[Collins] testing 51J4 mech filters

[kirklandb at sympatico.ca]

One of the 51j-4 manuals should have the selectivity curves that you need.
Typically IF filters are spec for their 6/60 dB points to give a "shape factor".

Ideally you want to terminate the filters with their "proper" impedances, i.e. that
which they were designed for. When we designed SAW filters we traded off insertion loss, pass band ripple and rejection/selectivity. e.g. if we matched the filter for lowest insertion loss, we didn't get the best selectivity or pass band ripple.

How much effort you put into this depends on what you are trying to achieve, e.g.
 -  do you want to know if you filter is broken or usable 
 - do you want to know if you filter is out of spec or not

For the latter, I would suggest loading the filter as in the 51j-4 design, possibly even
duplicating the pre and post amplifier circuits.  For the former, sounds like you all ready
have a circuit that works.

Bill

> From: w2hx at w2hx.com
> To: collins at mailman.qth.net
> Date: Sat, 25 Apr 2009 23:42:22 -0400
> CC: mondial1 at att.net; nerd at verizon.net
> Subject: [Collins] testing 51J4 mech filters
> 
> 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
> 
> 
> 
> 
> 
> 
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