[Radio Wrangler]

How to test an unknown filter....

Ideally you need a spectrum analyser with a tracking generator, or a network analyser.
You could use a signal generator, if you have a good, stable (probably synthesised) one and some sort of sensitive detector, but stepping through by hand is very very very tedious.

Just connect up and do some scans to find where the passband is. Slow the scans down because narrow filters do not respond quickly and your trace will be 'smeared'

It still won't look right, because they are critically dependent on having the right source and terminating impedance so their response will be all wrong, but at least you now know where to focus your scans in future.

Next you need to fiddle with the source and load resistors. This is where the spectrum analyser earns its spurs. There is ludicrous sensitivity, so you can afford inefficient, resistive termination pads to go from Zo of your test gear (50 ohms) towhere you guess the filter is set for. You also make sure your coax test cables are reasonably well matched.

You make many networks, refining your guess of the filter's design impedance, deciding on whether the filter shape is starting to look like something intentional... minimum ripple is the usual give-away.

I bought dozens of crystal filters at surplus prices in the eighties and had to sort them all out. As a result I have sets of just about all bandwidths I could want for 1.4MHz IF.

If fiddling with R load gets you close to something, but no cigar, mess around with adding a little capacitance.

This is a lot of work, but good fun and educational to boot.

David