[Collins] Collins filters...

[Dr. Gerald N. Johnson]

On Mon, 2009-06-08 at 10:18 +1000, Pete Williams wrote:
>   G'day.... I'm doing filter response curves of a selection of Collins stand
> alone  mechanical filters ---- 250, 500 and 455kHz  using my HP
> 8601/8601Asweeper combination.
> Display is on a oscilloscope via a diode detector.
> I would appreciate knowing how best to terminate and or resonate  input and
> output to achieve a more realistic response......a non terminated
> input/output shows ringing which doesn't appear to be  function of too fast
> a scan rate.  

Slow down the sweep rate. 60 Hz is plenty fast to lead to ringing. 10 Hz
would be better, and may still show some artifacts of ringing on the
departing frequency side of the sweep.

Typically the standard mechanical filters are terminated with 100K in
parallel with about 120 pf. The capacitor is often variable and when
adjusted for maximum response, tends to reduce the passband ripple of
the filter. That's a parallel connection, they can be series resonated
with that same 120 pf and work decently in a 50 to 500 ohm circuit.

> Collins must test them... question is how   !

SLOWLY! The vintage filters ring like a bell. They turn lightning clicks
into long crashes. They turn power line noise into a continuous signal
out of the receiver IF. Adjusting the termination probably won't affect
the ringing at all.

A ten second sweep time is not unreasonable to get it the same both
sweeping direction. Thats ten seconds in the passband.

> Thanks 
> Pete VK3IZ
> 

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