On Fri, 13 Sep 2019 at 12:44, Elliot Riley <elliot.j.riley@...> wrote:
Hello All
I find this to be an interesting discussion. I think I follow Dave's
discussion on considering the open, shorts, and loads as very short coaxial
lines that at low frequencies now have a significant R term. This makes
sense to me that these short coaxial lines now as a result have a different
characteristic impedance and the calibration standard no longer provides
the same overall impedance characteristics.
Dave, could you elaborate on your statement: " I guess we get away with
this, since st low frequencies, errors in the calibration standards are
insignificant."
My grammar was poor. If you are working at 1 MHz, where the wavelength is
300 m, with a coaxial short having a physical length of 15 mm, then the
short is 0.00005 wavelengths long. The maximum amount of impedance transfer
at 0.25 wavelengths. Since 0.25 >> 0.00005, that 0.00005 wavelengths is
probably insignificant.
Also, where is the cutoff frequency where one should use an impedance
analyzer over a network analyzer to determine the impedance of a device?
I think you need to define what you mean by “impedance analyser”. I have an
HP 4291B impedance/material analyser which works from 1 MHz to 1.8 GHz.
I believe in “Keysight language”, which is *not* uniformly adopted, an
“analyser” can work at any frequency in its range and has a graphical
display, whereas a “meter”, like my HP 4284A & 4285A LCR meters works at
discrete frequencies and show a numerical value with no graph on the
display.
I know my LCR meters work much better at very low and very high impedance
values than a VNA
One difference between my 4284A LCR meter (20 Hz to 1 MHz) LCR meters and a
VNA, is calibration consists only of open and short, with no need for a
load.
An accurately known value, close to that of a DUT can improve accuracy, but
is not needed
For instance, my friends who work with acoustics don't ask me to borrow a
VNA that works at 10 kHz (not that I have any anyway nor do I think it is
common to find that ) they just go and use their impedance analyzer. I have
always been interested in what fundamentally defines the lower frequency
limitations of VNA's. I hope this question makes sense and I do believe it
is pertinent to the current conversation.
Yes, it is, and I don’t claim to know the answer.
One trick I am aware of to determine the impedance of attenuators with just
a multimeter is to measure at one port with the other port open (call that
R), then short the other ports and again measure the DC resistance (call
that R2). The impedance of the the attenuator is then
Zo=sqrt (R1*R2)
It is conceivable that such a measurement could determine the impedance of
coax at low frequencies.
Thanks.
Elliot
This is all getting out of my “comfort zone”, where I am confident I know
what I am talking about. ?????
Dave, G8WRB.
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...
Telephone 01621-680100./ +44 1621 680100
Registered in England & Wales.
Company number 08914892.
Registered office:
Stokes Hall Lodge,
Burnham Rd,
Althorne,
Chelmsford,
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CM3 6DT,
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