[R-390] Filter test

[Bob Camp]

Hi

The capacitance of the input amp plus the capacitance of the cable is the issue here. Scope probes get around this by putting a bit of capacitance in parallel with the series resistor. Two feet of RG58 should be about 50 pf. If the scope input is about 25 pf you have a net of 75 pf. 

That will give you about 4666 j ohms at 455 KHz. Your 100 K ohm resistor will give you about 27 db of loss into the "detector". That plus the 25 db of the filter comes out to right around 50 db.

Bob

On Feb 23, 2013, at 6:08 PM, quartz55 <quartz55 at hughes.net> wrote:

> The input should be good, just a 50 ohm line to the N connector feeding the .01uF and the 100K ohm in series to the input of the filter and the associated caps. I'm thinking that should be good.
> 
> The output of the filter is loaded with a 100K resistor, per the testing instructions.  The output clips are attached to about 2' of RG58 to the input of the SM scope which has a 1Meg input.  I'm not sure I have the right vari caps though.  When I get the filter back together, I'll be more careful to see if I can get it to resonance.  So there's really no 'detector', the SM scope is just looking at the 455KHz out across 100K ohms as far as I can tell.  Now there may be some capacitance in the RG58 that could mess with it.  I could use a scope probe if that would make any difference?
> 
> I could also ground one side of the output to the case which I really have only done on the input side.
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