FM Transmitter: Where's the problem?

[DigiTan]

I tried building this FM voice transmitter yesterday and so far, I haven't gotten it to work at all. It's intended to work for the 88MHz-108MHz commercial radio band.

After my circuit failed the second time, I was suspicious of the hand-wound inductor in the tuning net with C5 and L2. I went to Fry's and replaced L2 with a 0.1uH coil that would supposedly target this band. This was able to create noise at certain frequencies but not a voice signal.

I also tested Q1 and Q2; and I tried two different electret mics. When I rebuild the circuit the third time, I played loud music into the microphone and watched the voltages across C4 and C3 with a voltmeter. I could not see any change in voltage; but there was an AC signal on the antenna.

Does anyone have advice for the type of circuit; or has anyone had trouble recreating this one in particular? If the tuning circuit is not at fault, where should I check next?

 

[Nigel Goodwin]

Have you built in on the PCB given?.

 

[David Bridgen]

The first thing to do is make sure it is oscillating in the correct frequency band. Forget the modulation until you have established that.

If it is within the broadcast band you should be able to "hear" it on you f.m. receiver which will go quiet when you tune it to the transmitter's frequency.

Your value of 0.1uH for L2, in combination with C5, doesn't calculate correctly.

0.3uH is more like it.

 

[audioguru]

The circuit is designed wrong like most circuits on that site.
The audio transistor is almost saturated all the time, especially if its gain is high. Use 680K for R4.

 

[DigiTan]

The circuit I built so far used a prototyping breadboard. I was hoping that the adjustable capacitor might let me comphensate for some of the breadboard capacitances.

0.1uH does seem too low for resonance. I have to admit the reason I went with 0.1 was because a similar circuit on a similar site used that value and I assumed I had miscalculated. (Shame on me)

I'll try substituting L2 and R4 like you said. If that doesn't work I'll just rip it up and try it in PSpice.

 

[Vaibhav Birit]

THe second stage i.e. the oscillator stage is proper. It will work properly, no doubt about that. You can retain your old 4-5 turn coils.

However the first stage i.e. audio amplifier stage is close to saturation, if you can plot the Ic-Vce characteristics, you will come to know. Hence you will get clipping in your audio.

But the biggest Mistake in the given circuit is that the FIRST STAGE is unnecessary. Since you are feeding your audio to the base of the second stage you really should not amplify it. By amplifying it, you are really going out of the active region on both sides. I would recommend you to feed the output coming from C4 directly to C3 input, eliminating the first stage. Also an RFC(5-6 turns) can be included between C3 & C4.

One more possible flaw is the antenna. The length of the antenna at these frequencies is highly unforgiving. one cm short or long & you should not expect anything from it. Just check your antenna length which according to me should be around the length of a half pencil.

 

[David Bridgen]

0.1uH does seem too low for resonance. I have to admit the reason I went with 0.1 was because a similar circuit on a similar site used that value and I assumed I had miscalculated. (Shame on me)

I'll try substituting L2 and R4 like you said.

10 turns of approx 0.5mm dia (not critical) wire, wound with an internal diameter of 5mm to occupy 10mm length should be about right.

 

[chemelec]

You are Far better off using the Hand Wound coil. Squeeze or Stretch it to do a Coarse Frequency adjust. Than the Trim cap for Fine Tuning.

I don't find the antenna Lenght that Critical, but a 10 pF cap in series with it, is a good idea. and an antenna about 6 to 8 inches in lenght is a good start.

Gary

 

[DigiTan]

Sounds like a plan to me.

Also, I was wondering if there is any quick way to test the electret microphones using only a volt/current meter. If I connected it to the supply and placed the volt meter across the leads, should there be noticeable voltage changes if noise is present; or is there more to it than that? I'm aiming to use whichever mic is most sensitive.

 

[audioguru]

Hi DigiTan,
Somebody else on this site had an FM transmitter circuit that didn't work. I wanted to help fix it but I don't have a good sim program. Since I knew that the capacitance between tracks on a breadboard are maybe 10 times higher than the circuit's lowest value cap, and that the "audio" transistor that is used as the 100MHz carrier oscillator hardly has any gain at such a high frequency, I built the circuit on a Veroboard.
I planned the layout and connections to be close and tight. I cut all extra track length. I used proper supply decoupling caps unlike your circuit, wound my own coil using heavy enamelled wire from an old transformer, and it worked!
Sort of.
It was nearly identical to your circuit and had all the problems of the person who complained about it:
1) No audio. Very similar to your project, with a new 9V battery, its audio preamp transistor was saturated and therefore didn't amplify and didn't even pass the mic's signal.
2) So I lowered the supply voltage and it worked for a moment but the audio preamp transistor became cutoff with a supply voltage of about 7.5V or less. When I touched the audio preamp transistor with my finger to warm it, it worked for a few seconds until it cooled.
3) With a supply voltage of about 8.2V the circuit would transmit my voice to my FM radio but was very distorted and muffled without any treble. When I touched the audio preamp transistor with my finger to warm it, it became saturated.
A wonderful circuit!

