[TenTec] 9Mhz 500 Hz filter

[Zivney, Terry L.]

The loss of crystal ladder filters increases as bandwidth
decreases because the narrower bandwidth filters are
more heavily loaded by the capacitors to ground at the
nodes between the crystals.  Thus, a 4 pole 600 Hz filter
will for sure have more loss than a 4 pole 1000 Hz filter
using the crystals with the same Q.

On the pitch offset, to what do you attribute the problem
you had with the proper INRAD narrow filter:
>> ...  you would likely lose one of
the CW passbands because of the limited range of PBT.  I already
tried inserting my Inrad 400 in the 1000 position but recall only
UCW would work with the range of PBT adjustment.

contrasted with:

>>BUT...you forget that Orion can compensate for any Pitch offset
from 100 Hz to 1200 Hz via software. Pitch, UCW/LCW, etc. are all
perfectly maintained because it is taken care of in software which
Omni cannot do. HOWEVER, Orion's software ASSUMES the 1000 Hz and
above positions are using a CF of 9.001500, so anything else requires
using PBT to correctly center. If you don't maintain the correct CF's
in Orion's IF, you will need to adjust PBT if you go between 1000 and
higher positions to the 250/500 positions which is very inconvenient. If
you don't believe me, try plugging an Inrad 400 or TT 500 into the 1000
slot and you will find you cannot use the radio in LCW (works for UCW
only because of limited PBT adjustment range). Do you really want to be
adjusting PBT by 750 Hz every time you switch between the 1000 and up positions and 500/250 positions?

That's why I suggest using the TenTec prescibed 9.000750 center frequency
and using the TenTec narrow filter slot.  In that case, no readjustment
in PBT is needed when diddling with your DSP bandwidth.  The Orion takes
care of it automatically. Equalizing the gain with the pad on the filter 
board will not degrade the IMD performance if the additional IF amplifier
after the filter is well designed.  I haven't located it on the schematic,
but the one in the OMNI VI looks robust enough.

In the OMNI VI, the 9MHz filter serves two purposes: 1) a roofing filter
for the subsequent 6.3MHz filter, in which case the 4 pole should be
just as adequate as for the Orion, and 2) provides variable bandwidth
with the PBT control as the two filters "slide" past one another.  The
4 pole will be somewhat inferior to the usual 6 pole 500Hz filter for
this purpose, but WAY better than the stock 2.4kHz filter one must use
if you don't buy the accessory narrow 1st IF filter.

I still haven't popped the top of my Orion yet, so haven't inserted any
of the filters from my OMNI VI, pending a decision on which OMNI VI to sell.

Terry Zivney, N4TZ/9