The home brew AM transmitter on the bench at the moment has thrown up the issue of either low or over modulation from my mic audio. Some time ago, I purchased a couple of interesting boards described as "SSM2167 Preamp Compressor Limiter Noise Gate Dynamics Processing Module DC 3V-5V Microphone Preamplifier". They are under AU$5 from AliExpress and really tiny.
To aid with experimentation, I've mounted mine on a carrier board.
R1 on the board controls the noise gate, which I'm not too interested in. R2 controls compression and came with a 1k resistor in place. The data sheet has this table:
With the un-modified board I fed a 1kHz tone in and get peak to peak voltages:
50mV -> 440mV
100mV -> 880mV
200mV -> 1.43V
Higher inputs do not give higher output. The output waveform looks pretty good, so not clipping. Gain is rather low for my dynamic mic however.
I replaced R2 with a 100k resistor and it has much more gain.
5mV -> 880mV
10mV -> 960mV
20mV -> 1.06V
Output level stops there and again gain is being reduced.
Stephen, VK2BLQ, brought this idea to my attention as it has been discussed as helping the uBitx radios.
3 comments:
I was thinking of trying this as AGC for a direct coversion receiver. What do you think?
It could be used to compress any audio but the time-constant might be a bit short to work as a normal AGC
Yes, that could be. One of the projects would be a fixed receiver for monitoring the International Beacon Project, especially on 28.2MHz, so the time constant should not be too critical. The only thing that could worry me a bit is the gate function, or maybe it could be used as a squelch function here.
Just an idea for experimentation.
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