AC9S:
The question - the manual recommends using a moderately wide CW roofing filter
(1 khz) for general, non crowded, weak signal dx - due to increased loss with
narrower filters. In the QST review of the original Orion, dynamic range
decreased with the narrowest roofing filters - the thought being the
compensating amplification was the culpret. Since Sherwood listed the same
95db dynamic range at 1 khz using the new 4 pole 300 hz filter as it did at 2
and 20 khz with wider filters is the manual's recommendation still valid?
1. - What is the narrowest filter for which there is no compensating amplifier?
As N4PY stated, Orion II solved this problem
by using ALL 4-pole filters in the first bank
without need for a compensating amplifier. In
retrospect, I believe the real problem was that
the original Orion used the following scheme for
the 500 and 250 positions:
20 kHz filter > amplifier > 500 or 250 Hz filter > amplifier
The problem with this scheme is that IMD products
created in the first amplifier stage were passed
to the narrow filter stage, which then could not
remove them since they already existed before
reaching the narrow roofing filters.
2. - Would that be the recommended filter roofing filter for general
weak signal
dxing?
Remember that there are two stages of variable
filtering in Orion. First is the roofing filter whose
main purpose is to prevent IMD/BDR products from
reaching further into the IF chain. Second is the DSP
filter whose purpose is to provide the final signal
filtering. For this reason, unless you are expecting
extremely close-spaced interference (such as in contests,
multi-transmitter environments, etc), you may not need
ANY optional filters. In other words, if IMD and BDR
products are not being generated from interfering signals
within the stock 1 kHz filter, the final DSP stage will
still allow you to narrow the final bandwidth all the
way down to 100 Hz for S/N improvement. What bandwidth
you actually like to use for weak signals is a matter
of personal preference but you have 10 Hz steps from
100-6000 Hz.
3. - With the new 4 pole roofing filters does it make any difference?
Not sure I understand exactly what you meant by
this question, but there is no way you can get into
trouble with any Orion II configuration. Whether you
actually even need ANY optional filters depends on
the severity of close-spaced QRM you expect (#2 above).
The O2 manual is definitely wrong as N4PY stated.
73, Bill W4ZV
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