WC1M:
>The Orion has a
famous flaw where it switches in an amplifier to compensate for losses in
the 500 Hz and 250 Hz roofing filters, but the gain is too high and the
dynamic range goes all to heck.
Dick I believe the flaw in Orion was actually routing the
signal first through the 20 kHz roofing filter plus following
12 dB amplifier before the 500/250 Hz filter and the second 12
dB amplifier. IMD products were generated prior to reaching
the narrow filter stage...similar to the problems up-conversion
rigs have using 15-20 kHz roofing filters.
>Hopefully, variable gain will compensate for losses with no other distorting
effects. But even if it does, what's to be gained by using an 8-pole filter
in the first place? Can it be demonstrated that the 8-pole filters improve
selectivity beyond what the IF DSP does? Have tests been run to determine
specific gain settings for each filter offered by Elecraft so dynamic range
won't be compromised?
As can be seen in the following IMD performance, the 8 and 5-pole
filters have similar performance (at similar bandwidths):
Filter 20kHz 10kHz 5kHz 2kHz
200 Hz, 5 pole 100+ 100+ 100+ 95
250 Hz, 8 pole 100+ 100+ 100+ 95
400 Hz, 8 pole 100+ 100+ 100+ 95
500 Hz, 5 pole 100+ 100+ 100+ 94
1 kHz, 8 pole 100+ 100+ 100 94
2.7 kHz, 5 pole 100+ 98 92 n/a
2.8 kHz, 8 pole 100+ 100 93 n/a
http://www.zerobeat.net/mediawiki/index.php/K3_Roofing_Filters
Gain compensation for the K3's 8-pole filters is set in the DSP stage
according to individual insertion loss measurements supplied with each
filter, per the filter installation procedure. Orion's gain compensation
for the 250/500 Hz filters was set much earlier in the IF chain which led
to the problems with those filters. There were no corresponding problems
with the 1.8 kHz 8-pole in Orion because its insertion loss was low enough
not to need additional compensation.
Regarding possible advantages of 8-poles versus 5-poles,
Elecraft claims the 8-pole may have better BDR performance, but I
suspect any such theoretical advantage would be overridden by
transmitted signal issues such as key clicks, phase noise, etc.
73, Bill W4ZV
P.S. Been in 6Y5 this week and *hoping* I have a big box waiting at my
Post Office today (supposedly shipped 11/26).
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