Someone responded directly regarding the
Omni's filtering scheme. The optional Inrad roofing
filter is inserted after the 1st mixer and BEFORE the
original 15 kHz roofing filter. The original filter
is not switched out but it basically has few ill IMD
effects because it is preceded by the optional filter
(600 Hz or 2.4 kHz). Here's a simple summary,
where "etc" represents the following stages of the IF
chain:
1. Omni (original):
1st mixer > 15 kHz roofing > etc.
2. Omni (Inrad roofing, either 600 Hz or 2.4 kHz):
1st mixer > Inrad roofing > 15 kHz filter > etc.
3. Stock Orion (for 20k - 1k filters):
1st mixer > 20k - 1k selectable roofing > etc.
4. Stock Orion (for 500-250 Hz filters):
1st mixer > 20k > 500 - 250 Hz selectable > etc.
(this is how I understand it was configured)
5. Orion (using Inrad 762):
1st mixer > 20k - 600 Hz selectable > etc.
6. Orion II:
1st mixer > 20k - 300 Hz selectable > etc.
A couple of issues come to mind:
1. Orion / Inrad 762 and Orion II / 2001 600 Hz (#5 and
#6 above) should have identical IMD performance using
filters from 600 Hz and up since their block diagram is
identical. Orion II may have a small advantage using
the 300 Hz at EXTREMELY close spacings (<300 Hz),
but perhaps not realizable due to phase noise (Orion's
and/or the offending station), clicks, etc.
2. In the #4 case of Orion with the 500 & 250 Hz
filters, it now seems fairly clear to me that the
IMD problems were introduced because the 20k
filter was left inline before the narrow filter stage.
However no one has ever actually determined the
source of the IMD problems so I cannot be sure.
In summary I think the definition of a
roofing filter being the first filter stage following
the 1st IF holds true.
73, Bill W4ZV
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|