[RTTY] K3 RTTY bandwidth already too narrow?

[Ed Muns]

Comments below, in line...

K9OM wrote:
> I understand the desirable effects of  reducing bandwidth... 
> but sometimes I wonder if the K3's transmit bandwidth  on 
> RTTY is already too narrow.

The K3's bandwidth (prior to the DSP-281 firmware, about to be released to
Beta) is on a par with all other radios' internally generated FSK.  And,
this bandwidth is significantly wider than it needs to be.  The wave-shaped
or filtered FSK signal in DSP-281 is an optimum balance between bandwidth
and intelligibility.  In other words, it is possible to be too narrow.
Thus, the new K3 FSK bandwidth occupies only enough bandwidth as needed for
reliable communication.  Hopefully, other manufacturers will follow suit.

> Case in point: I enjoy operating a lot of  RTTY contests, but 
> even when I'm running 1,500 watts... I continually have  
> stations that move in real close to me and cause me a  
> tremendous amount of received interference.

I suspect this is not due to the (current) K3 FSK signal being inherently
narrower than the other station's radio, but more likely due to improperly
adjusted AFSK or an improperly-driven linear amplifier.  If the other
station properly adjusted his transmitting system I think you'd find about
the same bandwidth in your signals.  And, perhaps the other station chooses
to tolerate your QRM more than you choose to tolerate his!

> (I often have to 
> QSY  which is frustrating when I've got a nice run taking 
> place)  And that's when I'm running my receive filters tight, 
> such as: 250hz roofing filter with DSP filter set at 350hz.  
> (though I prefer to use my 400hz roofing filter with a 400hz 
> DSP setting as it copies signals better)

Two points here:

1.  The K3 KFL250A is actually 370 Hz wide at the -6 dB points.  The KFL400A
is 435 Hz.  There is no reason to have both filters in the same receiver.

2.  The IF bandwidth is a function of the cascaded bandwidths of the crystal
and DSP filters.  For example, the resulting bandwidth of the KFLA250
(actually 370 Hz) and a DSP of 350 Hz, will be something less than 300 Hz.
The KFLA400 (really 435 Hz) and DSP of 400 will be nearly 300 Hz.

> So it seems to me 
> that: 1) either their receiver selectivity  is better then 
> that of my K3... which is unlikely,

They may also be using a K3.  There are more than 7000 out there!

> or 2) my K3 is already  
> transmitting a much cleaner signal then theirs.

Not yet, unless the other station is mis-adjusted or defective, at least for
the vast majority of RTTY radios in use.

> If my K3 
> transmit signal  is already much cleaner then theirs, then 
> I'm going to receive even greater QRM  if I narrow my K3 
> transmit bandwidth further.  Which is why  I sometimes wonder 
> if my K3 transmit b
> andwidth isn't already too narrow.

Yes, it is true that if you transmit a substantially narrower signal than
your neighbors on the band, that you are subject to more QRM from them than
they are from you.  This puts pressure on other manufacturers to follow suit
and narrow their transmitted signals down to the optimum needed for reliable
communication.  This is better than allowing the K3 to be adjusted wider so
as to "defend" your  run frequency.  ;>)

> Again, I appreciate the 
> effort to 
> reduce bandwidth as it's a good  thing, but more effort needs 
> to be made for this to happen on a  global scale. 

Absolutely.  A parallel history exists with CW bandwidth (key clicks) across
various manufacturers' radios.  Elecraft rightly chose not to allow user
adjustment of keying rise/fall times such that key clicks can be created.
They are about to do the same for FSK bandwidth by narrowing the K3 FSK
bandwidth to only what is needed.

Viewing K0SM's excellent work, it is easy to see that higher power FSK
signals are disproportionally worse than low power.  This is because the
skirts of the unfiltered FSK transmissions are not linear, but flare out
considerably.  Properly filtered FSK bandwidths are similar to good, and
properly adjusted, AFSK bandwidths.

Ed W0YK