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Re: AO40 U-AGC question



> Am I correct to assume that LEILA detects signals BEFORE the AGC?  That
is,
> does the uplink power necessary to activate LEILA stay the same as the
> U-band AGC changes?  It seemed to me that it took the same amount of
uplink
> power to activate LEILA during Field Day as it does at other times (~25W
> into a 436CP30).

I don't believe this is correct.  I did a search again through my Journal
CDs and the Tlm Spec.  It appears that LEILA is after the complete receiving
chain including AGC.  The Rx-Out is switched to LEILA-In and then LEILA-Out
is routed to the Tx.  The effect is that while LEILA can notch out an
offending signals downlink and thus hopefully remove the incentive for
running too much power, there's no relief to the AGC until they actually
stop transmitting.

> It didn't seem "10 dB harder than usual" for me to get into the AO40 U
> uplink during Field Day.  I ran my usual uplink power and was able to hear
> myself and a lot of other stations all the time with a 2-foot dish, patch
> feed, and low noise preamp.

I'm curious what your normal uplink power is.  I typically run just 5w into
the antenna.  It appears we were both running 14 dBic antennas so the only
difference would be our uplink powers.  I do know that during the weekends
prior to FD and even Saturday morning prior to 1800 UTC we were able to make
easily copied voice QSOs with 5w.  After 1800 UTC, our signal was barely
detectable in SSB and most of the time disappeared entirely.

> Do we know if there really is a linear
> relationship between AGC value and downlink power for a given uplink?  Has
> anybody measured downlink signal strength vs. AGC (with a fixed uplink
> power) to see if there is really a linear relationship?  That is, if the
> telemetry reports that the AGC jumped 10 dB, does the downlink power of
the
> test signal really drop 10 dB?   (not sure how this could be measured
> because the AGC data is not instantaneous)

Since there is a calibration curve for the AGC measurement in the Tlm Spec.
I believe that it has been measured.  There are several measurements such as
the S2-ALC and S2-Tx Power which were not calibrated and are noted in the
Spec. as "Use Raw".

There seems to be several things happening which all probably have some
interaction.
* LEILA of course does not recognize all signals equally.
* The AGC doesn't seem to reduce the gain on a 1:1 ratio above its
threshold.
* We don't know the AGC threshold or time constants
* AGC sample rate in the telemetry is very low, ~ 4 samples/minute

> All I know is that I ran my usual "not quite LEILA bait" uplink power
during
> Field Day, and it didn't seem 10 dB harder to hear myself or other
stations.
> If everybody was 10 dB weaker, I would have had a much more difficult time
> hearing everybody with my 2-foot dish.

Not necessarily.  The stations who were driving the AGC would not sound
weaker. Also all the stations on L-band would not be impacted and would be
at their normal levels.  Only the stations on 70cm that didn't increase
their power to keep up would be weaker.  Our station was definitely made
weaker by at least 10 dB.

Having said all that, there's a lot of things that haven't been published
about the various Rx AGCs and Tx ALCs.  If someone knows who has the
information I would be happy to work with them to put it together into an
article so that we would all have a better understanding of how things
really work.

73,
Lee-KU4OS

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