I re-designed the audio preamp, biasing it properly and using DC negative feedback to keep it immune from temp changes. I added AC negative feedback to reduce its distortion and added pre-emphasis so that my normal radio produces treble sounds from it. I also added a low-dropout voltage regulator so that it works when the 9V battery's voltage runs down to 5.3V.
It sounded great!

But it had more problems:
1) It changed its RF frequency a lot if I or anything conductive came close (10cm) to its antenna.
2) It changed its RF frequency a lot as the battery's voltage ran down. Connecting its RF oscillator to the voltage regulator easily fixed that.
3) It changed its RF frequency a lot if its temp changed.
4) It had a short range of only about 30m without walls or anything in its path.

So I added a tuned RF buffer stage to its RF oscillator to keep its antenna away from its oscillator's tuned circuit, which also improved its range enormously. I mounted it in a metal box and it works great!

Prime quality (not factory rejects, I don't know why people buy the cheap thingys) but inexpensive 2-wire electret mics are all about the same sensitivity. I have tried "normal-size" electret mics from toys and a tiny one from a cell phone with my transmitter and they sound the same. Since they have a FET transistor in them at their output they work a little when connected with backwards polarity but not very well.
They require only 0.1mA to 1mA so the value of its load/powering resistor (R6) in your circuit is low with a 9V supply. I always use 10K with them and measure about 4.3VDC across them. They produce an AC output level of about 10mV if you shout and eat them, which you probably can't measure with a multimeter. Since they taste bad they need a preamp. :lol:

 

[DigiTan]

While we're on the subject of mini-transmitters, does this **broken link removed** look legit? I haven't attempted it yet, but I'm suspicous of how the supply is connected to the 555 control pin and not the Vcc like normal.

Also, I was thinking of adding a calibration feature to the original FM Transmitter. Basically, I'd like to use another oscillator like a 555 to send an aubile tone to the radio instead of voice. That way, I wouldn't have a talk into the mic continiously to know if it's working, I'd just hunt down the tone. Voice/Tone mode would be controlled by a slide switch.

I'm pretty certain I'd have to send the audio tone to the base of Q1, but beyond that I'm unsure.

 

[audioguru]

Hi Digitan,
It's not a 555, it's an obsolete LM3909 low voltage LED flasher IC. It was unique because it used a charge pump as part of its oscillator to double the supply voltage.
A 555 won't work on only a 1.5V supply, which drops to 1.0V over the life of the battery cell. The LED won't work either.

I don't see how the RF oscillator can oscillate with its emitter grounded. Usually it has an emitter resistor so that it can receive positive feedback from its collector.

 

[DigiTan]

Hi Digitan,
It's not a 555, it's an obsolete LM3909 low voltage LED flasher IC.

Whoops! Looks like it's time for me that trip to the optomitrist again. :lol: Well, if it's a LM3909, a guess I'm not interested then. None of the shops have them anymore and from the sound of the shopkeep, they were discontinued many years ago.

So, I guess to sum up all the posts so far, this schematic is garbage and I need to overhaul the entire thing--starting with the tuning stage and then the biased amps?

 

[Vaibhav Birit]

I have made a single transistor (BF494) FM transmitter. It is crystal Controlled & hence extremely stable. It gives commercial quality sound without any distortion. Are you interested in the schematic as well as the pcb.

 

[Nigel Goodwin]

I have made a single transistor (BF494) FM transmitter. It is crystal Controlled & hence extremely stable. It gives commercial quality sound without any distortion. Are you interested in the schematic as well as the pcb.

Why don't you post it in the 'Electronics Projects' section, I'd certainly be interested to see how you can get wideband FM from a single transistor FM transmitter?.

 

[DigiTan]

So how do you guys calculate the operating frequency of this oscillator? I've got the resonant frequency as being w = (1 / L*C)^.5, but that result is a far from what the schematic and posts suggest. Those this formula not apply to this situation?

 

[audioguru]

Did you subtract the substantial amount of stray capacitance in the transistor and circuit board wiring from the capacitor in your calulation?

 

[Nigel Goodwin]

So how do you guys calculate the operating frequency of this oscillator? I've got the resonant frequency as being w = (1 / L*C)^.5, but that result is a far from what the schematic and posts suggest. Those this formula not apply to this situation?

The formula applies to EVERY situation, but your problem is finding the values for L and C. Stray capacitance and inductance will totally obscure the original values of L and C - it's best (only?) found by experiment and measurement.

 

[DigiTan]

I'm determined to figure out what's wrong with this rascal.

Assuming there are problems with both the 'audio amp' and 'RF osc' parts of that circuit, is it possible to remove the audio part to try and focus on generating a steady carrier using only the right part of the system?

From the looks of things, I would have to remove everything to the left of C1 (except the battery). Is this accurate?

 

[audioguru]

Of course the RF oscillator won't work without R2 and R3.
The wires on the circuit board must be very short (no prototyping breadboard).
A 0.01uF ceramic disc capacitor should be across the battery terminals on the circuit board.