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audioguru's FM transmitter

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zachtheterrible

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I'm revisiting your FM transmitter audio, I have it built exactly as in the picture, just using the oscillator and amplifier. My ipod is connected directly to the base of Q2. I'm running the whole thing off 9v. I didn't have the exact right values for the capacitors, so you can see the changes in the picture. I've made the circuit with the "ugly" UHF-friendly construction. I think it's called manchester or something like that?

Anyway, some weird things are happening with this circut. If I try to connect any kind of decoupling capacitors- I've tried .1uf, 100uf, and both of them together, the circuit quits transmitting at all.
I can hear it over the radio when there is no decoupling and the sound quality is nice, but the range is pathetic. Only about 30 ft.

It's only pulling about 20mA, which comes out to 180mW, I believe that it's supposed to be around 1W or so?

It should be alright running the whole thing off of 9v right?

Thanks
 

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I wonder what the output power is, probably 10mW is a bit optemistic. Audioguru's transmitter worked so well because it had a voltage regulator to keep the oscillator running at a stable frequency and pre-emphasis. You have removed both so is won't be as stable and it will sound crap too. What are you trying to achieve?
 
I just want long range, he said that his was able to transmit 2 km. The sound is great. It came over my truck stereo very clear. I'd rather have more range though.

I'm running it off of a variable PSU, so the frequency doesn't drift.
 
Hi Zach,
Connect a 330nF coupling capacitor at the base of Q2 for the audio input so the ipod doesn't upset the DC bias.
Use short wires from a 9V alkaline battery to power it.
Use a 100uF electrolytic and a 1000pF ceramic disc supply decoupling capacitors.
Use a vertical antenna about 80cm long.

Mine drew a current of 53mA from the 9V battery and its output transistor drew about 43mA. The output operates a little in class-C so it is about 70% efficient with 13mA for heat and 30mA times 9V= 270mW for RF.
Your oscillator operates from 9V instead of the 5V for mine so maybe the output transistor is over-driven into cutoff causing low power.

I had RF all over my neighbourhood. 2km with nothing in between. Houses blocked the signal.
Another guy made my circuit and it also worked well.
 
I made the changes that you specified audio. For one thing, my antenna was only about 30cm long. Now that it is 80cm, the transmitter seems to be very strong, registering about as strong RF signal as my walkie taklie, that is when it's working. It also transmits a mains buzz over my speakers that I have n the garage for listening to music. It even sends my PSU's digital voltmeter and ammeter berserk.

The only problem is that when I plug it into the ipod, oscillation stops. Then it is nearly impossible to get it oscillating again. The only way is to turn my PSU off and on until I see it oscillating on my o'scope. I dont have the scope connected to the circuit, it's just connected to a germanium diode.

Now this is something very strange- with my germanium diode connected to the scope, sometimes I simply see the voltage present on the diode rise when I turn on the transmitter, the PSU doesn't go beserk at this time. Other times, I can see the actual sine wave produced from the transmitter. This is when everything on the PSU goes beserk.

All in all, it seems that the transmitter is very unstable right now, and transmittingat two different power levels.
 
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Hi Zach,
Maybe the RF from the antenna is getting into the input along the ipod's wires which would cancel the oscillator's signal. I used a low value for the capacitor to ground at the base of Q2 for good pre-emphasis with my very short wiring. Try 2000pF there to filter out the high frequencies better.
 
Can I know what is decoupling capacitors? I know coupling capacitor, bypass capacitor but not decoupling.
 
I call them "supply bypass capacitors". They filter and hold up the supply voltage durning momentary voltage drops. They are a low impedance when a battery might be developing a high internal resistance. All electronic circuits need them.
I use a 100uF electrolytic supply bypass capacitor for audio, a 0.1uF ceramic disc for digital circuits and a 1000pF ceramic disc for VHF. Some of my circuits use all 3 capacitors in parallel.
 
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