> While making some measurements with an MFJ-259B impedance
> analyzer this evening, I noticed some strange behavior. The burnt
> plate choke I was sweeping looked like a Hi-Z (Z>650 ohms) over
> most of the HF spectrum except for the region c. 16 MHz, where
> the choked looked something like 200 + j 500. Thought that this
> might be a problem with the analyzer, so I removed the choke and
> replaced it with a 330 ohm carbon resistor. The 259B read c. 330 Ohms, R +
> j 100 over most of the HF spectrum. This seems a little strange as I would
> expect the reactive component to vary linearly with frequency. When I
> replaced the 330 ohms resistor with a 68 ohm resistor, the readings became
> more believable (e.g. ~68 + j 2). Any idea what could be causing this
> constant reactive offset at high R levels?
The 259B uses a 8 bit A-D converter. Even one or two bits error in
detector voltage can cause large reactance indications as you
move away from 50 ohms.
You can rock back and forth between the Gate and Mode buttons
rapidly when you first turn it on, and go into a test mode. The test
mode steps through some voltage measurements with the Mode
button and you wind up with a Vz Vs Vr display that reads bits as
a decimal number from zero to 255. Vr is reflected voltage.
A 50 ohm load should teeter between 127 and 128 bits on both Vz
and Vs.
Alignment is complex since you have a tracking control as well as
a level control on all three functions, and bias needs to be set also.
It sometimes isn't done perfectly at the factory, and as little as a
one or two bit error (0.4-.8 % error) can cause the effect you see.
If it is in warranty, send it back for recalibration (and expect it to be
gone for a month). I can send you an alignment sheet, but you'll
need a variety of precision loads to finish the measurements.
73, Tom W8JI
w8ji@contesting.com
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