Topband: Digital mode spurious issues

[Charlie Cunningham]

Well, I certainly have to agree, Tom, if the signal on the desired sideband
is just a single shifting tone. Might get messier if an sudio stage or A/D
is driven into limiting and producing harmonic distortion at audio, I guess.

73,
Charlie, K4OTV

-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Tom W8JI
Sent: Monday, December 30, 2013 10:56 PM
To: topband at contesting.com
Subject: Re: Topband: Digital mode spurious issues

Thanks everyone for the suggestions. I'm going to have to install some 
software to identify some of the signals.


>I would think that IMD products in a high-level PA that is over-driven
> beyond good linearity  limits could add some junk in the "undesired
> sideband"? FWIW
>

IMD **requires** two or more signals at once, and does not appear anything 
like sideband leakage. This was a single shifting tone, and the lower 
frequency signal went the opposite way but the same amount as the main 
signal with shifts. That is classic for inadequate sideband suppression.

There are multiple problems with using SSB to transmit audio tones and 
"thinking" it is a pure digi mode.

1.) things like this do not show on almost all digi waterfalls because they 
are out of passband of the other fellows receiver.

2.) SSB carrier, noise, and opposite sideband suppression is limited by the 
radio quality

3.) output purity is also limited by audio input purity, which includes 
audio line issues

4.) most digi operators do not have the low noise antennas most DXers have, 
and cannot hear some fairly strong signals. They are often on digimodes 
because of that!

5.) radios have terrible SSB transmitter performance compared to even fair 
receivers, so the transmitters often set the adjacent channel interference 
levels

Placing digimodes near weak signal areas is not very wise frequency 
planning, but there is nothing anyone can do about it. 

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