>From what the ARRL said in their review of the TS850, it was okay running
at reduced output at the level needed to drive a high gain amp. >The problem
IMD happened at rated output (100w).
> Frank
> W2FCA
Hi Frank,
While I have never specifically tested and 850, every radio I have tested
becomes pretty poor at low power. Radios go through an IM minimum as power
is increased from minimum, and eventually IM (as compared to the desired
signal) increases again at rated power.
In addition, virtually all radios have ALC overshoot caused by time delays
in loop response of ALC systems. These overshoots are particularly
devastating in tetrode amplifiers, where grid current changes perturb the
operating conditions long after the transients are gone. Low drive tetrode
amplifiers used in the amateur market are also some of the worse in IM, when
compared to GG triodes.
The ideal place to operate most radios is where the most time is spent at
minimum IM, and where the ratio of ALC overshoot to desired power is
minimized. Amplifier and transmitter combinations are generally cleanest
when the PA takes nearly 100% of the rated exciter power to drive.
Wide bandwidth Hi-FI systems are especially bad, because the rich
combination of frequencies that are NOT needed for communications provide
many new components to generate IM. These initial components are also spread
over a wide frequency range, and the boosted lows add seriously to power and
bias supply regulation woes.
The best solution is communications quality audio and running rigs into
amplifiers that reasonably match the exciter power.
73 Tom
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