[Elecraft] RTTY Filter for K3

[Ian White GM3SEK]

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