Zyg Skrobanski wrote:
>
>How do the IMD results show themselves in the real world, how far out
>will the splatter be objectionable? Splatter is bad, but splatter at
>10 KHz is worse than splatter at 5 Khz.
>
That's the key question, because you have been talking all the time
about 3rd order IMD, which is close to and underneath the speech signal
itself.
As you increase the drive level, various orders of IMD can both increase
and decrease. Typically, each one will go through a null at a particular
level of drive - that's how you get those "spectacular 5th order"
results, if that happens to be the level of drive that gives the rated
output.
When an amplifier is going through a static IMD test with a simple
two-tone input signal, a very small adjustment in drivel level can
sometimes "sweeten" its IMD performance without anybody noticing the
change in output power.
Higher-order IMD comes up very fast with increasing drive level, which
is why it clicks on and off like "buckshot". Higher-order IMD also
covers much more of the band than low-order, but all the specifications
concentrate on low-order products... wonder why...
A 3rd-order IMD figure on its own is no guide to what the amplifier
sounds like further out. Some amps have a steady taper in IMD levels
with frequency offset, so that the IM products die away into the noise.
But another amp can have the same low-order IMD performance, but the
higher orders just won't die - you keep tuning away from the main
signal, and the splatter is still there...
All these complexities mean that when someone talks about "IMD" without
going into very specific detail, it's likely to be incomplete or
meaningless. We all do it, but we also need to be aware of it.
Ironically, the most meaningful test of IMD performance for SSB is also
the simplest - just tune away from the signal, using a good receiver
that isn't being overloaded, and notice how far away you have to go
before the splatter dies away into the noise. Unfortunately this simple
test is also one of the most difficult to nail down as a reproducible
procedure!
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
|