N6RI wrote:
>I just sent in my order for my O2. I had a question about which roof
filters to get. I went fo the 1.8 and 300 but I might change this. I
figured that the 1.8 would work well for naroow SSB, and that for CW,
the 300 would be better. My thinking is that the 1k will work for many
situations where the 600 will, but when you need the 300, it will be
nice to have.
Bob, remember that you already have an
adjustable DSP BW which can be set in 10 Hz
steps from 100 to 6000 Hz. The primary purpose
of a roofing filter is to reduce unwanted effects
such as desense (BDR) and unwanted IMD products
caused by very close signals.
The 300 would not be my first choice on CW
for the following reasons:
1. 300 Hz is a little narrow for most of the
operating I do because I often get contest
callers more than +/- 150 Hz from my transmit
frequency (e.g. in contests I sometimes have
stations answer up to +/- 300 Hz). I would
not hear these using a narrow filter.
2. As a roofing filter, the 300 is theoretically
better than the 600 for signals spaced within the
roofing filter. This means a strong interfering
signal at +250 Hz would be rejected by the 300 Hz
filter, but would fall within the 600 Hz filter
and could create potential problems. The problem
is that there are other effects such as TX phase
noise, keyclicks, etc that will override any
improved IMD and BDR RX performance, and you will
never hear the improved IMD and BDR in our real
world of impure signals. This effect happens at
much wider spacings for lesser rigs and you will
often see "*phase noise limited" in ARRL tests
for spacings even as wide as 2 kHz.
In most contests, strong stations will
typically not get closer than 500 Hz, which
means the +/- 300 Hz of the 600 Hz roofing
filter can still perform its job nicely, while
not being too narrow for off-frequency callers.
BTW, the actual BW's of the roofing filters
in Orion (not II) are as follows on CW:
Inrad 762 - 640 Hz
TT 1000 - 1070 Hz
TT 1800 - 1400 Hz (in CW mode only, 1900 in SSB)
My 600 is installed in Orion's 1000 slot (see
reasons on Webpage below...which are now corrected
in Orion II). I do not miss not having the 1000
since my 1800 is actually not much wider than the
1000 in CW mode.
73, Bill W4ZV
http://users.vnet.net/btippett/inrad_.htm
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