I need for my project purposes to connect an external audio source ( provided by a MP3 player module : SOMO-14D )
The MP3 player will play a track when a call is on-going.
Please see schematic below.
When connecting the built-in audio module output to GM862 modem on pin MICMT+
the audio track played is polluted by the GSM carrier, then the sound is surrounded by "bip-bip / bip-bip / bip-bip" noise’ when it is been played.
I’ve tried to reduce the GSM carrier noise by adding a 12pF ceramic cap (C1
this helped to reduce a little bit that noise but also the cap also reduce too much the audio track level, hence it gets difficult to hear it.
Can you suggest me a filter solution to be added between MICMT+ pin and audio module output ?
Many thanks for your help,
12p is really too low for audio. substitute it with 1uF.
for what concern the RF noise. question is not so simple. You can try as a first attempt to connect a 33pF ceramic SMDfrom MIC+ to GND.
But a lot depends on PCB design.
Also never ground MIC pins! connect a 100nF from it to GND.
Thank you for your answer.
I’ve made some tests according to your advices, please see attached the schematic of the filter that provides best results.
I’ve found that adding a small cap (C27) from MIC-MT – to Ground gets better results than a 100nF cap. ( i do not know why … )
However RF noise still audible, but indeed less than before.
Could it be a good idea to insert a LC filter on the audio patch ?
something like this : http://fr.farnell.com/jsp/search/browse.jsp?N=2014+202464&Ntk=gensearch&Ntt=filtre+0805&Ntx=mode+matchallpartial
Or maybe add more passive components on the actual schematic in order to enhance the filter ?
filter to cut off RF from audio are really difficult to implement.
youi are using a single ended connection which is not optimized from noise point of view. Anyway, in order to balance a little the connection I think hte C27 should be substituted with a 27pF to balance the impedance.
Other that this, the layout is maybe the most important point. how do you realize the PCB? how many layers? is the audio line shielded enough?
Actually the audio comes from a mother board, Telit modem board is pluged on it.
To connect audio from mother board to Telit board, I need to use a very short cable to patch Audio+ pin to MIC_MT+ pin.
Here is the picture of the situation.
The mother board has 2 layers without ground plane,
the Telit modem board has 2 layers too, but that board has a ground plane.
The audio line connection from mother board to Telit board has been tested with flying grip cables, in order to tests differents capacitors values.
I’m aware that flying cables are not good, but it was for tests purposes.
even if your environment can be used for a test purpose, be aware from a noise poin of view is not optimum.
If you need to switch to a stable release you should re design the whole system.
Be also aware that at least the circuit from mp3 player to module must be designed very well, and a two layer without ground is not what can be tought a good design.
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