Jim,
As I have said many times, 250 Hz filters are trading off one set of
SNR compromises for another. Using 250 Hz filters is trading increased
ISI for decreased adjacent frequency QRM and potentially AC "pumping".
Ultra narrow filters work for very strong signals that have a high
SNR relative to the ISI products generated due to differential delay
at the filter knee.
If your goal is to work a bunch of 20 over signals in a contest, the
250 Hz filters may be of some value although others have reported that
such narrow *net* bandwidths provide rapidly diminishing results - to
the point that a net bandwidth of 200 to 240 Hz provides a net decrease
in overall copyability. Even at 270 Hz "net" bandwidth, tuning becomes
extremely critical for stations of modest strength as the slightest
mistuning causes ISI (and selective "fading") to increase significantly.
For most users a wider filter - e.g. 500 Hz - will provide better
results overall than an ultra-narrow filter - particularly if the user
takes steps to avoid receiver generated non-linearity and IMD.
73,
... Joe, W4TV
On 8/23/2013 3:29 PM, Jim W7RY wrote:
Joe...
I have been RTTY contesting for many (20+) years with some of the best
in the business. We ALWAYS used 250 Hz filters during crowed band
conditions.
And we have the wallpaper to prove it.
400 and 500 Hz crystal filters (and DSP filtering) is just not tight
enough. Period.
73
Jim W7RY
-----Original Message----- From: Joe Subich, W4TV
Sent: Friday, August 23, 2013 8:05 AM
To: rtty@contesting.com
Subject: Re: [RTTY] 300hz or 500hz IF filter?
No, the half bit makes the baud rate effectively 90.9 (the shortest
element is now 11 ms) thus the calculation is:
(2 * 90.9) + (1.2 * 170) = 385.5 Hz.
although the actual occupied bandwidth will be dependent on the
information content (how often/how regularly transitions occur will
effect the value of "K" in the previous formula).
Alternatively, observe RTTY signals on-air.
And most FSK signals are 370 Hz wide or more depending on the care
with which the manufacturer has designed the FSK circuits. The only
exception are later versions of the K3 firmware which generate very
clean FSK using DSP.
73,
... Joe, W4TV
On 8/23/2013 8:15 AM, Kai wrote:
Absolutely incorrect. Consult ITU-R SM.1138: BW = 2M + 2DK; D=shift/2;
M = Baud/2 K = 1.2 (typically)
BWrtty=2M+2DK = Baud + shift*1.2 =249.5 Hz
If you consider the effect of the 33 ms (1.5 bit) stop bit, that effect
has a narrower spectrum which is contained entirely within the 249.5 Hz
BW of the 22 ms start and Baudot bits. The shortest element is still
22 ms.
Alternatively, observe RTTY signals on-air.
Kai, KE4PT
On 8/22/2013 10:34 PM, Joe Subich, W4TV wrote:
On 8/22/2013 9:42 PM, Kai wrote:
The theoretical bandwidth of 170 Hz shift 45.45 baud RTTY is just
under 250 Hz.
Absolutely incorrect as 250 Hz does not account for the necessary
modulation sidebands or for the discontinuity (additional bandwidth)
generated by the 1.5 bit stop. Due of the half bit, the necessary
bandwidth for 170 Hz shift RTTY approaches 170 + (2 * 90.9 * 1.2) or
slightly over 370 Hz as the shortest element is now 11 ms.
73,
... Joe, W4TV
On 8/22/2013 9:42 PM, Kai wrote:
The theoretical bandwidth of 170 Hz shift 45.45 baud RTTY is just under
250 Hz.
73
Kai, KE4PT
On 8/22/2013 6:54 PM, Joe Subich, W4TV wrote:
The -6 dB bandwidth of the INRAD "300 Hz" filter is shown as 340 Hz
which is slightly less than the theoretical 370 Hz required for 170 Hz
shift 45.45 baud RTTY.
That said, performance will be a trade off between improved
selectivity and interference rejection - up to a point. If the
receiver can withstand AGC effects of close in interference, a 400
to 500 Hz filter will generally provide better copy than a 300 Hz
filter. Note: no amount of selectivity is useful when signals
overlap or the interfering signal includes distortion (spurious)
products that overlap the desired signal.
73,
... Joe, W4TV
On 8/22/2013 5:38 PM, David VE3VID wrote:
Hello everyoneI would like to outfit my FT-857D portable rig with an
IF filter on its 455khz stage. INRAD sells a suitable 500hz unit.
They also have a 300hz unit. I am leery about the 300hz filter
being too narrow.
Any opinions?
73Davidhttp://www.ve3vid.webs.com/
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