breadboard.

 

[Styx]

sometimes I am glad the highest freq I work with is 30kHz