The theoretical "necessary bandwidth" of a 170 Hz shift 45.45 baud
signal is around 370 Hz. 250 Hz filters work for the same reason
that a 6 or 10 dB attenuator will often improve copy of the low
bands ... the receivers have excess dynamic range.
In the case of "250 Hz" filters (the Inrad 455 KHz "250 Hz" filters
are really 270 - 280 Hz wide at - 6dB) the "necessary" sidebands
are somewhat down on the filter skirts but hopefully any close in
interfering signals are down much further which allows the receiver
AGC to "pull-up" the desired station. If one had steep skirted DSP
filters, using a 250 Hz bandwidth would not be a good idea - with
sharp filters 350 Hz is about as narrow as one wants to go before
decoder sensitivity starts to suffer (depending, of course on the
detector and filter design).
73,
... Joe, W4TV
On 11/8/2012 8:24 PM, Don Hill AA5AU wrote:
I'm not familiar with the INRAD 708, but 250 Hz on RTTY is not too narrow.
I use the 250 Hz filter in my PRO III and my Kenwood TS870's 100% of the time
on RTTY. When I was test driving the IC-7600, I
actually was able to use the 200 Hz filter in that radio on RTTY one time to
copy when QRM was invading into the edge of the
passband of the 250 Hz filter. And it worked. I wouldn't recommend 200 Hz for
RTTY, but 250 Hz should be fine.
73, Don AA5AU
-----Original Message-----
From: RTTY [mailto:rtty-bounces@contesting.com] On Behalf Of Stein-Roar
Brobakken
Sent: Thursday, November 08, 2012 5:28 PM
To: rtty@contesting.com
Subject: [RTTY] RTTY Filter INRAD filter # 708 250 Hz
Hi
I wonder if anyone have had success of using INRAD #708 for use with RTTY for
Yaesu ?
A friend of mine says it will be to NARROW, but I guess it will be ok as it
works with 250Hz filter on my Kenwood TS850.
PM to la6fja(a)nrrl.no for your opinion.
73s LA6FJA Rag
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