Pls correct me what is wrong with this FM transmitter

[thirumalai_pm]

Hi.

I am a new to electronics. I am learning by getting some electronics kit from shop and working with that. I got two kits, both are not working for me.

Here I am attaching the FM transmitter I was working, I could not get the audio in my FM receiver. Could you pls check the circuit and let me know What could be the problem.

Ward Regards,
Thirumalai M

 

[audioguru]

The BEL548 transistor is very old and is available only in India. So I could not find its datasheet.
If the transistor is similar to an ordinary and common BC548 then the transmitter should work but it is so simple that is frequency will vary all over the place and the transmitted sounds will be muffled.

I made a similar FM transmitter 47 years ago using an American transistor.

 

[thirumalai_pm]

Hi,

Pls let me know what is TRM. I seen in the kit mentioned that, by adjusting the variable capacitor and spacing between coil turns frequency can be set within 88 - 108 MHZ. But even I have not adjust any of these two, I could be able to get the RF at FM radio with in the range?

I doubt that, in the coil should i required to remove the brown color cote? Is it true. also pls let me know tuning the radio frequency near by the transmitter will not an issue.

Thanks. Thirumalai M

 

[Gary B]

The adjustment of the coil is done with a tool that can be home made out of a small stick of wood by tapering one end. While the system is operating, use the tool to pry one turn of the coil away from the others. This will lower the inductance of the coil and change the operating frequency. The easiest way to do this is with a spectrum analyzer so you can see where you are moving the operating point as you do it.

TRM would be a trimmer capacitor and I did not see it on your drawing, but it would normally be in parallel with a larger cap and be used to adjust the frequency instead of bending the coil.

 

[transistor495]

Hi,

I'd assume you've got the pcb and all components with the specified FM transmitter kit. TRM is the TRI in the circuit, that is a TRIMMER. Look on to the picture and search whether you've that in the box. It has a total of 3 terminals out of which two(just opposite to each other) are shortened as one terminal and the middle one as the second terminal for using as the variable capacitor. Look on to the TRI in the circuit and examine it with the actual component.

Inductance L is 4 turns of 22SWG enamell coated wire(insulated). You've to scratch and remove the brown coating on each end of the coil before inserting and soldering. Finally adjust the TRIMMER(TRI on circuit) and spacing between the 4 turns of coil L1 to get in to the range.

Trimmer cap(variable).

 

[mbarazeen]

you are telling you got the RF on FM band but only audio is not comming. are you sure its from your circuit? just disconnect power and check. if its from your circuit then there may be a problem in your condensor microphone. try to feed some audio from a tape player with a preset to adjust the level.

single transistor modulators never shows such behavior to dump the audio. you still can remove the microphone and touch by your finger and realise wheather you get any audio or not.

if it doesnt work there are many other circuits even with two transsisotrs you can try that.

if you have BC548 its good, or use C1975 or C930 or C9014.

 

[thirumalai_pm]

Thanks everyone.

As transistor495 told, I have to remove the brown coating and then test. I will try today and come back.

@mbarazeen : I was not getting any RF on FM band, but compare to normal I was getting lots of distrubance while tuning.

If I get that to work, I will get confidence and will go for another one.

Many Thanks. Thirumalai M

 

[colin55]

There is only one fault with the circuit. The 47E should be 470 ohms. I would make the 4 turn coil 5 turns 3mm diameter.

 

[thirumalai_pm]

Hi,

Thanks for your guidence. I was testing the circuit with breadboard. But it was not working, then I thought I should try with the PCB which comes with the kit. I soldered all the component and when I was testing it, I could here some sound (like gigigigigigigigigigigig) at one frequency (94.2) which does not have any voice (I played some songs infront of MIC). After I disconnected power from the transmitter also I could here the sound (?). But whenever I was touching the antenna, I could see somechanges in the sound.

What could be the issue? As colin55 I have not replaced the 47E with 470 ohms as I does not have it. I can anyway change the same in this week.

Further I wish to know, as the capacitors will be damaged by soldering, how do I know a capacitor is damaged or not? Is there a way to test it after soldering/before soldering.

Thanks Thirumalai M

 

[colin55]

You cannot test the capacitors. Put in 470R.

 

[audioguru]

The extremely simple circuit will not work when made on a breadboard because the breadboard has too much capacitance between its tracks and wires. Also the tracks and wires have too much inductance. A breadboard might work if the frequency is less than 100kHz, not at 100MHz which is 1000 times higher.

It might not work with an emitter resistor value as low as 47 ohms. Most FM transmitter circuits use 220 ohms or 470 ohms.

Of course the frequency changes if something moves toward or away from the antenna or if you touch the antenna because the antenna is connected directly to the tuned circuit where a few pF changes the frequency a lot. Since the circuit does not have a voltage regulator then the frequency will also change as the battery voltage runs down.

A solder joint takes only 1 second if the soldering iron is clean and the solder has a rosin flux core. A soldering iron that gets too hot is never clean and burns away the flux before it works, so soldering takes a longer time and is poor.
Ceramic capacitors are made from something like glass so soldering does not harm them.

 

[thirumalai_pm]

Thanks Audioguru,

Actually I tried with 560R as I dont have 470R resister. But I can try with 220R today. I will buy this weekend 470 and try that also if not working.

I am not sure what is the issue after soldering with PCB, I could not here the sound yet. Can you please let me know I am not hearing the voice because the poor circuit quality or issue wth PCB/soldering/component.

