[RTTY] K3

[Ian White GM3SEK]

J. Edward (Ed) Muns wrote:


>> I suspect Lyle is generating both PSK31 and FSK in the DSP and not
>> creating external audio.
>
>Yes.  In the 15kHz IF.  One frequency that is shifted for MARK and 
>SPACE.
>
>Ed - P49X (W0YK)
>
>*******************************************************************
>
>There is one frequency that is generated in the 15 kHz IF and 
>MARK/SPACE are created with "phase-continuous frequency shifting".
>
Thank you for the additional information.

>> The increase in bandwidth to 260Hz makes the filter much more practical
>> for RTTY under heavy QRM conditions, without losing its effectiveness as
>> a roofing filter for CW.
>
>Unless you tell the K3 that this 260 Hz crystal filter is, say, 500 Hz 
>so that it switches in with the DSP bandwidth much wider, the cascade 
>effect of the crystal filter and a narrow DSP bandwidth (250 or 300 Hz) 
>will be narrower than 200 Hz.

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.

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.

[...]

>Furthermore, there is no need for a crystal filter that narrow.  The 
>DSP can select 200, 250 or 300 Hz bandwidths and the 370 Hz bandwidth 
>of the so-called 250 Hz bandwidth 8-pole filter is plenty narrow enough 
>to protect the DSP at these narrow DSP bandwidths.
>In fact, there will be no operational difference by using any of the 
>250, 400 or 500 crystal filters for RTTY.  A 260 Hz crystal filter 
>doesn't make any sense for RTTY.

I respect your vast experience, Ed, but that statement is not true for 
the whole world.

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.

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.

The FT-1000MP with two cascaded "250Hz" filters performed very well, and 
was rarely defeated by strong QRM. But on changing to the K3, I 
immediately noticed pumping of the hardware AGC by strong RTTY tones 
that were outside the bandwidth of the DSP but inside the bandwidth of 
the 400Hz roofing filter. It didn't happen often...  but noticeably more 
often than it used to with the 1000MP.

The 260Hz roofing filter has solved that problem, and has restored the 
K3 to the top of the heap.


-- 

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