On Thu, Oct 20, 2011 at 2:30 PM, Dave Beumer WØDHB <d...@w0dhb.net> wrote:

> Ed
>
>
>
> Using a transmit profile with a 250Hz wide filter centered at 1500 will
> help .
>

Well, actually, it won't. The only reason to run a narrower TX filter is if
the signal being presented to the transmitter already has too much THD or
IMD. With the flex radios that is not the case because the digital mode
program is sending the signal in digital form to PowerSDR via VAC. Since it
never gets converted to analog, there just isn't a need to narrow the TX
filter.

Now it is possible, if some program along the way (VAC or PSDR) has to do
sample rate conversion, to get some artifacts in there but usually they are
at least 60dB down. (In my case they were almost 80dB down.) Yeah, you can
see them on the pan display but I can almost guarantee you that no one is
going to hear them and they certainly aren't going to show up on the
wattmeter.

So, narrowing your TX filter might make you feel good but it doesn't really
have any effect on the quality of your signal. And if you think you are
reducing IMD, the bulk of the IMD is created in the PA just prior to
reaching the antenna. Nothing you do with the TX filter is going to change
that.

-- 
Brian Lloyd, WB6RQN/J79BPL
3191 Western Dr.
Cameron Park, CA 95682
br...@lloyd.com
+1.767.617.1365 (Dominica)
+1.916.877.5067 (USA)
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