[TenTec] 9MHz 500HZ filter

[Bill Tippett]

N4TZ wrote:
 >International Radio (www.qth.com/inrad) sells a kit with 4 - 9Mhz
crystals and the necessary capacitors to make a 600Hz filter
with 200 ohms termination impedance, which I believe is the same
impedance as the Ten-Tec filters.  This kit, without a circuit
board, is only $24.

         Terry (and Rick), this brings up an interesting point.  The
250 and 500 Hz filters DO have higher insertion loss so Ten-Tec
compensates for this by using an additional stage of amplification.
BUT this also causes the 3-5 dB DR degradation ARRL noted in their
Expanded Test Report (see note at the bottom of page 16) when using
the 500 Hz roofing filter.

         The 250 and 500 Hz positions also use a slightly different IF
of 9.000750 instead of 9.001500 for the 1000 Hz and above positions.
The Inrad kit is actually for 9.000000 MHz as it stands unless you
make a special order (10 minimum).  You could use the 9.0 MHz kit,
but you would have to use Orion's PBT to shift either 750 or 1500 Hz
depending on whether you wanted to put the filter into the "500"
position or the "1000" position, and 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.

         What I would like is to replace my Orion's 1000 Hz filter with
the 600 Hz filter.  The insertion loss is equivalent, so there should
be no gain compensation issues and you should improve DR over the
stock 1000 Hz filter at closer spacings.  If you make the center
frequency 9.001500, everything should center properly without need
for any PBT (except possibly slight compensation of the center since
there is not a compensation adjustment for the 1000 position).  The
cost of this would be $24 plus $10 for the plug-in board Inrad uses
for all Ten-Tec filters.  Although this filter has a relatively poor
shape factor, that is really not critical in Orion beyond about 20 dB
down on the skirts.  The 600 Hz filter has a BW of ~900 Hz at 20 dB
down versus the stock 1000 Hz filter which is ~1070 Hz at 6 dB down,
so there should be some noticeable improvement in dynamic range
performance at very close spacings.

         I thought about ordering 10 kits centered on 9.001500 myself
and trying to sell the other 9.  However, if there are 9 other Orion
owners who would like to try this, please reply direct and I will be
broker if we can collect 10 orders.  The total cost should be ~$40
for the kit, the board and shipping via First Class.  If I get 9
others interested, I will then ask for your check before placing the
order with Inrad.  BTW, W2VJN says the kits are simple to build,
instructions are included and you don't need a special soldering iron
to handle the SMT capacitors.

         The kit description and filter plots are here respectively:

http://www.qth.com/inrad/exp_filt.htm (no engineering fee per W2VJN)
http://www.qth.com/inrad/graphs/350.gif

         If any Orion owners are interested, please let me know off-list,
and I'll let you know if we get sufficient response.

                                                 73,  Bill  W4ZV

P.S.  Another alternative is to order kits for 9.000750 and use them
in the "500" position.  The problem here would be that the 600 has
about 6 dB less insertion loss than TT's 500 or Inrad's 400, so gain
would be over-compensated, likely resulting in even more degradation
in DR than there already is.  I'm not interested in this alternative
myself but only mention it in case someone else is.