[RTTY] 300hz or 500hz IF filter?

[Ian White]

Chen wrote:
>
>Think of the crystal or audio filter as being a "roofing" filter
>for the sound card, and that will be the right way to think about what
>crystal filter to use.
>

That is correct, but Chen's previous sentence ("Any hardware filter
should only be narrow enough to keep the sound card from clipping")
overlooks many other problems that a narrower hardware filter can help
to avoid.

We cannot ignore the problems of QRM. When trying to copy a fairly weak
RTTY signal in a heavily occupied band, an over-generous hardware (IF)
bandwidth will often let through single tones from strong nearby
signals, which activate the receiver AGC and suppress the wanted signal.


So my answer to the OP is: it depends strongly on your personal
operating habits. If you often experience problems of "AGC capture" by
nearby strong signals, then by all means consider a narrower crystal
filter. Understand that in rejecting close-in QRM, you may also have to
accept some degradation on copy of weak signals... but you may still end
up ahead in the game.

Also understand that a narrower filter requires greater tuning accuracy,
both on your part and by the stations calling you.

Another important point is that the "bandwidth" stated by the filter
manufacturer is often incorrect or misleading. You need to examine the
actual passband plots and determine the bandwidth for yourself, taking
into account the narrowing effects of any other filters within your
transceiver.

In terms of practical experience, here in NW Europe I can generally
operate RTTY contests on the higher bands with an Inrad "400Hz" filter
(actual bandwidth about 430Hz at -6dB) without undue problems of QRM or
AGC capture. But night-time on 40m is a very different story - a wall of
intense QRM from the large numbers of single-hop signals out of Central
Europe. For that situation I often need to use a 260Hz crystal filter.
The narrower bandwidth requires a little more care in tuning, and I know
there is some loss of readability of wanted signals; but those problems
are completely outweighed by the advantages. With the narrower filter, I
can beam straight into that Great Wall of QRM and still copy weaker
stations in the Far East and Oceania that would be totally unreadable
with the 400Hz filter. 

One more data point: I had also found that a filter of about 220Hz
bandwidth was too narrow, requiring extremely precise tuning to avoid
distorting the RTTY signal. So close to the theoretical limit of
bandwidth those extra few tens of hertz made a big difference to the
usability of the filter; 260Hz proved usable where 220Hz was not.
(Passband shape also makes a difference, but few of us are able to
evaluate those effects separately from the bandwidth.)

As I say, it all depends on the user's operating habits. If any of the
OP's operating involves intense QRM, then narrower bandwidths are well
worth considering.


73 from Ian GM3SEK