J. Edward (Ed) Muns wrote:
I agree with Joe's summary. Note that the "250" Hz 8-pole filter is
really about 370 Hz at the -6 dB point. Any of the current "500",
"400" or "250" Hz crystal filters will be fine for RTTY. With any of
them, the DSP can be narrowed to 300 Hz without rolling off the outside
edges of the two tones.
Note that the Dual-Tone Filter for RTTY is cascaded with the crystal
and normal DSP filters and if either of the latter two filters are 250
Hz (at -6 dB) the three-filter-cascade will have significant roll-off
on the outside skirts. This has the undesired effect of moving the two
filter peaks much closer together than the 170 Hz split.
That said, I have been running the 370 Hz crystal filter (aka "250 Hz
8-pole") with 200 Hz DSP and the DTF for several months now in RTTY
contests. While the resultant passband was indeed rolled off as
described, this was overall a net benefit in heavy pileup conditions
because it filtered out the pileup better, producing a clear callsign
quicker, despite the rolled-off passband. In other than heavy pileup
conditions, it is best to keep the DSP at 300 Hz or higher, especially
for copy of weaker signals.
In the future, there will probably be a 200-400 (or 200-500) Hz
variable 5-pole crystal filter that will be ideal to track with the DSP
for high performance RTTY reception, ranging from weak signal to heavy
pileup conditions.
Thanks to both Joe and Ed. I hadn't realised just how wide Inrad's
so-called "250Hz" filter is. 50% wider than nominal is frankly
ridiculous.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
