Some interesting comments from Bob Sherwood (Sherwood Engineering) speaking
about the Orion II and the ICOM 7800:
The newest narrow roofing filters became 600 and 300 Hz.
Do you really need a 300 Hz roofing filter? Probably not, but it
allows a 1 kHz or 500 Hz dynamic range test.
Personally I think 600 Hz is more than adequately narrow in the real
world.
What was noted when really looking carefully at the dynamic range of
the Orion II at very close 2 kHz, 1 kHz and even 500 Hz test spacings?
The IMD of the radio became limited by the roofing filter itself.
The numbers were great, 95 dB at 1 kHz, but this was somewhat affected
by which filter you had plugged in, and which way you plugged it in.
Was the filter inserted in left to right or right to left?
Which sample of several that I had on hand was I using?
So now we are at the limit of the crystal filter itself, not the
electronics.
The same is true on the 7800 as far as the limit ultimately coming down
to the individual filter in the radio, at very close spacings only.
The IMD in the 3 kHz filter is slightly worse than the 6 kHz filter.
This is not a surprise, but an acceptable trade off in a worst-case
pileup/contest conditions. At least you are keeping more of the QRM out of the
DSP. Call the
roofing filter a DSP protection filter. Now the Orion II is about 15 dB better
in close-in dynamic range at 1
and 2 kHz than the 7800.
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Is this the whole story? Of course not, as there are many other things
about the Orion that are inferior to the 7800. I think the DSP code of the 7800
is superior to the Orion II code.
Both can be flashed, so let the two compete. Are the ergonomics of the 7800
vastly better than the Orion?
Absolutely yes. S2 signals on the 7800 are less noisy, less fatiguing and thus
more
pleasant than on the Orion II.
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There is no doubt that Bob Sherwood has a full understanding of all of the
controls on a radio like the Orion II (I don't think he is suffering from too
high an RF Gain setting or that he doesn't know how to adjust the AGC
settings). It is interesting that he finds the Orion II - more noisy and
fatiguing - then the ICOM 7800. The DSP code, and the algorithms implemented in
the DSP, certainly can make a huge difference in the performance of the radio.
73, Merle - W0EWM
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