[Boatanchors] Re: IF Filter testing...

[Barry L. Ornitz]

Jordan Arndt, VE2SWL, asked:

> Hi.. is there a simple method of testing IF filters for 
> throughput and resonant frequency..??
> Specifically mechanical filters, ceramic filters etc..??
> Anyone out there familiar with out of circuit testing of 
> these...? Please  any advice would be greatly appreciated... 
> 73 de Jordan...

Tony, K1KP, presented an excellent response but did not 
mention one important thing.

In a completely unknown filter, finding the proper frequency 
can be difficult.  Mechanical and crystal filters shown a very 
large number of spurious responses outside the passband of the 
filter.  Finding the correct response is sometimes difficult.

I had access to a Hewlett-Packard 3577A network analyzer (5 Hz 
to 200 MHz) when I tested about a dozen "hamfest specials".  
The only ones that were trivial to measure were those marked 
with their frequency - and one special unit from a NMR machine 
(4 kHz center frequency, passband 2 Hz).  If you scan the 
filters over a wide frequency range, you have to tune VERY 
slowly or you will miss the response.  Even once you know the 
center frequency, you have to scan very slowly in frequency to 
get the real response curve. [Remember that what you are 
exciting the filter with is really a frequency modulated 
signal, and FM signals have an infinite number of 
sidebands spaced at the modulating frequency (scan rate).]

Determining what is a real and what is a spurious response can 
be difficult.  Usually the desired response characteristic 
will have the lowest attenuation and a passband shape that is 
most "square."  The filter terminations are important here too 
since if the filter is not terminated properly, you will not 
see its true response.  On new filters, the manufacturer will 
often provide data on the location of the spurious responses 
in frequency, but with an unknown hamfest filter - who knows?

With the Collins mechanical filters, the center frequencies 
tended to be relatively standard, 250, 300, 450, 455, and 500 
kHz - except the filters designed for telephone services where 
a large number of frequencies below 100 kHz were used.  For 
crystal filters, the best places to look for responses are 
between 2 and 6 MHz, and around 9.0, 10.7 MHz and 21.4 MHz.

If the filters are currently installed in surplus equipment, 
the resonant frequencies of any transformers or tuned circuits 
used with the filters are a good hint of the filter frequency.

        73,  Barry L. Ornitz     WA4VZQ     ornitz@tricon.net