[Elecraft] RTTY Filter for K3

[Ron D'Eau Claire]

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