Hello all !
Just a short question: Has anyone succesfully connected a microphone to the GM862 module ?
I’m trying to do this for days and no matter what i do, i still get that annoying gsm buzz. I’ve even built the circuit described in the module’s Hardware Guide and nothing…
It’s driving me crazy.
Please help.
Thank,
James
It’s probably due to poor PCB layout.
Leon
Hi !
The board is the GM862 Evaluation Board - USB from Sparkfun.
All the best,
James
It’s still probably due to poor layout, if the circuit is otherwise correct. Did SFE design it for microphone input?
Leon
Ok…
Poor layout on the Sparkfun board or on my regulator/buffer board ?
James
It could be either.
Leon
Ok then…
I’ll rephrase the question:
Has anyone using the Sparkfun GM862 Evaluation Board - USB successfully connected a microphone to it ? Without picking up the annoying GSM buzz and obtaining decent audio quality !
Thanks,
James
I have not had any experience with using the GM862 or the Eval board in an audio application, but I’ve worked with several different wireless audio circuits on a regular basis. I’ve got some thoughts, which you may or may not find helpful:
First off, as has already been discussed, poor trace layout can pick up a lot of noise.
Also, different types of microphones pick up different types of noise. I had one circuit that would buzz like mad with an electret microphone, but when I used a dynamic microphone, it was crystal clear.
Single-ended vs differential (or unbalanced vs balanced) audio paths make a difference too. Single-ended (unbalanced) pick up noise more than differential (balanced). See the GM862 datasheet, it should have a few pages on this.
Finally, the GM862 datasheet also shows ‘pi’ filters in the audio paths. It says it’ll help filter the GSM noise.
Hopefully that helps, and good luck.
Hi Andrew,
Actually i’ve built the exact circuit as in the GM862 datasheet for a balanced microphone. I’ve tried every single microphone i had and it’s still buzzing a lot.
I’ve even tried to power the microphone circuit from a separate battery and nothing.
It’s driving me crazy. Do you think that my PCB layout for the microphone could be the cause ?
Thanks,
James
It’s more likely to be the PCB for the GM-862. I doubt if SFE expected anyone to use the microphone input when they designed it. Perhaps you should ask them about it.
Leon
Hello Leon,
Ok then…
How about if I try to redesign the PCB for the GM862 module ? I have some friends who could make the boards along with one of theirs and I could have them ready in a couple of days.
Can you help me with this ? I don’t have much to offer but i can promise you a beer (or more), tapas and a free guided tour if you visit Barcelona 
Thanks a lot,
James
I’ve only used the GM863, and that didn’t have a microphone. It’s probably going to be quite difficult, and might require two or three iterations of the board to get it working properly. It would be a good idea to make it a four-layer board.
It would be cheaper to buy the Round Solutions evaluation board. That might be better than the SFE one, and you can get support from them.
Leon