I will try after replacing 47E to 220E/470E and will come back if any issues.

Thanks, Thirumalai M

 

[colin55]

The audio is not coming through because either the microphone, transistor audio amp or the coupling between the amplifier and the oscillator is not working.
I have sold 30,000 simple FM transmitters and they all work first go, so they are very reliable.
To make sure the carrier is working, switch on the transmitter and locate where it is transmitting on the FM dial 88MHz to 108MHz. Then switch it off to hear the hiss in the radio. If this does not occur, the RF oscillator is not working.

 

[thirumalai_pm]

Hi colin55,

You sold 30,000 FM transmitter!!! really great.

I am very new to electronics.. Just reading the concept and circuit, getting hand on experience. Can you please give me a FM transmitter which having good quality output. I will work on it.

Many Thanks,
Thirumalai M

 

[colin55]

I have 15 different FM transmitter circuits
Here are a few:
THE WASP Page1
Code Puzzle
Voyager MkII Page 1

 

[audioguru]

On some of these FM transmitters:
1) The antenna is connected directly to the oscillator's LC tuned circuit so the radio frequency will change when something moves toward or moves away from the antenna.
2) The circuits do not have a voltage regulator so the radio frequency will also change as the battery voltage runs down.
3) The transmitters do not have pre-emphasis (treble frequencies boost) which all FM radio stations have so the sounds heard on an FM radio (they all have de-emphasis (treble frequencies cut)) will sound muffled and awful.

My FM transmitter circuit does not have these problems and sounds great.

 

[transistor495]

I'll tell you my way of debugging it.

Swich on your FM radio and place it near to the transmitter anywhere on the dial between 88-108MHz. Crank up the volume.

Connect your transmitter to the battery and remove, if you can hear the disturbances -that means your transmitter is oscillating. It can be outside of the band so you can't track it on the dial.

Experiment on the LC components until you reach on the dial.

 

[davidbear]

Mod4

@audioguru - In an earlier post, you said that to make your design accept stereo encoded input that one would need to remove the pre-emphasis and allow for 54 kHz signals. You further said that your transmitter had a cut-off at about 17 kHz and it would require re-design. At that time you said that you worked on it years ago, and that you were not interested in re-designing it (I paraphrase). Since you brought up your design again, I was hoping that maybe you might reconsider. I would like to learn about how to go about doing such a redesign so that perhaps I would not have to ask in the future . I don't think that I want to a micromitter, using the BH1417F chip ($10), I'd rather learn some theory.

From what I can gather, C4 is responsible for the pre-emphasis above 3 kHz by reducing the impedance at the emitter of Q1. I still have no idea what C2 is doing. I am thinking that C5 is responsible for the high frequency cut-off, but I have not figured out how since I really don't understand the RF oscillator. I have read about Clapp and Colpits oscillators, but I have yet to understand them especially in terms of how the audio signal at the base of Q2 modulates them. If you would care to take the time to explain your superior circuit, I would enjoy reading about it. If you would like to point me towards a reference, I would be happy to read it. If you could not be bothered, I'll understand and continue to muddle through.

thanks,

DavidBear

 

[audioguru]

@audioguru - In an earlier post, you said that to make your design accept stereo encoded input that one would need to remove the pre-emphasis and allow for 54 kHz signals. You further said that your transmitter had a cut-off at about 17 kHz and it would require re-design.

Yes, R4 in parallel with R6 and in parallel with the input impedance of Q2 with C5 cause the 17kHz cutoff frequency. If Q1 is replaced by an opamp (with a very low output impedance) then the response can be flat up to the highest stereo subcarrier frequency of 53kHz.

I would like to learn about how to go about doing such a redesign. I'd rather learn some theory.

Simply learn about an FM stereo encoder.

From what I can gather, C4 is responsible for the pre-emphasis above 3 kHz by reducing the impedance at the emitter of Q1.

Yes. A stereo transmitter has two pre-emphasis circuits for each channel separately.

I still have no idea what C2 is doing.

C2 shorts the input of the preamp transistor at radio frequencies so that the radio frequency is not amplified nor rectified by Q1.

I am thinking that C5 is responsible for the high frequency cut-off, but I have not figured out how since I really don't understand the RF oscillator. I have read about Clapp and Colpits oscillators, but I have yet to understand them especially in terms of how the audio signal at the base of Q2 modulates them.

C5 must be about 470pf so it is a very low impedance to ground at radio frequencies so that Q2 is a common-base oscillator (its base is grounded by C5).
R4, R6 and the input impedance of Q2 together with C5 form a lowpass filter. If R4 is replaced by the extremely low output resistance of an opamp then the cutoff frequency can be much higher.

Q2 is a "Colpitts" common-base oscillator. C7 provides positive feedback from the collector to the emitter to keep it oscillating at the tuned frequency of L1 and C6.
The audio signal at the base of Q2 causes it to conduct more and less. The varying conduction causes its collector voltage to increase and decrease (it is AM) and causes the capacitance of the collector-emitter to increase and decrease producing FM. FM radios ignor the AM that is also produced.

 

[davidbear]

@Audioguru - Thanks! You have greatly increased my understanding of your circuit (I think). I still need to work on the "colpitts" common-base oscillator, but there are plenty of resources for that. This is what I have come up with as a possible front end without "hitting the books." I am anticipating 0.2 Vpp 53 kHz input, so a gain of ~19 should leave plenty of head room for the MCP6L72 opamp. The RF amp will be what is in your Mod4 design. Any comments will be appreciated.

DavidBear