Dani, you are right.
What you are seeing is the combination of the attack and decay time of the AGC with a strong signal. You can see the AGC bringing the noise back up as soon as the carrier is gone. The transponder is fast-attack and slow decay on the AGC.
Since the downlink passband is a bit stronger as compared to FUNcube-1, you can now more clearly see the passband.
Wouter PA3WEG
On Sun, Nov 13, 2016 at 6:20 PM, Dani EA4GPZ daniel@destevez.net wrote:
El 13/11/16 a las 17:55, Scott escribió:
I have uploaded a screen shot of my waterfall showing how the wideband stream comes & goes in response to activation of the transponder at: https://dl.dropboxusercontent.com/u/31460496/ham/2016-11-13--EO-79.png
Hi Scott,
I think that what you're seeing is just the noise floor of the transponder. I have no experience with FUNcube-3, but your picture looks very much like the noise floor I have seen on FUNcube-1. On overhead passes, it's quiet noticeable, even with only a 3 element yagi.
Keep in mind that (usually) a linear transponder is running all the time. It doesn't have to be activated by an uplink signal.
In general, AGC action may cause the transponder's noise floor to vary with input signals. One expects that if there are strong signals the transponder's noise floor will become weaker. This behaviour depends on lots of design parameters, so this need not always be the case.
73,
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