Ian, > In the intense QRM conditions of RTTY contesting in Europe, I > routinely use two cascaded 250Hz filters at 8215kHz and 455kHz > in the 1000MP. (Unfortunately the new Inrad website doesn't > include filter plots, so it isn't possible to estimate the > cumulative bandwidths of these cascaded filters.)
The "nasty little secret" is that all of the INRAD "250 Hz" filters in the 8 - 9 MHz range have -6 dB bandwidths around 325-375 Hz. That makes the original analysis accurate since your FT-1000MP is really using cascaded 370/300 Hz filters. The old Inrad site is still available at www.qth.com/inrad see the Yaesu "250" filters at: www.qth.com/inrad/graphs/704.gif (455 KHz) www.qth.com/inrad/graphs/708.gif (8125 KHz) 73, ... Joe, W4TV > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Ian > White GM3SEK > Sent: Friday, May 16, 2008 4:30 AM > To: [email protected] > Subject: Re: [Elecraft] RTTY Filter for K3 > > > Frank Lammel wrote: > >Joe Subich, W4TV schrieb: > > > >> After making extensive tests and plotting the selectivity > curves of > >>the 200 Hz filter in my K3, I'm convinced that it is too > narrow for > >>reliable weak signal RTTY operation, particularly in > conjunction with > >>dual-tone filter. The measured -6dB bandwidth of the 200 > Hz crystal > >>filter was 209 Hz and the composite bandwidth of the 200 > Hz crystal > >>filter, 200 Hz DSP and dual tone fitter was less than 170 Hz. A > >>standard 170 Hz shift 45.45 baud RTTY signal needs at least 235 Hz > >>(170 + 3*22.7) and the dual tone filter represents a 220 Hz > (two 50 > >>Hz filters with center frequencies separated by 170 Hz). After > >>extended discussion with a well know RTTY op and K3 beta tester, I > >>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 > >>bandwidth of about 250 Hz - about optimum for narrow RTTY. The > >>currently available, "250," "400" or 500 Hz filters in combination > >>with a 300 Hz DPS bandwidth and dual tone filter are very close to > >>optimum. 73, ... Joe, W4TV > > >> > >Hi Axel, > > > >full ack with Joe. I use the 400 Hz 8 Pole with a DSP of > about 300-350 > >in dual tone. Perfect! > > > > Many thanks to Joe for that analysis. However, that analysis applies > only to weak signals in the absence of QRM. > > In the intense QRM conditions of RTTY contesting in Europe, I > routinely > use two cascaded 250Hz filters at 8215kHz and 455kHz in the 1000MP. > (Unfortunately the new Inrad website doesn't include filter > plots, so it > isn't possible to estimate the cumulative bandwidths of these > cascaded > filters.) When the channel is clear, copy of weak signals is not > noticeably degraded compared with cascaded 500Hz filters; but > when QRM > appears, the narrower filters often make a decisive difference by > preventing a strong unwanted tone from capturing the AGC. > > A 200Hz 5-pole filter for the K3 is arriving next week, and > then I'll be > able to do some A/B/C testing against the K3's own 400Hz > filter and the > 250+250 filters in the MP. > > > -- > > 73 from Ian GM3SEK > http://www.ifwtech.co.uk/g3sek _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
