N4TZ wrote:
>I see that INRAD is taking orders for the OMNI VI roofing filter kit.
The performance measurements on their site show that, the unmodified
stock OMNI VI has better intermod performance at 2kHz spacing than the
FT1000MPs modified with the similar INRAD kits (79 dB DR for the
stock OMNI VI, vs 71 for the FT1000MP (79 for the more expensive
FT1000MP Mk V after modification). But, the modified OMNI VI showed
94 dB DR with a 600 Hz roofing filter, 85.5 dB with a 2.4 kHz roofing
filter.
You are seeing the difference between a 4 kHz filter
(MP mod) versus a 600 Hz (Omni mod). At 2 kHz spacings, the MP
has one interfering signal falling inside its +/- 2 kHz passband,
but at the same spacings, all signals fall well outside of Omni's
+/- 300 Hz passband.
Based on Sherwood Engineering's measurements of Orion
using the standard 1000 Hz roofing filter, 2 kHz IMD was 92-93 dB
depending on the unit tested (his table says 93 dB but his Dayton
paper says 92 dB). To compare apples to apples, Sherwood's tests
using the 600 Hz Inrad #762 were "6-7 dB improvement at 2 kHz and
closer to 10 dB at 1 kHz spacings". This would imply Orion's 2 kHz
IMD with the #762 (same 600 Hz BW as the Omni #763) would be in the
neighborhood of 98-100 dB (92-93 plus 6-7 equals 98-100).
It will be interesting to see published results for
the Omni and Orion 600 Hz filters by an independent third party.
I'm sure there will be 2 Ten-Tec rigs at the top of that chart
but it will be interesting to see in which order they fall. My
guess is Orion will be a few dB better based on Sherwood's results
above, but it is really impossible to compare results between
different testers using different equipment. This is one of the
real benefits of ARRL, RSGB and Sherwood publishing results using
internally consistent test methodologies by the same tester.
73, Bill W4ZV
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