On 2015-05-22 1:20 PM, Jim Thomson wrote:
Somebody mentioned the FT-990. Take a look at the yaesu 767GX from
years ago. It had superb imd. But it used 200 watt devices...run at
100w out.
The FT-990 and FT-767GX probably used the same devices as they were
contemporary rigs from the same manufacturer. But this is the point,
the devices were rated for much higher power when meant they were not
being pushed into compression (saturation) at the 100W level.
Too many modern radios have marginal devices in push-pull, run very
close to class B which creates distortion both at cross over and on
peaks. Then when some idiot uses a battery that is not fully charged,
things get even worse with voltage down around 11V instead of a nice
stiff 13.8V. The output impedance of the transistors drops from 1.9
Ohms to 1.1 Ohms to maintain the same output power and suddenly the
devices are working into a substantial (2:1) mismatch even if the SWR
is good. That makes the IMD even worse.
73,
... Joe, W4TV
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