[Elecraft] RTTY Filter for K3

[Ian White GM3SEK]

Frank Lammel wrote:
>Joe Subich, W4TV schrieb:
>
>> After making extensive tests and plotting the selectivity curves  of 
>>the 200 Hz filter in my K3, I'm convinced that it is too  narrow for 
>>reliable weak signal RTTY operation, particularly in  conjunction with 
>>dual-tone filter.    The measured -6dB bandwidth of the 200 Hz crystal 
>>filter was  209 Hz and the composite bandwidth of the 200 Hz crystal 
>>filter,  200 Hz DSP and dual tone fitter was less than 170 Hz.  A 
>>standard  170 Hz shift 45.45 baud RTTY signal needs at least 235 Hz 
>>(170 + 3*22.7) and the dual tone filter represents a 220 Hz (two  50 
>>Hz filters with center frequencies separated by 170 Hz).   After 
>>extended discussion with a well know RTTY op and K3 beta  tester, I 
>>believe the combination of a 300 Hz 5-pole crystal filter,
>> 300 Hz DSP and the dual tone filter will have a composite -6 dB 
>>bandwidth of about 250 Hz - about optimum for narrow RTTY.  The 
>>currently available, "250," "400" or 500 Hz filters in combination 
>>with a 300 Hz DPS bandwidth and dual tone filter are very close to 
>>optimum.    73,      ... Joe, W4TV

>>
>Hi Axel,
>
>full ack with Joe. I use the 400 Hz 8 Pole with a DSP of about 300-350 
>in dual tone. Perfect!
>

Many thanks to Joe for that analysis. However, that analysis applies 
only to weak signals in the absence of QRM.

In the intense QRM conditions of RTTY contesting in Europe, I routinely 
use two cascaded 250Hz filters at 8215kHz and 455kHz in the 1000MP. 
(Unfortunately the new Inrad website doesn't include filter plots, so it 
isn't possible to estimate the cumulative bandwidths of these cascaded 
filters.) When the channel is clear, copy of weak signals is not 
noticeably degraded compared with cascaded 500Hz filters; but when QRM 
appears, the narrower filters often make a decisive difference by 
preventing a strong unwanted tone from capturing the AGC.

A 200Hz 5-pole filter for the K3 is arriving next week, and then I'll be 
able to do some A/B/C testing against the K3's own 400Hz filter and the 
250+250 filters in the MP.


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek