audioguru FM transmitter

[mdanh2002]

Hi,

I finally got the time to construct audioguru's FM transmitter (MOD4) after so long. It worked upon first power-on. I am using a 80cm wire as an antenna. The wire is mounted on my window on level 5. Approximately 20m opposite the window is a tall (12-level) and wide building. Within our apartment, the reception is very clear and sound quality is excellent.

The distance I got when testing the transmitter - by walking around on ground level with my pretty sensitive PLL radio - is approximate 300m, from the transmitter to the point where the transmitted sound can no longer be heard.

Also when walking around, I notice that signal strength is not consistent. Only within a few meter's radius, there are points where the audio can be heard clearly while there are also places where it is severely distorted.

Is this expected due to the obstacles between the transmitter and the radio? or is there more to be done to improve the range?

 

[audioguru]

The frequency of my MOD4 FM transmitter is not crystal controlled so maybe its frequency drifts a little but your PLL radio does not drift. Then they are both not on the same radio frequency. I had this problem with my home stereo FM tuner and my car radio. My Sony Walkman FM radio has AFC so it follows the drifting frequency of my transmitter.

I think your radio is overloaded by the nearby FM transmitter project and by local FM radio stations. Then phantom intermodulation "images" appear at a few points on the dial and you tuned the radio to one phantom image instead of to the real signal. My Sony Walkman FM radio gets severely overloaded and strong signals appear all across its dial.

If your radio has a poor "capture ratio" then it will mix a multipath reflected signal with the direct signal which produces multipath distortion.
A good capture ratio ignors multipath interference because it locks onto the signal that is slightly stronger without mixing.

The receiver antenna must be parallel to the transmitter antenna. No obstacles between them. The 9V alkaline battery must be brand new.

 

[mdanh2002]

Hi audioguru,

By AFC do you mean Automatic frequency control? I am aware that the frequency might change and have tried to tune the radio to nearby frequency when the audio seems weak but that does not help. I also notice the frequency drift, but only as much as +/- 0.2MHz from the main frequency.

What are the symptoms when I do not tune to the main frequency? I guess it would be reception loss within a few meters' range. My transmitter goes pretty far so I hope I am tuning to the correct frequency. I have a few radios here, both analog (manual tuning) and digital. In 2 of my analog radios, the transmitter appear at multiple places. Other stations are also affected (distorted transmitted audio can be heard in the background). On the other radios, including the PLL radio, the transmitter appears at 1 place only (88.6MHz).

I will mount the transmitter on a different location and try the parallel antenna method tomorrow, just to test if it works better

Also what is the purpose of the 2nd (rightmost) LC oscillator of the circuit? Must it be tuned to the same frequency as the first oscillator? I though it acts purely as an RF amplifier. So far, my range and frequency do not seem to be effected by tuning C13.

 

[audioguru]

The antennas must be parallel and both pointing in the same direction. Both pointing straight up or both horizontal and pointing in the same direction.

The second tuning is a broad peak of the RF amplifier for maximum range. It covers one-third the 88MHz to 108mHz broadcast band. This trimmer capacitor will have its capacitance set a little different to the frequency trimmer because the stray capacitances are different.

 

[mdanh2002]

Thanks audioguru. So effectively, both oscillators need to be on same frequency for maximum range. Is there a way to tune this easily? Because within the premises of my apartment, the signal is strong even without the RF amplifier stage. I assume I can only place the radio much further away (at least 100m) from the transmitter and tune C13 until the strongest signal is received, which is difficult with only 1 person working on the circuit.

 

[Mikebits]

Wow AG, your circuit is global, your famous

 

[audioguru]

So effectively, both oscillators need to be on same frequency for maximum range. Is there a way to tune this easily? Because within the premises of my apartment, the signal is strong even without the RF amplifier stage. I assume I can only place the radio much further away (at least 100m) from the transmitter and tune C13 until the strongest signal is received, which is difficult with only 1 person working on the circuit.

The transmitter has only one oscillator. The second LC tuned circuit is in the RF amplifier at the output.
I made a field strength meter with a 1N34A geranium diode, a couple of capacitors and a resistor to feed DC into a voltmeter.
Some field strength meters (look at Talking Electronics or in Google) add a transistor, battery and LED. When the LED is the brightest then the tuning is peaked.

 

[mdanh2002]

The transmitter has only one oscillator. The second LC tuned circuit is in the RF amplifier at the output.
I made a field strength meter with a 1N34A geranium diode, a couple of capacitors and a resistor to feed DC into a voltmeter.
Some field strength meters (look at Talking Electronics or in Google) add a transistor, battery and LED. When the LED is the brightest then the tuning is peaked.

Thanks audio guru. Will try to make my own field strengh meter.

I retuned C13 a bit today and mount my transmitter on another place, looking out to an open field. The results are great. Distance is approximate 1km line of sight. Could probably be higher if there are no trees in between.

One thing I notice is that the transmitter consumes lots of power. A new 9V battery (measures 9.7V) reduces to 8V within 1hour, and to 7.5V within the next 15 mins. Perhaps I need to run it from a wall-wart or from 6xAA batteries instead.

 

[audioguru]

My FM transmitter uses a current of about 53mA. Then a new little 9V alkaline battery drops to 8V in about half an hour and is 7V in about 3 hours.