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AO-16 & FM receiver thoughts (was: AO-16 report fromCopenhagen)




On Jan 23, 2008, at 11:14 PM, Tom Clark, K3IO wrote:

> The receiver has a 15 kHz wide crystal filter with sharp skirts. So if
> your NBFM xmtr is set with a ±5 kHz deviation, you may well find your
> signal hitting the filter "walls". You may get better performance if  
> you
> crank the deviation back a bit.

Lots of folks who haven't worked on FM repeaters (or repeater  
coordination) don't realize that a 5 KHz deviation signal actually  
occupies 16 KHz, Tom.  This is a great reminder, and knowing there's a  
sharp-skirted 15 KHz filter is great info for folks trying.

(Sadly, lots of repeater users don't know the difference between  
"deviation" and "modulation" these days, either -- but that's a whole  
different rant...)

This would also mean that if you're way off on correcting for doppler  
on the uplink you could ram into the skirts of the filter too... on  
modern rigs, go into those menus and set that FM step for as small a  
number as it'll go, and play around folks... you might find that fine  
tuning things a bit the correct direction for the doppler on the  
uplink might help a bit too.

A little lower deviation and a little more tuning and fiddling as  
needed with the uplink frequency, and voila!

Anyway this leads me to a thought, Tom -- for those who have modern FM  
rigs that have so-called "narrowband" mode (usually max 2.5 KHz  
deviation) would the satellite's FM receiver be fairly linear when fed  
with low deviation levels?  I know it hurts on S/N ratio on the DSB  
downlink, but would 2.5 KHz deviation yield 50% modulation of the DSB  
transmitter, or is the FM receiver's audio output non-linear to some  
extent (like most are) and 2.5 KHz deviation would really be down to  
something like 30-40% modulated on the downlink?

It'd keep people from hitting the filter skirts as much, but if it  
yields really low modulation levels of the DSB transmitter, it'd  
probably hurt more than it would help.  What do you think from what  
you know of the ol' girl's FM receiver audio setup?  Any thoughts?   
Worth experimenting with the feature if folks rigs have that setting?

Neat stuff seeing the reports of those playing with the bird since she  
came back to life, both the control stations folks who did their  
magic, and now the end-users.  Cool to read along!

--
Nate Duehr, WY0X
nate@natetech.com



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