[Collins] Collins mechanical filters.

[n7hr]

Recco : Use sig gen at filter passband freq, measure input vs output on dual 
trace scope.
----- Original Message ----- 
From: "Gerald" <geraldj at ispwest.com>
To: <collins at mailman.qth.net>
Sent: Sunday, December 18, 2005 9:57 AM
Subject: Re: [Collins] Collins mechanical filters.


> 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
>
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