[TenTec] 500 HZ Roofing Filter Attenuation-Orion

[Bill Tippett]

Hi Toby,

W4CAK:
 >Pete,  I agree, there seems to be little or no drop except for very weak
signals, signals that don' t move the meter.  I worked for three hours
tonight recentering both filters and think they are set OK. As I said, you
will not notice it unless you compare copying a very weak signal with and
without the filter engaged.

         A few comments on this.  The problems of the 500 Hz filter having
poor IMD has been fairly well documented:

page 16 of ARRL's Expanded Test Report:

Notes:1. On the Orion, the DSP bandwidth was set to 500 Hz, but the first 
IF filter was set to 1000 Hz bandwidth. Although the
optional 500 Hz IF filter (designed for use in earlier Ten-Tec 
transceivers) was installed, it was not used in the ARRL Lab
measurements because the dynamic range was degraded (by 3-5 dB) when it was 
selected. The 500 Hz filter has some
additional insertion loss, and there is an additional amplification stage 
that is used in the receiver to compensate for this loss.
Apparently, that amplification stage is responsible for the degraded 
performance.

http://www.arrl.org/members-only/prodrev/pdf/lab/orion_etrr.pdf

...and on page 5 of YT1NT's paper:

For 700 Hz spaced interfering signals, Sinisa measured the 500 Hz position
having 23-24 dB worse IMD performance than either the 1000 or 250 filters.

http://www.geocities.com/va3ttn/UsingOrionRX.pdf (second table down)

         I no longer have my 500 Hz filter, so I cannot really check what you
are hearing.  However, my guess is that you might be seeing something related
to the IMD issue.  If you're listening to noise floor signals, and have some
S9 signals or stronger nearby, they could be masking weak signals with IMD
products.  However, I would think that would make the noise increase rather
than decrease.

         At any rate, the 1000 Hz roofing filter is always the best one to use
on a crowded band.  If you want better performance, Inrad will be offering
their #762 600 Hz which will plug into the 1000 slot and give even better
performance.  This is a commercial version of the #352 kit which several of
us built earlier this year.  See http://users.vnet.net/btippett/inrad_.htm
and http://www.qsl.net/n1eu/orion/352.htm

         Regarding centering the filters, if you simply tune for a peak, you
may not actually center the -6dB points if the filter is assymetrical.  You
can see this in Inrad's plot of their 250 Hz filter here
http://www.qth.com/inrad/graphs/760.gif where the peak is on the high side of
the center.  This will cause you to have problems if you switch CW passbands.
A better way to center the filters for equal -6 dB attenuation on each side
is to use anaudio DSP program such as Spectrogram (see under Files Related
Elecraft Kits about 80% of the way down this page http://www.qsl.net/n0ss/
...or, much more simply, use a signal generator Orion's 6 dB attenuator and
adjust the offset so that the -6dB points are equidistant from the center.

                                                 73,  Bill  W4ZV