J. McLaughlin wrote:
"Roy Lewallen" wrote in message
...
A very capable engineering colleage of mine mentioned some time ago
that
an AEA he had purchased worked very well except for one thing -- the
reference point appeared to be inside the instrument. He said it was
as
though there were another length of line (about 5 cm as I recall)
inside. For example, measurement of a short circuit would indicate the
amount of positive reactance you'd expect from such a line. This might
not be too important for general purpose HF use, but would be a
problem
with some measurements, especially at high frequencies. It could be
removed by mathematical adjustment of the measurement results, of
course.
I mentioned this problem to the AEA folks at Dayton, not long after it
changed hands, and they might have fixed it. Do you see this
phenomenon?
I saw a similar thing when I spent a few minutes playing with the very
first model of Autek. I don't see this with my MFJ.
Again, the problem might have been fixed, but it's something to look
for, particularly on an older used unit.
Dear Roy:
Very interesting!
It just did not occur to me to test. Now that the EMC book I have
been helping with is finally being printed (over ten years in the
crafting) I shall put an appraisal of the AEA on our list of activities.
It occurs to me that almost all of my use of the instrument has been
below about 10 MHz. At those frequencies, my suspicions probably were
not tripped by a discrepancy of half a degree (or smaller).
I did buy the instrument after at least one change of hands.
Thank you very much for the heads-up. I am shaking my head in
wonder that that property I never thought to check.
Warm regards, Mac N8TT
A few years ago, I was evaluating an AEA-CIA for a possible magazine
review. One of the tests involved a load consisting of some metres of 50
ohm coax terminated in three paralleled 50 ohm chip resistors. When the
frequency is swept, this load walks around the SWR=3 circle on a Smith
chart, giving |Z| values ranging between 16 ohms and 150 ohms with a
progressively rotating phase; or equivalent results in terms of
(R+/-jX). In other words, the test involves only moderately high or low
impedances with no nasty surprises.
The AEA-CIA gave good results as a frequency-sweeping SWR meter, and the
graphical display is unique in this price range; but unfortunately but
it did not give sensible results in the R-X mode (the mode that gives
the "Complex Impedance Analyser" its name). In a frequency range where
the true value of X was falling progressively through zero, the
indicated value came down correctly to about 30 ohms - and then suddenly
jumped to 0.0. The R readings continued to change with frequency exactly
as expected, but the X reading stayed 'stuck' at precisely 0.0 until the
sweep reached the frequency at which X changed sign, whereupon the X
readings started to make sense again. This behaviour was totally
reproducible. Also, the AEA-CIA is also supposed to be able to resolve
the sign of the complex impedance (which it presumably does by changing
frequency and noting what happens to X), but perhaps not surprisingly
this didn't work reliably either. At a constant frequency where the
value of X was nowhere close to zero, the instrument was often unable to
make up its mind about the correct sign.
All these symptoms looked like firmware problems to me. Since R and X
are both computed from the same analog voltage readings, and R was
correct while X was not, the problem had to be in the computation. The
AEA management at the time were quick to respond through the UK dealer.
They sent me schematics, and analog-type mods to try, and even replaced
the entire instrument... which behaved exactly like the one before.
However, they didn't seem to understand what I just wrote above, and
didn't want to go anywhere near the firmware.
In the end, I abandoned the effort and the UK dealer didn't import the
instrument. The magazine decided we should review the MFJ-269 instead -
which handled the same test load with good accuracy.
Sorry, I don't recall what specific firmware versions gave these
problems with the AEA-CIA, and have no information whether they have
been fixed in later versions. As Roy said:
the problem might have been fixed, but it's something to look
for, particularly on an older used unit.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek