single transistor FM transmitter

[mdanh2002]

Hi,

I have built the following transmitter

Simple 30 meters range FM transmitter

It works well as-is using an ECM mic. Now I want to to replace the mic with the audio out output from a laptop/DVD player. What changes do I have to make? The quality is good enough and the range is about 20m line of sight (good enough for me, I only want to use it inside my room)

 

[audioguru]

The old FM transmitter is extremely simple so it has poor performance:
1) Its antenna is connected to its tuned circuit so its radio frequency changes if something moves toward or away from the antenna.
2) It doesn't have a voltage regulator so its frequency changes as its battery voltage runs down.
3) It doesn't have pre-emphasis (treble boost) like all FM radio stations have so the de-emphasis (treble cut) in all FM radios will make its reception sound muffled without high audio frequencies.
4) It is mono (like in 1960) not stereo.

You should remove the 5.6k resistor that powers the mic then add a 1k resistor to ground between the 10k resistor and the 0.33uF capacitor. Feed the line level signal to the 10k resistor.

 

[carbonzit]

You know, Audio McDuck, you are a real piece of work.

Someone posts a question about a circuit that they got to work, a one-transistor FM transmitter that runs on one 1.5V cell, mind you, which is kind of a miracle unto itself, and all you can do is poke holes in it. Gosh, it's low-fidelity, it's not stereo, no preemphasis, for chrissakes! What the hell do you expect of a one-transistor 1.5V transmitter? 30-15KHz stereo, 1%THD?

Sheesh.

To the O.P.: sorry, I can't help you. I would if I could. If you wait, other, more helpful folks than Mr. McDuck will be along to give you advice.

 

[audioguru]

CarbonZit,
Please go away! What is wrong with you?

 

[mdanh2002]

Hi audioguru

Thanks for your suggestions which helped me. My circuit works upon first power on. BUT, the transmitted audio is not satisfactory. The audio is not distorted or sound muffled - the quality seems actually quite good. However, low frequency components seem missing. For example, when I played an MP3, I could hear the background music clearly but little of the singer's voice. When I play a MIDI, the transmitted audio seems to be a sactisfactory copy of the original. Female voice tends to be heard but male voice goes missing.

Do you have any ideas how I can improve this? Which component values are critical? Since I do not have the exact values, I use a 27pF instead of 28pF, a 56p instead of 62p, a 0.1uF instead of 0.33uF. I omit the 10n capacitor since it's too far on my board to connect and did not seem to serve any purpose when I built the ECM version of the board.

 

[carbonzit]

I think you're pretty much at the limit of quality you're going to get out of that circuit. As I said, it's a wonder it even works. You really can't expect much more out of a 1.5V-powered circuit where one little transistor does the job of amplifier, oscillator and modulator all at once.

So far as component values go, probably the only two that are all critical are the inductor and the tuning capacitor, which form a resonant circuit to establish the transmitting frequency. The 0.33µF capacitor is absolutely non-critical, as it's only used for coupling.

It is pretty cool that such a simple circuit can work at all ...

 

[mdanh2002]

Ok, I think i figured it out. It's not that I have reached the capability of the circuit. It's because the computer output is in Stereo and the cable alone does not get a satisfactory mono output. So for stereo file that have slightly different audio output between the left/right channel, the transmitted audio will be distorted. I am currently working on a way to sum up the 2 output channels and get a mono output satisfactorily. Using windows volume control to move the audio to one speaker only, and the transmitted audio is good.

The 10k resistor also need to be changed to a higher value, say around 33k. Otherwise the audio output from the computer needs to be kept at a minimum 10% for the transmitter to work.

 

[audioguru]

Increasing the value of the audio coupling capacitor gives better bass. You reduced its value which reduced the bass.
It is easy to mix stereo into mono with two resistors.
Instead of increasing the value of the 10k resistor an attenuating 1k resistor was added.
The 10nF (0.01uF) capacitor is important and should be a ceramic disc with very short wires.

I attached the modified schematic.
I also attatched a graph showing the amount of treble boost used by FM radio stations. I am in North America so 10kHz is boosted 14dB and in Asia you have a boost of 10dB which is a lot. Your simple transmitter does not have the boost so the de-emphasis in all FM radios does the opposite and cuts 10kHz by 10dB in Asia which sounds muffled like your stereo with its treble tone control turned all the way down.
Maybe your hearing or tweeters are bad?

 

[mdanh2002]

Hi audioguru. Thanks for your info. Now I understand what the coupling capacitor is for. What is the purpose of the 10nF capacitor? I left it out and the transmitter still seems to work as intended.

I tried to use a 10k resistor with a 1k to ground like what you suggested. But since my laptop does not have LINE OUT output, only headset output, the attenuated signal is still high and audio will be distorted unless I increase the 10k resistor to around 33k or kept the output volume at a minimum. I ended adding a 50k trimmer to replace the 10k resistor to change the attenuation factor as intended.

I think the distortion is because I use a mono 3.5mm socket with a stereo cable. I will replace it with a stereo socket using your schematics and try again. Many thanks.

What surprised me about this transmitter is that it's quite stable (no or little frequency shift or audio distortion when something moves close to it). Audio quality and distance is good considering it's running on 1 single transistor with 1.5V. When played on both a manual tuning radio and my phone radio, the sound is crystal clear. Many other simple transmitters I have tried are horrible, for some the signal couldn't even be received by my iPod radio, only by a manual tuning radio.

I will try your FM TX mod 4 after this

 

[Ratchit]

mdanh2002,

As a matter of curiousity, what is inductance of the coil, the capacitance of the variable capacitor, and the type of microphone?

Ratch

 

[mdanh2002]

The coil has 2 loose loops around 1cm in diameter. I never measured the inductance, just adjusted the loops until it worked. The variable capacitor is 2-35pF. The original schematics used an ECM microphone but I modify it to work with an audio source.

 

[audioguru]

mdanh2002,

As a matter of curiousity, what is inductance of the coil, the capacitance of the variable capacitor, and the type of microphone?

Ratch

Stray inductance and capacitance of the wiring affect calculated values. 2-3 turns for an open-air coil is probably 30uH, the variable capacitor has a 27pf capacitor in parallel plus about 5pF of stray capacitance so the trimmer capacitor value is very low at 2pf to about 15pf and the microphone is biased so it is an electret type that many dumbos in stores call a condenser mic.

 

[mdanh2002]

Hi audioguru. Just one more question, what is the purpose of the 10nF capacitor? I left it out and the transmitter still seems to work as intended.

 

[audioguru]

The 10nF capacitor couples the positive supply to ground. All electronic circuits need it but lower frequency circuits use a higher value.
Maybe the stray capacitance of your wiring causes some capacitance to ground.

The requirement for the capacitor increases as the battery runs down because the internal resistance of the battery increases.

50 years ago I had a calculator that used an LED display. I still have it and it still works. Its 9V battery powered it for only a few minutes. I added a 100uf capacitor on the circuit board from +9V to 0V then the battery lasted for many hours.

 

[mdanh2002]

Thanks, I suspect it's for coupling too but since you say it's very important I thought it served some other purposes. I will add it in for the sake of completeness.

Also, I discover that tapping on the circuit board while operating creates a tapping sound (or something that sounds like it but at higher frequency) on the transmitted audio! The board uses the computer line out and has no microphone to capture any sound so how is this sound generated?

 

[audioguru]

Tapping on the circuit board vibrates parts which causes their capacitance through the air to nearby parts to change which creates FM in the oscillator.
A ceramic capacitor can be "microphonic" when it is vibrated.