FM transmitter

Roff · Oct 26, 2005

mstechca said:
here's the interesting part.

at the point where the emitter inductor and the resistor meet, I hooked up a basic RC common emitter amplifier, and the two transistors are both PN3563's. It seems that I am obtaing better results, but I still need to obtain the perfect pull-up resistor and coupling capacitor.

Not bad, considering that I am literally using a common emitter amplifier to attempt (quench?) frequency modulation. :wink:

I thought this was supposed to be an FM transmitter.

mstechca · Oct 27, 2005

I wanted it to be FM, but hey, if the thing can transmit in any mode, why not?

after all, it works a little better than the 1-transistor FM transmitters out there.

mstechca · Oct 27, 2005

The transmitter works fine with a standard LC tank circuit, but when I replace it with my digital LC circuit (see the 1st circuit in this discussion), I can't seem to make a transmission unless I consistently adjust the positive feedback capacitor, andeven then, the transmission is not as strong.

It makes me think that the digital circuit is adding or removing something.

Can you give me some insight as to what it may be adding or removing to prevent the transmitter from transmitting?

I even tried adding a trim capacitor from collector to VCC inthe oscillator circuit, and that made no difference.

audioguru · Oct 27, 2005

Your tank is tuned to VHF that needs extremely short wires between the tuning cap and inductor to eliminate the inductance of wires longer than about 1" and eliminate the capacitance between the wires or anything nearby.

Your counter doesn't switch the caps in and out, it just connects them to the supply or to ground which are at the same RF potential. The capacitance between the output pins on your counter far exceeds the value of your tuning capacitor.

Why don't you tune the tank with a variable DC voltage feeding a vari-cap or ordinary reversed-biased diode or transistor junction? You could even modulate the vari-cap's DC voltage to produce pure FM without any AM.
The variable DC voltage for the vari-cap could be located anywhere.
Your counter could drive adjustable resistors to produce the variable voltage for the vari-cap and make a sequenced change of RF frequencies like you want.

Roff · Oct 27, 2005

The 4024 has significant output resistance, in either the high or the low state. Try replacing the "digital cap" with a cap and a 500 ohm resistor in series, which is a rough equivalent. Oscillation will probably stop unless you increase the feedback capacitance. The resistor severely impacts the Q of the tank circuit.
We have told you repeatedly that the 4024 outputs do not switch between short circuit and open circuit as they change states. In the low state, they connect to GND through an equivalent resistance of several hundred ohms (an N channel MOSFET). In the high state, they connect to VCC through an equivalent resistance of several hundred ohms (a P channel MOSFET). I hope you understand by now (do you?) that, to AC (RF), VCC and GND are equivalent. The capacitance change of your "digital cap" will be essentially zero as outputs change state. The series resistance will be a little different, because the 4024 does not have identical output resistance in the two output states.

EDIT: Audioguru, I was typing while you were posting.

audioguru · Oct 27, 2005

Ron H said:
EDIT: Audioguru, I was typing while you were posting.

I beat you, Ron. You're too slow! :lol:

Roff · Oct 27, 2005

audioguru said:
Ron H said:

EDIT: Audioguru, I was typing while you were posting.

Click to expand...

I beat you, Ron. You're too slow! :lol:

And getting slower and slower and s l o w e r .

audioguru · Oct 27, 2005

I think I'm getting faster as I get older. Does that mean my days remaining alive are numbered? :?: :?: :?:

Roff · Oct 27, 2005

audioguru said:
I think I'm getting faster as I get older. Does that mean my days remaining alive are numbered? :?: :?: :?:

I don't know if that's what it means, but I'll guarantee you that your days are numbered.
If I'm wrong, you can collect from me when you're still alive and time=infinity. :lol:

audioguru · Oct 27, 2005

I've always been a slim and trim guy and the news says that today's fat kids will not last as long as their slim parents. My slim father lasted a very long time and I'm expecting to last a very long time, but maye not 'til infinity. :lol:

mstechca · Oct 27, 2005

Ron H said:
The 4024 has significant output resistance, in either the high or the low state. Try replacing the "digital cap" with a cap and a 500 ohm resistor in series, which is a rough equivalent.
Oscillation will probably stop unless you increase the feedback capacitance. The resistor severely impacts the Q of the tank circuit.
We have told you repeatedly that the 4024 outputs do not switch between short circuit and open circuit as they change states. In the low state, they connect to GND through an equivalent resistance of several hundred ohms (an N channel MOSFET). In the high state, they connect to VCC through an equivalent resistance of several hundred ohms (a P channel MOSFET). I hope you understand by now (do you?) that, to AC (RF), VCC and GND are equivalent. The capacitance change of your "digital cap" will be essentially zero as outputs change state. The series resistance will be a little different, because the 4024 does not have identical output resistance in the two output states.

