FM (Decent Transmitter) Question

[DigiTan]

A few months ago I found this schematic for a miniature FM transmitter. I can't find the original thread anymore, but I managed to print the schematic while the topic was still fresh. I built two of these, but when I had trouble with the first, I sent second one to the oscilloscope. Here is the circuit I'm referring to...

**broken link removed**

I'm powering it off a 3V coin battery. When I read the antenna's voltage from the Q1 collector (with respect to the negative battery terminal), I see a damped sinusoid that is 104.17MHz, but only 100mV peak-to-peak. Then it dies out within a half dozen cycles, disappears for 12.8ms, and repeats.

I had to substitute a few parts that we're available to me:
1. Q1 PN3563 was replaced by an NTE108.
2. I substituted the tuning capacitor C2 with one that had a 0.5pF - 8pF range instead of 1pF - 6pF. In case I could tune the entire FM band.
3. Originally, I tried an electric mic. When it didn't work, I read from Nigel or audio that its impedance would be too high. So I now have an 8ohm miniature speaker as the "audio input."


That damped sinusoid seems accurate enough frequency-wise. I got readings ranging from 98MHz to 107MHz with just a partial turn of the tuner; so that's all good. Any idea why the amplitude is only 100mV or why this damped sine only happens every 12.8ms? I have some screenshots from the scope if it will help...

 

[audioguru]

It is a very poor FM transmitter.
It was designed by MStechca who didn't know anything about electronics.
I have corrected a few of its errors so you can see how its frequency drifts all over the place if anything gets near it, and how bad it sounds.

The capacitance of your 'scope's cable de-tuned the frequency to be much lower than without the scope's cable. Make an antenna for your scope so it can measure the tiny signal (it might reach across a street) without de-tuning.

 

[DigiTan]

Since this one de-tunes easily, what's a highly-compact design you'd recommend for the same uses? (The smaller the better)

 

[audioguru]

My FM transmitter uses an RF amplifier to isolate the RF oscillator from the antenna, a low dropout voltage regulator for the RF oscillator and mic preamp to keep their voltage constant as the battery runs down and pre-emphasis (treble boost like FM radio stations have) for excellent sound.
It has a range of over 2km directly to a very sensitive FM tuner.

 

[DigiTan]

So about that antenna... Would you just use a wire loop, or is there more to it than that?

 

[audioguru]

I have never seen a loop antenna for FM or for VHF TV frequencies.
For my FM transmitter I used an 80cm piece of wire hanging straight down from an upstairs window, and my very sensitive car radio has a vertical whip antenna.

 

[DigiTan]

But for the oscilloscope's antenna...how would I build that? Is it just wire?

 

[audioguru]

The antenna for the 'scope is just a piece of wire, about 1m from the antenna of the FM transmitter.

 

[DigiTan]

Right now I'm in the process of ordering components for the 2km design. As far as the corrected version of MStechca's radio, what improvements should I get over my current one? Does the change solve the carrier amplitude and modulation problem I saw?

 

[audioguru]

With the increased value of the trimmer capacitor it should be able to be tuned to the FM band with an antenna at the collector of the transmitter. Keep away from the antenna. Anything near the antenna detunes it. Anything near the circuit board detunes it.

It should transmit FM to across the street.

 

[DigiTan]

Is it also possible to mod the improved decent transmitter to transmit audio from something like a 555 timer or a PIC?

 

[audioguru]

A tranmitter can transmit any kind of sound signal.
The output level from a 555 or from a PIC would need attenuation with two resistors.