Zyg Skrobanski had written:
>> >If the system/amplifier is IMD'ing in the audio frequencies, then
>> >intuitively, the problem causing that IMD is probably also causing
>splatter
>> >products outside the audio bandwidth too.
I replied:
>> Probably, but the audio frequencies are the worst possible place to look
>> for the IMD.
>
Tom Rauch wrote:
>Actually they are meaningless Ian because of all the harmonic
>distortion, amplitude changes, and IM that occurs before and after the
>narrow filters. The rest of the system dominates the distortion we hear
>on frequency, while troublesome IM products are only noticeable off
>frequency.
>
What I had meant to say was the audio frequencies <of the received
signal> are the worst possible place to look for the IMD... so we agree
100% on that.
>The biggest single mistake people make when checking for bandwidth
>complaints like clicks and IM distortion is to listen on frequency, or
>to use a bandscope.
Absolutely. Let me quote at a little more length from the web page, at:
http://www.ifwtech.co.uk/g3sek/cleansig/no-splat.htm
===============================================================
Your mission... if you choose to accept it... is to build a chain of
operators:
* whose own SSB transmission quality is good
* who can report reliably on other people's signal quality.
Here's how to get an SSB quality report:
Step 1
======
Find a strong local station when the band is quiet. Ask for a report on
the quality and width of your SSB signal.
You aren't asking for an audio quality report, in the "hi-fi" sense. You
want to know what your signal sounds like when he's *not* listening to
your frequency!
Step 2
======
If you ask for a report, but then continue talking normally, he won't
tune around your signal properly - he'll try to keep on listening to
what you're saying, and you won't get a meaningful report.
So make it a bit more formal: tell him that you're going to count slowly
up to 20 and back down again. He now knows that he won't miss anything
interesting that you're about to say, and he also knows roughly how much
time you expect him to take, tuning around your signal.
Step 3
======
Ask for comments on how soon he loses your signal on the HF and LF
sides, and also on carrier suppression, spurious sidebands or any other
unusual features.
Then start counting...
===============================================================
There's more on the web page - particularly about how to set up a speech
processor so that you *never* overdrive the transmitter.
In the heyday of VHF DX over here, we built a very effective chain of
stations who had good signals and could comment usefully on other
people's signals too. It actually worked - most people could be
persuaded to take some pride in their signal quality, shown how to do
it, and recruited into the chain.
It was *so* nice to be able to tune a few kHz away from an enormous
local signal and be able to hear weak DX, right down into the band
noise.
Showing people how to do it is the key feature, but this really isn't
rocket science.
Occasionally, someone would whine "But I'm only an amateur"... as if
that was some kind of excuse for ****ing on other people's QSOs. My
standard reply was: "There's a guy in the next town who left school at
14 and chops meat for a living. He cleaned up his signal, so why can't
you?"
Well, why can't anybody?
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
|