No subject

[Peter Chadwick]

Joe, KM1P asks:


> Tom, what is your opinion of generating two audio tones say, 2 Khz
apart
> and feeding them into the mike of an ssb transmitter?
>
> -Joe KM1P


I'd say 'it all depends'. If you want to evaluate your entire SYSTEM
under two tone test conditions (and I'd say that's a useful, but not an
infallible guide), then that's a good start. However, it's worth looking
at the method of generating two AF tones without IMD on them, and that's
not always too easy - lots of attenuation is useful before the combiner,
or make a 2 to 4 wire hybrid in the same way as the telephone people do,
using a transformer and passive components.

Tom says:

>I don't think much of that method, because the exciter always has
>IMD distortion and less than perfect sideband and carrier
>suppression. Most tube PA's will go far beyond what a modern solid
>state rig will do in IMD performance.

I'm not sure I agree totally, but Tom has a very valid point, especially
if the exciter is running hard.
Of course, the other point is that the exciter might be reasonably clean
into a 50 ohm DC to daylight load, but get appreciably worse into a load
that's only 50 ohms at the operating frequency.

If you're driving  a power amplifier on its own to evaluate it in the
limit, then two signal sources with some sort of hybrid combiner to
prevent inter-transmitter intermodulation is a good idea.  There are
combining and outphasing methods, too. If you can get hold of a copy of
the second edition of the Plessey Semiconductors Broadband Amplifiers
Applications book (long out of print and unavailable - I'm not even sure
that I've got a copy still) there's an App note there that I wrote on
the subject. Aimed mainly at low power receiving type circuits, a fair
amount of it is applicable.

I guess the bottom line is that it depends on what you're doing. For
straight forward run of the mill DXing or ragchewing, I guess an
oscilloscope looking at the output envelope and making sure that it
doesn't clip too hard on peaks is adequate, bearing in mind the effects
of any speech processor.  If you want to be fancy, hook the scope up
with detectors on the input and the output of the amplifier to plot the
transfer characteristic under dynamic conditions - see Pappenfus et. al,
Single Sideband Fundamentals and Circuits, McGraw Hill, 1964.

For contesting, it appears that the usual practice for many is not to
give a cuss, and wind  everything up to keep the frequency, and the
channels each side by splattering as much as possible..........the
'ideal contest linear' I guess uses a tetrode with a weak bias supply,
supposedly AB1 but driven into grid current, with an unregulated screen
supply! No, guys, I'm only joking............just there are too many
contest linears like that.

For ragchewing on 80meteres, I'd suggest a spectrum analyser at the
operating position, because someone, sooner or later is going to
complain that you're spreading when they've got a crap receiver.

The 80 metre  Law: If you're interfering with me, it's your crap
transmitter. If I'm interfering with you, it's your crap receiver.

73

G3RZP

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