Re: Saab mic

Posted by czabel on Jul 11, 2019; 4:20am
URL: http://bluesaab-forum.90.s1.nabble.com/Saab-mic-tp6p1466.html

Alright, made some progress tonight.

I've got the mic on the bench, running through one of our electret mic preamps (see schematic below)


My mic measures as follows:
Pins 1 & 3 = 6.5kOhm (High Impedance)
Pins 2 & 3 = 1.1kOhm (Low Impedance)

When using the low impedance configuration I can barely get a signal out. It's there, but almost in the noise. I'll keep playing with it, but the high impedance pins are where I had the best luck. I've uploaded the .WAV file I recorded out of the mic, seems to enjoy working from +4.5VDC on up (I stopped at 5.1V). The output amplitude doesn't change noticeably beyond 4.8V. As you can hear, it barely works at 4.4V, but after that is pretty clear. SaabMic_HighImp_VoltageTesting_01.wav

By closely examining the waveform, you can see that the amplitude of the background noise (equipment fan hum) does increase with voltage, but I would be inclined to run as low a bias voltage as possible, not knowing what the microphone module really wants.

Anyway, next step for me is to send Microchip an email asking for a dev board sample. They have a few methods of connecting a microphone on the board and I'd like to try them and mess around with their Mic_Bias line. Their datasheet leaves a lot to be desired... Sure, I get 1.5dB steps, but dB relative to what? 1V? Who knows.

Or, if they don't want to send me a sample, I'll just buy the stuff for a full BlueSaab and mess with it that way. Is it possible to interact with the RN52 directly when using the full BlueSaab? I see the UART headers, I'm guessing there's something I'll need to use to convert to USB?