Speaking of roofing filters and DR3... I was browsing through the spec sheet
of the relatively new Hilberling PT-8000 "super-rig" yesterday, just to see
how it compares. This very impressive transceiver sells right now for around
15,000 USD. It uses conventional up-conversion to a 1st IF frequency of ~70
MHz, but splurges for a 2.7 kHz roofing filter at that frequency -- not an
easy (or inexpensive) thing to do. As a result, its claimed numbers are about
as good as it gets for this kind of receiver design.
But compare with the K3, with its ~8 MHz 1st IF, at a cost (fully loaded) of
about 3,000 USD:
IMD3 dynamic range
at 2 kHz spacing: PT-8000 86 dB, K3 95 dB
at 5 kHz spacing: PT-8000 97 dB, K3 100+ dB
Because of its 70 MHz 1st IF, a 500 kHz or 200 kHz roofing filter (as in the
K3) is out of the question. And that 9 dB difference is significant. Suppose a
guy a couple kHz up from you running 100W is S9+40 on his own frequency, but
is causing no problems for your K3 copying a very weak signal 2 kHz down from
him. That same guy, if you were using the PT-8000 receiver, might have to QRP
down to as little as 12 watts in order not to cause IMD problems for you, all
other parameters being equal, or QSY up the band another 3 kHz or so --
neither of which is very likely.
I don't expect my K3 until some time in February or March, but I'm sure
looking forward to it. :-) It will be a long wait. I'm really enjoying the
reports you guys are sending in as you receive yours and put them through
their paces.
And you 6M guys, don't forget the winter Es season should be starting any day
now! Check the band out often, especially during early evenings, when winter
Es is more likely to occur.
Bill W5WVO
Bill Tippett wrote:
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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