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 [email protected]