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Mark963

CM-707 2.4GHz Receiver RSSI

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I picked up a couple "CM-707" 2.4GHz audio/video receivers pretty cheap. I've had a 10mW transmitter out past a half mile with some video still coming through... but it wasn't pretty. Still I think that means there is some pretty good sensitivity here.

There's a 15 ohm resistor in series with the input to an LM7805 regulator. Apparently it provides a little filtering and burns off some heat if running it off of 12V as intended. I shorted that out and run it off a 600mAH 2S (7.2 to 8.4V) lipo. Without it shorted, it still works, but there's only 4.3V coming out of the regulator and I'm not sure that's enough to make everything work fully right.

So... I poked around the module and I don't see any unused pins with anything that looks like an RSSI signal. I opened the module and there's an SL1461 "Wideband PPL FM Demodulator" that appears to have an AGC output signal on pin 7. A datasheet on it indicates specs like 400-800MHz operating frequency, -40dbm sensitivity, 0db input overload and there's a graph that shows AGC voltages of about 1.5V for -45db and lower input to about 0.25V for signals over -30db. Can this be used as an RSSI signal for simple (comparator, fet video switch) diversity system? Can I hook a series resistor on to it and pass that out to a high impedance circuit as an RSSI signal?

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The SL1461 AGC is used to control the front end attenuator. So, be very careful how/what you attach to it or noise will be introduced into the RF chain. For example, install a tiny buffer amp inside the Rx's metal tuner that isolates the AGC from your external use.

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Well, there isn't a lot of room to add much in there, and I'd hate to screw up the RF tuning by adding stuff that wasn't there when it was tweaked at the factory. Darn. Hmm... maybe a SOT23 jfet... no, probably not. Thanks.

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