[R-390] Mechanical filter tests?

[John Kolb]

That was me - I've got a copy of the Collins specification document for the
2 kHz filter, 526-9163-000/009, F455N-20, 1970.  The 4, 8, and 16 should
have similiar characteristics except for filter bandwidths.

The insersion spec is 25 db max.  Doesn't say anything about min or variation
between filters.

I can mail out copies of the spec if I have a limited number of 
requests, or maybe
I can get it scanned at work and have pdf's available.

Ed, your proceedure sounds like it should work fine.  I've been 
thinking of something
similiar - rather than use the scope probe for isolation, I'm 
thinking of a 10:1 divider,
900K over 100K on both sides of the filter from hot to gnd.  On the 
output side, there
would be a 110K resistor in parallel, making a total 100K load.  On 
the generator side,
there would be a 110K series resistor, again making a 100K source driving
the filter.  Variable caps both sides to adjust resonating C.  Connecting the
400EL across the 900K/100K junction then should have hardly any 
effect on the tuning,
avoiding all the retuning as the voltmeter is moved from one side to 
the other.  Was
looking at ebay, but decided I couldn't afford a second 400EL  :)

At 02:37 PM 2/17/2007, you wrote:
>Thanks for all the suggestion on filter testing, both on and off the
>reflector.  Here's what I finally did, which seems to make sense in 
>light  of all the
>comments:
>
>I connected the URM-25 high level output through a 100K resistor to the
>input of the filter, and tacked a 100 pf air variable across the input.
>Similarly, I put a variable cap across the output, along with a 100K 
>load.   I used my
>HP 400EL AC voltmeter, which has an input resistance of 10M, shunted  by 25
>PF.  To decouple the meter more, I used a scope probe at 10X to  connect the
>meter to the setup.
>
>To obtain a reference level, I connected the meter to the input of the
>filter, then adjusted the trimmer cap for a peak.  I then 
>transferred the  meter to
>the output, and peaked the output cap.  Finally, while measuring  the output,
>I repeaked the input to compensate for the effect of the meter in  setting
>the reference.
>
>I measured about 9 dB of insertion loss on my spare filter, reasonable
>ripple in the passband, and a sharp falloff outside the 
>passbands.  I  couldn't
>measure levels of filter rejection because I was too close to the 
>lower  limit of
>the meter. I'm pretty happy with this setup- the filter resonated  as
>expected, the loss is reasonable and the filter shape looks OK.  My 
>only  problem is
>one responder said the loss should be about 25 dB.  I'll have to  try and
>tracl down the original filter specs.
>
>Anyone have any further criticisms/suggestions?  Now I have to decide  if I
>want to go to the trouble of replacing the filter, given that I 
>never use  it.
>Or should I put the spare in the "someday" drawer with my  spare 3TF7s.
>
>Ed  WB2LHI
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