Measuring high impedance filters

Nick Kennedy
 

Since I do have a parallel universe version of the PHSNA working, I've been measuring a few things.  Most of my filters I'd already characterized point by point before developing the ability to auto scan.  But I have several commercial filters in my junkbox from hamfests: Heath, Kenwood, Yaesu, ICM etc, which I've not checked.  Some don't specify their design terminating resistance requirements and some are pretty high, like one from Heath that's 2000 ohms.
To match it to my 50 ohm system, I could use some sort of resonated transformer or maybe an L-match but that sounded complicated and might affect the filter's response.  Another way is to use minimum loss resistance pads.  Minimum loss is pretty high when you're going all the way from 50 to 2000 though.  It looks like 22 dB on the input and 22 on the output , so -44 dB total.  That could put the signal down in the dirt, so I need some amplification.  A while back I built a little broadband amplifier with 38 dB gain, using three MMICs in a line.
With this much difference in resistances, you just about don't have to calculate the matching pad.  Just put 50 ohms shunt to ground at the input and output connections, and 2000 ohms series from there to the filter In/Out terminals.
I had a fair amount of trouble with my noise floor while using the amplifier.  Initially it was only -25 dBm but just by moving components and cables around physically I got to -40 dBm.  Still way too high to show ultimate attenuation, but the shape and flattness and 3 dB BW ought to be OK.  I don't know though, this filter is marked 2.1 kHz but I showed about 1.2 kHz.  Is it my measurement method, or is it the filter?  Guess I should try some others.
What about filters that don't reveal their termination resistance requirement?  From what I understand, you  make measurements with various resistance values until the shape looks "right" and say, "that's it".  Sounds like a lot of work to me.
I wonder if we'll be able to do 455 kHz filters with this gizmo?  Hope so.

73,
Nick, WA5BDU

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