N4PY wrote:
>Why not just go with the tentec 250 hertz filter?
Carl, I've had cascaded 250 Hz filters in my MP
for over 6 years and I find I seldom use them. 250
Hz is too narrow, especially in contests, because you
will miss many guys who call +/- 500 Hz from your
frequency. Even dual 500 Hz filters are often
too narrow and my normal mode of operation is with
shifted 2.0 kHz filters which give an effective BW
of about 1 kHz (this is on 10 meters...not 160 where
500 Hz is about all you can get away with in a contest).
The only place 250 Hz filters are very useful for me
is for extremely weak signals in noise, but often I
actually can hear those better with a wider BW and let
my ears do the DSP filtering.
Inrad's roofing filters have the following bandwidths:
BW@-6 dB BW@-60 dB Shape Factor
"400 Hz" 455 Hz 955 Hz 2.1
"250 Hz" 315 Hz 700 Hz 2.2
http://www.qth.com/inrad/ and click "Performance Curve"
There is actually only about 140 Hz difference between
them so I think the 400 Hz is a nice tradeoff.
Regarding Mark's question about the relationship
between the crystal roofing filters and DSP bandwidths,
The way I understand Orion works is as follows:
1. When you select a DSP BW, the next higher roofing
filter that is enabled is automatically selected. However,
these MAY be over-ridden. For example if I had the optional
500 Hz roofing filter installed, and set my DSP BW to 400 Hz,
I could manually choose any roofing filter above 400 Hz, i.e.
2400 Hz or 1000 Hz or 500 Hz. Why would I want to do this?
To make the combination a little sloppier to catch those guys
who call off frequency. Off-frequency signals would be a bit
weaker but should be more readable than if I used the narrowest
roofing filter possible. I don't see anything in the manual
about adjusting the DSP shape factor, but that might also
be desirable for some cases. I'm sure the manual is very
basic at this time so it may be possible and I may be simply
missing whether shape factor can be adjusted or not.
2. Orion assumes a 500 Hz filter is in the 500 Hz roofing
filter position when enabled. If my Inrad filter is actually
455 Hz, I probably want to make sure the DSP BW is not set to
more than 450 Hz. I'm not sure if there would be any artifacts
(false signals) of setting DSP wider than the actual analog
bandwidth (roofing filter) but there could be. Maybe someone
at Ten Tec will comment. Remember that the DSP BW's are
continuously adjustable from 300-6000 Hz in 10 Hz steps.
3. The simplest way to think about Orion's filtering is two
distinctly separate stages: analog roofing filters at 9 MHz
followed by DSP filters at audio frequencies. The analog
roofing filter does the heavy lifting by first removing
unwanted signals outside its passband and then the DSP filter
goes to work with fine tuning what's left. The combination
is the best of both worlds of analog and digital filtering.
I don't have my Orion yet but this is the way I understand
Orion's filtering scheme works. Of course I could be all
wet so hopefully someone from Ten Tec will straighten me out!
73, Bill W4ZV
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