[RTTY] Kenwood IF filters ???

[Joe Subich, W4TV]

> That would be me and the majority of RTTY contesters. I will 
> accept a 6 dB loss on the desired signal if I can attenuate 
> the other station by 20, 30, or 40 dB. It can mean the difference 
> whether you can copy the guy on your frequency or not.

The concern is not so much the loss as the phase distortion 
that occurs when the signal is on the skirts of the filter.  
Phase distortion causes "smearing" of the bit transitions 
much like multipath and is a particular problem when the 
desired signal is closest to the noise floor.  

The difference  between 400 Hz with an effective shape factor 
of 1.6 (cascaded 400 Hz) and 250 Hz with an effective shape 
factor of 1.8 (cascaded 250 Hz) at -30 or -40dB is less than 
the width of an RTTY signal.

If you want something sharper than the cascaded "400 Hz" pair, 
replace the 8.83 MHz filter with the "250 Hz" filter which is 
really 367 Hz according to the curves on the INRAD site.  The 
combination of 367/450 Hz should give a well behaved 350 KHz 
"nose" with a sharper roll-off than the 460/450 pair.  

73, 

   ... Joe, W4TV 
 


> -----Original Message-----
> From: rtty-bounces at contesting.com 
> [mailto:rtty-bounces at contesting.com] On Behalf Of Robert Chudek - K0RC
> Sent: Thursday, February 18, 2010 7:44 PM
> To: Joe Subich, W4TV; 'David VE3VID'; 'RTTY contest group'
> Subject: Re: [RTTY] Kenwood IF filters ???
> 
> 
> Joe is correct on the calculated characteristics of cascading filters.
> 
> An additional consideration is actual on-air circumstances 
> that you will 
> encounter while working a RTTY contest. Stations pack 
> themselves into the 
> available spectrum tighter than a can of sardines. This was 
> the case last 
> weekend with the huge participation in the CQWW WPX RTTY contest.
> 
> What you will find in this environment is one fellow calling 
> you that is +20 
> dB over S-9 but there's another station that is calling 
> someone else, maybe 
> 200 Hz away. His signal is also +20 dB over S-9. This would 
> be the situation 
> where Joe says "some [RTTY operators] use them [250 Hz 
> filters] and accept 
> the additional loss".
> 
> That would be me and the majority of RTTY contesters. I will 
> accept a 6 dB 
> loss on the desired signal if I can attenuate the other 
> station by 20, 30, 
> or 40 dB. It can mean the difference whether you can copy the 
> guy on your 
> frequency or not.
> 
> Outside of contesting, when stations have plenty of room to 
> spread out, 
> using 250 Hz filters is not a critical tool that will make or break a 
> contact. The wider bandwidth will reduce the chance of 
> decoding errors 
> because you are not "giving up" part of the signal.
> 
> 73 de Bob - KØRC in MN
> 
> 
> ----- Original Message ----- 
> From: "Joe Subich, W4TV" <lists at subich.com>
> To: "'David VE3VID'" <ve3vid at hotmail.com>; "'RTTY contest group'" 
> <rtty at contesting.com>
> Sent: Thursday, February 18, 2010 5:48 PM
> Subject: Re: [RTTY] Kenwood IF filters ???
> 
> 
> 
> > Is my logic correct?  RTTY signals are "supposed" to
> > be 170hz wide, so a 250hz filter would be optimal...... 
> > yes/no.......(circle one hihi)
> 
> No.  The RTTY shift is 170 Hz but there are keying
> sidebands that are necessary to proper decoding.  As
> a rule of thumb you can add the shift plus three times
> the baud rate to get the bandwidth of an FSK signal.
> 
> For 45.45 baud/170 Hz shift RTTY that works out to just
> about 300 Hz.  Since the cascaded 250 Hz filters are
> supposedly -6dB at 250 Hz, that makes them just a bit
> too narrow for RTTY (although some use them and accept
> the additional loss).  Cascaded 400 Hz filters would
> be better on a theoretical basis ... both tones and
> the keying sidebands will be within the "flat" part
> of the filter response and not down on the skirts.
> 
> 73,
> 
>    ... Joe, W4TV
> 
> 
> 
> 
> 
> > -----Original Message-----
> > From: rtty-bounces at contesting.com 
> [mailto:rtty-bounces at contesting.com] 
> > On Behalf Of David VE3VID
> > Sent: Thursday, February 18, 2010 4:30 PM
> > To: RTTY contest group
> > Subject: Re: [RTTY] Kenwood IF filters ???
> >
> >
> >
> >
> >
> > Hmmmm ???  Okay.  I am using MMTTY.  WPX last week was my first 
> > contest.  I wasn't aware MMTTY did filtering.  My TS-450s has the 
> > stock filters for SSB:  6khz in the 8.83mhz passband an 2.4khz in 
> > 455khz IF.  Obviously those filters are frustrating for RTTY.
> >
> > Show of hands please from users with TS-850s & TS-450s - 2 x 400hz 
> > filters for both IF stages, or 2 x 250hz filters in both.  
> Is my logic 
> > correct?  RTTY signals are "supposed" to be 170hz wide, so a 250hz 
> > filter would be optimal......yes/no.......(circle one hihi)
> >
> > Al (KE1FO) - why did you find the narrow filters 
> frustrating?  Can you 
> > comment on "MMTTY filtering"?
> >
> > David
> > VE3VID
> >
> >