[R-390] Testing Collins Mechanical filters:

[Jon Schlegel]

Frank

Oops.  Guess I should have checked the spec instead of relying on 
memory (thanks, John).  25 dB loss would then translate to about an 
18:1 voltage factor maximum filter input to filter output instead of 10:1.

Jon



At 10:20 PM 4/24/2009 -0700, John Kolb wrote:

>The Collins data sheet for the F455N-20 filter used in the R-390A
>lists the maximum insertion loss as 25 db.
>
>John
>
>
>At 03:58 PM 4/24/2009, Jon Schlegel wrote:
> >Hi Frank,
> >
> >Another way to check the mechanical filters is to actually measure
> >the RF signal going in and out to determine insertion loss.  You will
> >need a Hi-Z AC voltmeter like one of the HP410 model VTVMs or a scope
> >with with a 10x probe to make this measurement.  I think you should
> >probably see an insertion loss of 20 dB or perhaps better.  Assuming
> >the input and output impedances of the filter are the same, this
> >means you should see the ratio of filter input voltage to output
> >voltage of no greater than about 10:1.
> >
> >This is a dicey measurement because of the impedances involved.  If
> >the probe is not high enough impedance, it's loading effect on the
> >circuits could make the measurements invalid.  Also, the trimmer caps
> >may need re-peaked due the extra shunt C that probe presents to the circuit.
> >
> >For reference, I've measured filter impedances in the rather wide
> >range of 10k to 50k.  The standard Collins measurement setup requires
> >a 100k source and load on the filter.  In-circuit, the filter sees
> >the plate resistance of V501 which as I recall is listed at around a
> >1 Megohm and the output sees only the grid input impedance of
> >V502.  It's clear Collins didn't want any more loading on these
> >filters than absolutely necessary.
> >
> >Regards,
> >Jon Schlegel WA3MVM
>
>
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