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raptorbasher

disable AGC for RX antenna testing?

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Hi All,

Does any one know, or have suugestions for how to disable the agc side of a reciever for the purpose of using the RSSI, as Mr RC Cam did during his antenna tests??

I wish to replace the long wire on the Rx with a base loaded whip( for a micro heli ), and then test for best range etc, but as Mr RC Cam had no agc on his test he got the figures he needed.

the FM chips in most of my Rx's is the 31136 and so i know where the RSSI output pin is, is it simply a case of lifting the signal after the cap, and cutting the trace there??

I would use a commercial deans type of antenna, but here in the UK we are on 35Mhz, and as such there is already an inductor at the rx end( on another note here, would it be best to chop the wire down to size and then change this inductor to suit?)

all suggestions welcomed thanks

Tim

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For the test, it would be best to choose a Rx that does not employ AGC. Luckily, most don't.

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Actually I think the RSSI shouldn't be affected by the AGC - usually it's the later IF stages that get affected. As such I think that RSSI is a good measure of receiver performance (frontend and early-IF of course). In fact, the RSSI is usually the measure of the AGC adjustment required to bring the signal back to par.

Daniel

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It seems that some R/C Rx's use the RSSI for the AGC. I don't recall which, but I have run across them. But overall, AGC is a rare feature in R/C Rx's.

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the only kind of agc I have seen was implmented with a transistor that shortcircuited the antenna input when rssi was getting too strong, neat :-)

to prevent overdrive, seen this on a few schulze 35Mhz types,

but simply perform the test and see if rssi signal changes as you think it should !

also if signal is too strong, rssi will not change lineary, so stay on the straight part of the curve when testing, else your conclutions might be a bit wrong.

I hane bench tested nearly 70 RC RX units so far, and opened them all

see:

http://www.webx.dk/rc/RX-measurements/

and

http://www.webx.dk/rc/RX-measurements/komp...plet-listen.htm

Edited by ThomasScherrer

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