[Collins] testing 51J4 mech filters

[W2HX]

Hello all, 

About a year and a half ago I put out a query on this list to find out how I
might sweep/test a 51J4 filter to see if its ok or not.

Well, I finally got around to trying it (boy does the time fly). I am using
a sweep generator and a spectrum analyzer.

My question to the group is: When one of these mech filters are rated as
BW=3.1 kHz is this the -3dB bandwidth or -6dB bandwidth? I ask because my
measurements seem to be very close to the stated bandwidth using -6dB, but
further off using -3dB.

My set up is rather crude. I am not resonating the input or output of the
filter, or attempting to match impedances, just in and out. 50 ohm sweeper
in and 50 ohm SA out.

In case anyone is interested, here is a plot of a 1.4 kHz BW filter 

<http://www.hertzmail.com/51j4/collins%201.4%20kHz.gif>

As you can see, this -6dB plot is still a little narrower than 1.4 kHz and
the -3dB is narrower still.

Perhaps if I used the proper input and output impedances, would my results
differ?

Thanks
Eugene W2HX