Building my first fm transmitter

[inklen]

Hi!

I build this FM transmitter using MAX2606 but since I'm still new to electronics, I'm not even sure how to check if it works... :/

Here's the schematic:

1) I'm not sure the schematic is ok. I took it from the web somewhere and slightly changed (well, according to my level of knowledge)
2) I built it and checked all the traces, all the connections on the PCB - seems to be ok. When I turn it on, the only thing I can do is to measure voltages on the IC, etc. That also works. As I don't have anything except my Fluke, is there anything I could do to understand if it works or not?

I tried to find any presence of transmission with RTLSDR but no, can't find anything in the range.

Please help me out here.

PS. What's interesting I don't seem to find a good book on radio and FM radio in particular. I've read about types of modulation and some basic stuff. Then I run into something called "pre-emphasis" and got lost. So I'd appreciate any suggestions on the radio books.

 

[danadakk]

Board layout critical, is this representative of what you did ?

Regards, Dana.

 

[Audioguru]

FM radio produces almost high fidelity sound from a low audio frequency to 15kHz. The very high frequencies above 15kHz are rolled off so that the 19kHz stereo pilot tone is not heard.
ALL FM radio stations have pre-emphasis which boosts high audio frequencies a lot then ALL FM radios have matching de-emphasis that cuts high audio frequencies down to normal and also cuts hiss sounds.
The world has two different amounts of pre and de emphasis, American and European.

The circuit you have has no pre-emphasis to it will sound awful on any FM radio since the de-emphasis circuit in the radio cuts away all high audio frequencies worse than an old AM radio.

The CircuitsToday.com website you copied the circuit from is in India and they do not seem to know any details about audio circuits and many other electronic circuits. There is also an American copy of the circuit at ElectroSchematics.com.

 

[inklen]

@danadakk - I don't have any connectors, I just soldered everyrhing on the PCB, including antenna (basically, a piece of wire - 1mm thick, not sure if this is correct too...). Thus, my PCB is a lot smaller.

@Audioguru thanks for the explanation. I'd really like to hear *anything* for now. And this is not the case.

I guess I will need to start from scratch I will omit all the volume trimmer just to simplify things a bit.

The question still holds - how to test ... anything in this circuit? Are there any helpful tools that do not cost a fortune (it seems everything more or less professional for electronics cost a fortune )

 

[danadakk]

Post a picture of your prototype.

Regards, Dana.

 

[inklen]

Not something I'm proud of...

 

[danadakk]

You are pretty sure no shorts or opens on pins....

What test equipment do you have ?


Regards, Dana.

 

[Audioguru]

On your pcb I do not see a 390nH inductor and I do not see a voltage regulator.
Is the thing with an X an adjustable inductor? What is its inductance?

 

[inklen]

@danadakk I'm pretty sure there are no shorts. I have Fluke 116.

@Audioguru I use 0603 fixed inductor with high Q. But I'm not sure if this is a correct one.

 

[FvM]

I see that you are using a chip inductor. Can you tell the exact type?

If the transmitter is working at all, you should see a carrier with SDR stick.

 

[inklen]

@FvM it's LQM18PN1R0MFRL. And I just looked at the Frequency characteristics (Q) - it's very low close to 100 MHz. Could it be a problem?

 

[Audioguru]

Your inductor is 1uH which is 1000nH which is too much. You need 440nH for the 88MHz to 108MHz FM broadcast band on a MAX2606 IC.
I think the inductor must have a self-resonance much higher than 100MHz.

 

[inklen]

@Audioguru how much higher? ok, I'm going to refresh my memory on inductors again tomorrow.

Thank you! I think that might be the problem. I'll buy some other and try it again.

 

[Audioguru]

The inductor you selected is "For Power Lines" not for the tuning coil in a radio.