[Collins] Collins mechanical filters.

[Gerald]

On Sun, 2005-12-18 at 22:18 +1030, L.D. Ritta wrote:
> Howdy Folks
> 
> How do you test mechanical filters? I have a few but don't know if they are
> any good or not, is there any simple test?
> 
> 73 Lee.
> 
I use a signal generator and sensitive scope. An RF millivoltmeter or
wideband (like HP 400 series) AC VTVM can do the indication too.
Generally the impedances of mechanical filters are high, but need
tuning. I'd tune the input series resonant with a 100 pf capacitor and
drive that with the low impedance signal generator output. Then I'd tune
the output with a 100 pf parallel capacitor and hook up the scope or
meter probe. Them I can measure the insertion loss and the pass band of
the filter. Its handy to have a counter to check the signal generator
frequency to do a good response curve plot.

An alternative would be to cobble the filter into a receiver and use the
calibrator, the receiver tuning, and the receiver S-meter to compare
loss and to run a response curve. 75S3B would be my first choice for 455
KHz filters.

It would be handier and fancier to use a sweep generator with the scope
for the response curve but the vintage mechanical filters tend to ring
on pulsed inputs so the sweep rate has to be slow, slower than used for
IF transformer alignment. It does work and is probably the test
procedure in the Collins component specifications.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer