[AMPS] IMDs

[G0MJW]

I happened to have an FT690R on the bench and the posts regarding IMDs
prompted me to look at the results from a "typical" solid state driver. The
FT690R is not renound for its quality output but I expect many older solid
state rigs are similar. The test involved a 2-tone test at 1.2 kHz and
2.2kHz, with adjustments made to make both tones equal in level. The values
in the tables are levels of IMDs relative to one of the tones. They are 6dB
better relative to PEP. The ALC was used to set the output.

Here are some results.

12.5V supply.

Power    3rd    5th    7th    9th    11th    13th    15th    17th
2.5W    -28    -29    -41    -47    -49    -53    -62
3.0W    -24    -30    -37    -48    -53    -53    -57    -64
4.0W    -16    -30    -35    -37    -43    -53    -58    -61

At 5W I only measured the 3rd order, as it was only -13dB !

13.8V Supply
Power    3rd    5th    7th    9th    11th    13th    15th    17th
2.5W    -30    -32    -41    -47    -51    -57    -64    -68

10.1V Supply (I.e. NIMH battery)
Power    3rd    5th    7th    9th    11th    13th    15th    17th    19th
2.5W    -18    -29    -31    -40    -51    -53    -54    -55    -59


The results show two things.

1. It is only sensible to extract 2.5W from the FT690. 2.5W set on FM
measures an average power of 1.5W under 2 tone testing, that is 3W PEP.
Under these conditions, the rig is really not all that bad and could be used
to drive a good quality linear amplifier without offending the locals too
much.

2. When running of the internal battery, the IMDs are poor. This is OK as
long as there is no big amplifier following the rig but is a strong warning
for those intending to go out portable with a car battery and a small linear
as the signal will be awful once the battery starts to go flat.




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