W0YG wrote:
>The ARRL review indicates that by manually selecting the 500 Hz filter, the
radio noise comes up appreciably. Mine does the same. Any ideas as to why?
The noise goes back down when I select the 250 Hz roofing filter????
This is due to overcompensation of gain when the 500 and 250 Hz
filters are switched in. The 500 Hz filter has nominal insertion loss
of 7.5 dB and the 250 has nominal loss of 14.0 dB according to Ten-Tec
specs on their filters (P/N 74089-C). Apparently they compensated gain
for +12 dB which makes the 500 Hz position ~5 dB high and the 250 Hz
position ~2 dB low compared to the 1000 Hz position. Ten-Tec is aware
of this issue because we exchanged messages in early October when I was
getting some strange results because of a defective 250 Hz filter.
It WAS very interesting to me that apparently IMDDR3 was degraded
by the additional amplification stage for the 500 Hz roofing filter (per
ARRL's review). By the way, there are also some very strange gain
increases in the DSP BW around 120-100 Hz. Tune in a steady signal
and slowly decrease DSP BW from 150 to 100 and you will see what I mean.
I have also notified Ten-Tec about this and know I am not the only one
to notice it.
73, Bill W4ZV
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