On Thu, 09 Jun 2005 20:43:06 GMT, "Harold E. Johnson"
wrote:


I'm sorry, Harold, I don't know that 'TRF' stands for. Is that
somehting similar to TSDR 'true software defined radio'? That is, an
A/D hooked directly to an antenna?
Glenn

I HATE it when I do that! No, it's an OLD term. Actually, one of the few
that's older than me. Stands for Tuned Radio Frequency, the topology from
before Armstrong came along. Tuned RF amplifiers at the signal frequency,
followed by a detector and audio. No oscillators.

IMD, in particular odd order IMD, is readily identified by tuning to the
opposite sideband of a SSB signal with a selective receiver. If you can hear
the signal weakly but clearly, you're listening to the suppressed sideband
of the transmitting station. If everything is garbled, (AND MUCH stronger
than the suppressed sideband which you probably CAN'T hear due to the
garbage ) You're hearing odd order IMD.

If it's a REALLY strong signal, it may be being generated in your receiver.
If it's not terribly strong, (Like maybe "S"-9 although this is being pretty
subjective) it's likely that the transmitting station is producing the IMD.

We've been sold down the river to some extent, published tests of amateur
equipment in this country, now examine the IMD with respect to a single tone
of two tones used in the test to generate the IMD. This makes the equipment
look like it's 6 dB better than it would be if you tested it against BOTH
tones which is the benchmark our military and most other countries use to
evaluate IMD. Commercial offerings these days, particularly in more modestly
priced gear is really pretty sorry in this respect.



IMD should be expressed referencing a single tone of a two tone test
signal. That is the way most tube manufacturers do it.

Some of the radio manufacturers reference IMD to PEP which makes it
look like the IMD is 6 db further down!

The PEP of a transmitter is 6 db higher than either tone of a two tone
signal.
If the IMD is 30 db below the level of one tone of a two tone signal
then it will be 36 db below PEP.

73
Gary K4FMX