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Re: AO-40 Beacon and Transponder noise calculations



>The transponder noise is highest near the beacon, 
>and falls off towards the edges of the passband. 
>73 Charlie 

Charlie et al.,

regarding the receiver/transmitter combination
noise floor shape, you need to be aware that the
transmitters are basically very broadband compared
to the receievers. The V band TX (QRT) and the K band
TX's are are a separate issue with their crystal
filter IF's, but other TX's are wider. What this
means is that the RX/TX combination passband
response shape will be influenced by the individual
receivers. The HF receivers and the V band receiver
are narrowband with crystal filters for shaping,
whilst the others exhibit a broader passband.

The individual shapes vary. Even the L1 and L2 are 
different in shape, and regarding the current case,
the U band receiver passband is also slightly
Gaussian in shape. I believe the rolling off of the
passband noise level we see currently is due to the 
U band receiver passband shape, not the transmitter.
Please note here that the L1 receiver passband noise
is also influencing the shape.

With the different combinations of the possibly
multiple receivers, the passband noise floor
shape will vary. Each receiver (when the relevant
AGC's are active, which we can take for granted),
will output a nominal IF power level to the matrix.
With a matrix gain of 1, these power levels
will appear noise combined at the matrix output.

This is the reason why each individual TX has an
ALC (or AGC) circuit in the design. The TX in use
needs to supply the specified output power irrelevant
of the input noise floor (say 1, 2 or even 3 receiver
noise bandwidths combined).

Transponder (if we can call the unique AO-40 TX/RX
combination such a thing) passband noise floor shape
will in other words also be dominated by the action 
of AGC/ALC in each individual receiver and transmitter
in the combination in question. Coming from different
AMSAT parties around the globe, each receiver and
transmitter module have individual solutions 
implemented in the AGC designs. This means that
each individual AGC/ALC will also have a different
behavioral pattern (peak, average or some other
response, different attack/decay times, even thermal
considerations).

I am trying to keep the message concise and to the
point, but as you can see, the noise floor shape
can turn to be a complex issue and very dependent
on the receiver combination in use for each case.

We have a truly versatile spacecraft in our use
and the complexity of the working innerds will take
a while for even techies to grasp :-)

The bird is very unique in many aspects and I know
for a fact that professional satellite operators that
get to hear about AO-40 are fascinated by the 
technology that a bunch of international amateurs
have put together.

73's Michael, OH2AUE, AMSAT-OH

www.oh2aue.pp.fi


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