EDIT: Audioguru, I was typing while you were posting.

Actually, I got the digital LC circuit working. My breadboard wasn't holding the wires together too well. I know in AC that VCC and Ground are equal.

Would I get much better results if I replaced the cap(acitor)s in my digital circuit with inductors and the inductor with a cap?
That idea is more expensive, but if it provides signifigant improvement, I just might try it.

mstechca · Oct 27, 2005

I almost forgot to ask.

When I hooked up the transmitter to a normal 8 ohm speaker, the circuit appears to work, and I can transmit some sounds to the receiver.

As soon as I replaced the speaker with a speaker extension cable connected to the output of my soundcard, I couldn't get sound. I even jacked up the volume on my PC to maximum and I was trying to play a music CD. If I just plugged in a set of speakers to my computer, I can hear the sound.

why is this so?

It's not like I short-circuited anything.

audioguru · Oct 27, 2005

You don't seem to understand that your digital circuit doesn't switch anything off. You need a circuit that changes the capacitance by switching-off some of it.

You would get repeatable and much better results if you use Veroboard (stripboard) or a pcb instead of a breadboard with its loose connections, high capacitance and high inductance.

mstechca · Oct 27, 2005

I understand.

I thought that each output pin would act as a small resistance to ground, or a small resistance to VCC. (depending on the logic output).
I don't think there is an impedance state at all on the connected outputs.

but then maybe it works well, because the proper outputs (the 1 bits) can form the correct LC tank circuit. and the unwanted outputs (the 0 bits) make the opposite filter which opposes unwanted frequency.

I think I may have to adjust some capacitors.

audioguru · Oct 27, 2005

These capacitors are easy to adjust:

Roff · Oct 29, 2005

mstechca said:
Ron H said:

The 4024 has significant output resistance, in either the high or the low state. Try replacing the "digital cap" with a cap and a 500 <a href="#">ohm</a> resistor in series, which is a rough equivalent.
Oscillation will probably stop unless you increase the feedback capacitance. The resistor severely impacts the Q of the tank circuit.
We have told you repeatedly that the 4024 outputs do not switch between short circuit and open circuit as they change states. In the low state, they connect to GND through an equivalent resistance of several hundred ohms (an N channel MOSFET). In the high state, they connect to VCC through an equivalent resistance of several hundred ohms (a P channel MOSFET). I hope you understand by now (do you?) that, to AC (RF), VCC and GND are equivalent. The capacitance change of your "digital cap" will be essentially zero as outputs change state. The series resistance will be a little different, because the 4024 does not have identical output resistance in the two output states.

EDIT: Audioguru, I was typing while you were posting.

Click to expand...

Actually, I got the digital LC circuit working. My breadboard wasn't holding the wires together too well. I know in AC that VCC and Ground are equal.

Would I get much better results if I replaced the cap(acitor)s in my digital circuit with inductors and the inductor with a cap?
That idea is more expensive, but if it provides signifigant improvement, I just might try it.

I'm guessing you're smokin' the kind of organic cigarettes that make you believe you got the digital LC circuit working. :wink:

mstechca · Oct 29, 2005

I got it working. The only problem now is that the long wire coming from the clock pin happens to be some kind of an antenna. I'm using it in place of the push button sometimes, because I want to ensure that the circuit is working.

Why do I know it is an antenna? because when I hooked it up to the receiver, the receiver produced pulses, and when I cut the wire in half, the frequency has changed!

mstechca · Oct 29, 2005

audioguru said:
These capacitors are easy to adjust:

Ron, do you think I'm still smoking an organic cigarette? I think Audioguru took my cigarette and is now smoking it :lol:

audioguru, those numbers 1N4001 and 1N4148 are diodes, not capacitors. Yes, I understand that diodes can have some capacitance, but diodes function differently than capacitors do.

and I can't see how diodes will make a difference.

Roff · Oct 29, 2005

mstechca said:
audioguru said:

These capacitors are easy to adjust:

Click to expand...

Ron, do you think I'm still smoking an organic cigarette? I think Audioguru took my cigarette and is now smoking it :lol:

audioguru, those numbers 1N4001 and 1N4148 are diodes, not capacitors. Yes, I understand that diodes can have some capacitance, but diodes function differently than capacitors do.

and I can't see how diodes will make a difference.

Do a search on "varactor" and "varicap". Audioguru is spot on. You just have a lot to learn. You would learn a lot faster if you weren't so danged hardheaded.

chemelec · Oct 29, 2005

mstechca: audioguru, those numbers 1N4001 and 1N4148 are diodes, not capacitors. Yes, I understand that diodes can have some capacitance, but diodes function differently than capacitors do.

and I can't see how diodes will make a difference

.

Yes you do have a lot to learn.

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