[R-390] Testing Collins Mechanical filters:

[John Kolb]

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