>Message text written by "Steve Thompson"
>
> Mine certainly offends with a nasty little overshoot of a few 100 us on
> rising edges whenever there is any ALC action. It depends on the excess
gain
> in the transmit chain and varies with power and frequency. When wound down
> to drive a transverter, it makes the rig too dirty to use in our contest
> station with spitching covering about 30kHz. Over 24 hours, the gain
varies
> somewhat and it's asking for trouble to have all the ops keep tweaking the
> mic gain.
>
> The problem is that the ALC loop response is crudely slowed with a
> capacitor, presumeably to keep it stable. One day I'll find some spare
time
> to try and improve it.
Hi Steve et.al
I find it very interresting that You report problems with the FT990.
Recently
we had a serious problem here with an amateur that started using his FT990
as a backend for a 144MHz transverter.
When the power was turned dovn to 2W for the tranceiver the phasenoise from
the 990 was big enough to put out a -60dBc. With a 1000W PA that resulted
in S9 broadband noisefloor covering several MHz!
With 100W/2W in/out from an attenuator/dummyload the noisefloor was lowerd
the equivalent 14dB which made it possible to tolerate with good antennas
pointing away from him.
Tests with a spectrunanalyzer pointed out the FT990 as the offender. During
keydown-
no modulation the 28MHz signal had a 60db S/SN relation and with modulation
the ratio
improved with the rising signal. The problem may be related to the internal
AGC/ALC.
He sold the FT990 immediately.
Now, a -60dBc noisefloor may be a very small problem on HF, but on VHF and
up
I believe that -90 dBc for an avarage station is OK, and -100 to -110dBc for
a "contest-
grade" station will do.
Any comments?
73 de OZ5TG, Verner
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