[RTTY] K3

[Joe Subich, W4TV]

> > For 40m contesting in Europe, many weak and distant multipliers 
> > can only be worked by beaming directly into a wall of extremely 
> > strong QRM. If you want those mults, the receiver bandwidth must 
> > be reduced to the minimum possible *AND* the receiver must 
> > maintain a very high dynamic range at this narrow bandwidth. 
> > The second part of this requirement is where the problems arise.
> 
> Yes, but I'm surprised that a 260 Hz vs. a 370 Hz crystal 
> filter will improve things noticeably.  I'll reserve final 
> judgment, though, until I try it.  I never built up any 260 
> or 280 Hz crystal filters because I didn't believe it would 
> help.  At these minute differences other attributes of those 
> strong signals will both you the same with either crystal filter.

The "interference window" ... that is the difference between the 
250 Hz DSP bandwidth for RTTY and the -30 dB response point for 
the roofing filter is very small.  Using measured values (the 
500 Hz numbers are from the four filters in my two K3s, 8-pole 
values are from http://www.elecraft.com/K3/K3_8_pole_plots.htm, 
and the 260 Hz numbers are a combination of measurements of my 
200 Hz filters with Ian's measurement of his "260 Hz" filter) 
results in the following comparisons:  

  Filter     -30 dB     "window"   Notes: 
  -------------------------------------------
  500 Hz     775 Hz      262 Hz   Stock 5-pole
  400 Hz     520 Hz      135 Hz   KFL3A-400 
  250 Hz     465 Hz      108 Hz   KFL3A-250
  200 Hz     520 Hz      135 Hz   Modified 260 Hz/5-pole 

These values show the relatively small difference among the 
four options ... and insignificant difference among the 
three narrowest options ... if one assumes that -30dB is 
sufficient protection for the hardware AGC/DSP.  The 
additional selectivity would not appear to be sufficient 
to make any difference in IMD DR3 since the "window" on 
each side of the DSP passband would not accommodate even 
one complete RTTY signal.  

Based on this data, the "400 Hz" filter might be a good 
all around choice for RTTY with the (unmodified) 200 Hz 
filter as a narrow option for CW (or critical PSK31) use. 
A 1000/400 Hz filter combination might be a better choice 
than the 400/250 Hz option for any user who is primarily 
interested in digital/RTTY modes.   

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: rtty-bounces at contesting.com 
> [mailto:rtty-bounces at contesting.com] On Behalf Of J. Edward (Ed) Muns
> Sent: Wednesday, January 07, 2009 12:02 PM
> To: 'Ian White GM3SEK'
> Cc: rtty at contesting.com
> Subject: Re: [RTTY] K3
> 
> 
> > Sorry, I had forgotten to mention that. My K3 has a 400Hz 
> filter, and 
> > is configured to use the 260Hz filter at all bandwidth 
> settings below 
> > 350Hz. This gives a reasonably smooth reduction in passband width.
> 
> How is this a "smooth transition"?  Your IF bandwidth goes 
> from 350 Hz to 260 Hz in one step (that would normally be a 
> 50 Hz step). Then it stays at 260 until you get below 200 Hz, 
> or so, on the DSP.  From your next statement, I guess you 
> don't really care about smooth transitions and consistent steps.
> 
> > For RTTY I never use front-panel WIDTH settings below 300Hz, so the 
> > passband width is normally determined by the 260Hz xtal 
> filter while 
> > the DSP handles the skirt selectivity.
> 
> That's fine.  I've just never had a problem with a slightly 
> wider crystal filter and using the DSP for final bandwidth.
> 
> > For 40m contesting in Europe, many weak and distant multipliers can 
> > only be worked by beaming directly into a wall of extremely strong 
> > QRM. If you want those mults, the receiver bandwidth must 
> be reduced 
> > to the minimum possible *AND* the receiver must maintain a 
> very high 
> > dynamic range at this narrow bandwidth. The second part of this 
> > requirement is where the problems arise.
> 
> Yes, but I'm surprised that a 260 Hz vs. a 370 Hz crystal 
> filter will improve things noticeably.  I'll reserve final 
> judgment, though, until I try it.  I never built up any 260 
> or 280 Hz crystal filters because I didn't believe it would 
> help.  At these minute differences other attributes of those 
> strong signals will both you the same with either crystal filter.
> 
> > In this intensely competitive situation it is often 
> necessary to use a 
> > 250Hz bandwidth for hours at a stretch, because very strong 
> stations 
> > are operating close by. Stations that are using 250Hz filters can 
> > coexist quite comfortably. But with wider bandwidths such as 500Hz, 
> > strong nearby tones are highly likely to capture the AGC 
> and de-sense 
> > the receiver.
> 
> I haven't seen this in Aruba with the 370 Hz crystal filter, 
> but like I say above, I'll give the narrower crystal filter a 
> try and see for myself.
> 
> Ed - P49X (W0YK)
> 
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