[Elecraft] RTTY Filter for K3

[Joe Subich, W4TV]

> It seems like Inrad may be using -3 dB for some of their 
> measurements. The corner of the bandpass is covered by the 
> data, but it looks very much like that 455 kHz 300 Hz filter 
> is 300 Hz wide at the -3 dB points. 

Collins specified their mechanical filters at -3 dB.  However, 
crystal filters in amateur literature have been specified at 
-6 dB for at least 30 years.  "Shape factor" has been the 
ratio of -6 to -60 dB bandwidth for at least as long.   

73, 

   ... Joe, W4TV 
 




> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ron 
> D'Eau Claire
> Sent: Friday, May 16, 2008 7:40 PM
> To: elecraft at mailman.qth.net
> Subject: RE: [Elecraft] RTTY Filter for K3
> 
> 
> One thing I don't see mentioned much - something that makes 
> ALL the difference in the world - is exactly *where* the 
> bandwidth is measured on the response curve. 
> 
> Years ago I saw a lot of bandwidth measured at the -10 dB 
> points (that was probably before many K3 owners were born - I 
> haven't seen it for a long time but it was pretty common at 
> one time). 
> 
> Then a *lot* of bandwidths were specified at the -3 dB or 
> "half power" points. 
> 
> Now -6 dB seems popular. 
> 
> It seems like Inrad may be using -3 dB for some of their 
> measurements. The corner of the bandpass is covered by the 
> data, but it looks very much like that 455 kHz 300 Hz filter 
> is 300 Hz wide at the -3 dB points. 
> 
> Ron AC7AC
> 
> 
> 
> -----Original Message-----
> 
> 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 
>