[RTTY] 300hz or 500hz IF filter?

[Ed Muns]

Since this thread is approaching infinite length, I guess my post won't
extend it much.

While I wouldn't try to debate the technical aspects of this topic with the
likes of Kai, Andy, Joe and Chen, I can share my experience with receive
bandwidth for RTTY operation.

First of all, one has to be clear about what the actual receive bandwidth
is.  Choosing a 500 Hz or 300 Hz filter will certainly affect the bandwidth
but other filtering in the receive chain needs to be considered to know what
actual receive bandwidth results from the cascade effect of all the
filtering.

Second, the marketing name of a filter, e.g., "300 Hz" may not be the actual
-6dB bandwidth.  Many of the INRAD filters are wider than their marketing
name.  The 8 MHZ IF filter labeled "250 Hz" is actually 380 Hz wide at -6dB
because it was first used in the 1st IF of YAESU radios where another "250
Hz" filter was used in the 2nd IF of 455 KHz.  Both filters needed to be
wider than 250 Hz so that their cascaded bandwidth came down to 250 Hz,
which was the most common use case.  When an INRAD, or Elecraft branded,
"250 Hz" roofing filter is installed in a K3 radio, the actual bandwidth of
380 Hz needs to be considered with the DSP bandwidth in order to know the
actual IF bandwidth of the radio.

Third, for many years when my primary RTTY decoder was MMTTY, I found that
narrow radio bandwidths, e.g., 250-300 Hz, worked best in most contesting
scenarios.  When QRM was extreme, 200-250 Hz decoded a bit better.  I
strongly advocated 250-300 Hz for RTTY contesting.  Still, there were times
when a signal was obviously very strong, yet decoding was difficult.

Fourth, when I tried opening up the bandwidth on difficult decoding
situations, many times decoding improved.  When 2Tone came along I found
that a 500 Hz receiver bandwidth decoded much better than a narrower
bandwidth, even in high QRM and pile-up conditions, most of the time.  I
very seldom reduce receive bandwidth below 500 Hz, even with extreme
QRM/pile-ups.  So, what decoder you use will effect the optimum receive
bandwidth in the radio.

Fifth, there are other parameters, besides receive bandwidth, that impact
decoding integrity.  One of the biggest is the receive chain gain for a
given situation.  The RF Gain control, pre-amp settings or attenuation
settings all effect the ultimate receive gain and decoding accuracy.  Then
there is the AGC settings which vary greatly between radios.

Because of all these variables, most of the posts in this thread are "right"
(even though they may seem to be contradictory) for a given set of radio and
signal conditions.  This has been an excellent discussion and should provide
ideas for receive improvement in all of our stations.

73,
Ed - W0YK
-------------------------
Ed Muns
www.w0yk.com 

 

> -----Original Message-----
> From: RTTY [mailto:rtty-bounces at contesting.com] On Behalf Of 
> David VE3VID
> Sent: Thursday, August 22, 2013 2:38 PM
> To: RTTY contest group
> Subject: [RTTY] 300hz or 500hz IF filter?
> 
> 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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