[Elecraft] RTTY Filter for K3

[Joe Subich, W4TV]

Ian,=20

> In the intense QRM conditions of RTTY contesting in Europe, I=20
> routinely use two cascaded 250Hz filters at 8215kHz and 455kHz=20
> in the 1000MP.  (Unfortunately the new Inrad website doesn't=20
> include filter plots, so it isn't possible to estimate the=20
> cumulative bandwidths of these cascaded filters.)=20

The "nasty little secret" is that all of the INRAD "250 Hz"=20
filters in the 8 - 9 MHz range have -6 dB bandwidths around=20
325-375 Hz.  That makes the original analysis accurate since=20
your FT-1000MP is really using cascaded 370/300 Hz filters.=20

The old Inrad site is still available at www.qth.com/inrad=20
see the Yaesu "250" filters at:=20
   www.qth.com/inrad/graphs/704.gif   (455 KHz)=20
   www.qth.com/inrad/graphs/708.gif   (8125 KHz)

73,=20

   ... Joe, W4TV=20
=20



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net=20
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ian=20
> White GM3SEK
> Sent: Friday, May 16, 2008 4:30 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] RTTY Filter for K3
>=20
>=20
> Frank Lammel wrote:
> >Joe Subich, W4TV schrieb:
> >
> >> After making extensive tests and plotting the selectivity=20
> curves  of=20
> >>the 200 Hz filter in my K3, I'm convinced that it is too =20
> narrow for=20
> >>reliable weak signal RTTY operation, particularly in =20
> conjunction with=20
> >>dual-tone filter.    The measured -6dB bandwidth of the 200=20
> Hz crystal=20
> >>filter was  209 Hz and the composite bandwidth of the 200=20
> Hz crystal=20
> >>filter,  200 Hz DSP and dual tone fitter was less than 170 Hz.  A=20
> >>standard  170 Hz shift 45.45 baud RTTY signal needs at least 235 Hz=20
> >>(170 + 3*22.7) and the dual tone filter represents a 220 Hz=20
> (two  50=20
> >>Hz filters with center frequencies separated by 170 Hz).   After=20
> >>extended discussion with a well know RTTY op and K3 beta  tester, I=20
> >>believe the combination of a 300 Hz 5-pole crystal filter,
> >> 300 Hz DSP and the dual tone filter will have a composite -6 dB=20
> >>bandwidth of about 250 Hz - about optimum for narrow RTTY.  The=20
> >>currently available, "250," "400" or 500 Hz filters in combination=20
> >>with a 300 Hz DPS bandwidth and dual tone filter are very close to=20
> >>optimum.    73,      ... Joe, W4TV
>=20
> >>
> >Hi Axel,
> >
> >full ack with Joe. I use the 400 Hz 8 Pole with a DSP of=20
> about 300-350=20
> >in dual tone. Perfect!
> >
>=20
> Many thanks to Joe for that analysis. However, that analysis applies=20
> only to weak signals in the absence of QRM.
>=20
> In the intense QRM conditions of RTTY contesting in Europe, I=20
> routinely=20
> use two cascaded 250Hz filters at 8215kHz and 455kHz in the 1000MP.=20
> (Unfortunately the new Inrad website doesn't include filter=20
> plots, so it=20
> isn't possible to estimate the cumulative bandwidths of these=20
> cascaded=20
> filters.) When the channel is clear, copy of weak signals is not=20
> noticeably degraded compared with cascaded 500Hz filters; but=20
> when QRM=20
> appears, the narrower filters often make a decisive difference by=20
> preventing a strong unwanted tone from capturing the AGC.
>=20
> A 200Hz 5-pole filter for the K3 is arriving next week, and=20
> then I'll be=20
> able to do some A/B/C testing against the K3's own 400Hz=20
> filter and the=20
> 250+250 filters in the MP.
>=20
>=20
> --=20
>=20
> 73 from Ian GM3SEK
> http://www.ifwtech.co.uk/g3sek