dansawyeror wrote:
All,

What is the best way to base impedance measurements, by voltage and
current phase or by forward and reflected power?

Measurement or calculation of forward and reverse power don't give you
enough information to determine impedance. There are an infinite number
of impedances which result in any given set of forward and reverse powers.

Likewise, voltage and current phase don't provide enough information.
Again, there are an infinite number of impedances possible for any set
of voltage and current phases.

First I assume both methods provide the data to measure impedance.

No, neither one does. Among adequate sets of measurements which will
allow you to determine impedance are both magnitude and phase of both
voltage and current; or the ratio of voltage and current and the
difference between their phase angles; or the magnitude and angle of the
reflection coefficient.

It would seem that both suffer from the real effects of the taking the
measurement. Measuring voltage suffers from the effects of the reflected
signal, I would assume current does as well. Couplers have a built in
cross talk parameter.

It's much more difficult than most people realize to make accurate RF
measurements. Any measurements have numerous potential sources of error.
If you need accurate results, you should always measure a number of
known impedances approximately equal to the measurement being questioned
to verify that the system is sound. Measuring antennas brings the
additional problems of coupling among the antenna, test equipment, and
you; disturbances from other signals being received by the antenna;
common mode currents; and properties of connecting feedlines.

Roy Lewallen, W7EL

Roy,

Thank you. Yes, I understood both magnitude and phase are required for both.

The question arose after looking into the block diagram of an HP 4805a Impedance
Meter. It measures voltage and current as the basis for determining impedance.

My work with a vector voltmeter has been through using directional couplers and
measuring forward and reflected power.

My question is: Is there an inherent accuracy benefit to one measurement basis
over the other?

Or: Are current and voltage probes inherently easier to build and more accurate
then directional couplers?

- Dan


Roy Lewallen wrote:
dansawyeror wrote:

All,

What is the best way to base impedance measurements, by voltage and
current phase or by forward and reflected power?


Measurement or calculation of forward and reverse power don't give you
enough information to determine impedance. There are an infinite number
of impedances which result in any given set of forward and reverse powers.

Likewise, voltage and current phase don't provide enough information.
Again, there are an infinite number of impedances possible for any set
of voltage and current phases.

First I assume both methods provide the data to measure impedance.


No, neither one does. Among adequate sets of measurements which will
allow you to determine impedance are both magnitude and phase of both
voltage and current; or the ratio of voltage and current and the
difference between their phase angles; or the magnitude and angle of the
reflection coefficient.

It would seem that both suffer from the real effects of the taking the
measurement. Measuring voltage suffers from the effects of the
reflected signal, I would assume current does as well. Couplers have a
built in cross talk parameter.


It's much more difficult than most people realize to make accurate RF
measurements. Any measurements have numerous potential sources of error.
If you need accurate results, you should always measure a number of
known impedances approximately equal to the measurement being questioned
to verify that the system is sound. Measuring antennas brings the
additional problems of coupling among the antenna, test equipment, and
you; disturbances from other signals being received by the antenna;
common mode currents; and properties of connecting feedlines.

Roy Lewallen, W7EL

On Thu, 04 May 2006 06:02:32 -0700, dansawyeror
wrote:

Or: Are current and voltage probes inherently easier to build and more accurate
then directional couplers?

Hi Dan,

Probes have the benefit of being more sensitive, but you pay a price
in a far greater probablity of distorting the measurement. Unless, of
course, you are only looking for an indication which is qualitative
instead of quantitative.

Directional couplers are quite robust, accurate, and in my experience
I've never seen one that fell outside of its tolerance - except at the
band edges.

73's
Richard Clark, KB7QHC

Thanks for the clarification.

The HP4805a isn't an impedance meter. It's a vector voltmeter, which
measures voltage amplitude and phase. As you know, it requires external
directional couplers to measure impedance. (Or you could use current
measurements using current probes in addition to the voltage measurements.)

When you use directional couplers with the vector voltmeter -- or with a
network analyzer for that matter -- you're not measuring forward and
reflected power. You're measuring forward and reflected voltage. As I
mentioned in my last posting, power measurement doesn't give you enough
information to determine impedance.

The answer to your question is that you can't measure impedance at all
using only the voltage probes of the 4805a, and you can't measure it at
all by measuring forward and reverse power with directional couplers. If
you want to compare using voltage *and current* probes with measuring
forward and reverse *voltages* with directional couplers, the former is
probably better when impedances are considerably different than 50 ohms
resistive, the latter better in a near 50 ohm system. Making accurate
measurements with either method is much more difficult than most people
realize, and it helps to have an understanding of what you're actually
measuring. Both methods have numerous sources of possible error which
have to be understood and controlled.

Roy Lewallen, W7EL

Network

dansawyeror wrote:
Roy,

Thank you. Yes, I understood both magnitude and phase are required for
both.

The question arose after looking into the block diagram of an HP 4805a
Impedance Meter. It measures voltage and current as the basis for
determining impedance.

My work with a vector voltmeter has been through using directional
couplers and measuring forward and reflected power.

My question is: Is there an inherent accuracy benefit to one measurement
basis over the other?

Or: Are current and voltage probes inherently easier to build and more
accurate then directional couplers?

- Dan


Roy Lewallen wrote:
dansawyeror wrote:

All,

What is the best way to base impedance measurements, by voltage and
current phase or by forward and reflected power?


Measurement or calculation of forward and reverse power don't give you
enough information to determine impedance. There are an infinite
number of impedances which result in any given set of forward and
reverse powers.

Likewise, voltage and current phase don't provide enough information.
Again, there are an infinite number of impedances possible for any set
of voltage and current phases.

First I assume both methods provide the data to measure impedance.


No, neither one does. Among adequate sets of measurements which will
allow you to determine impedance are both magnitude and phase of both
voltage and current; or the ratio of voltage and current and the
difference between their phase angles; or the magnitude and angle of
the reflection coefficient.

It would seem that both suffer from the real effects of the taking
the measurement. Measuring voltage suffers from the effects of the
reflected signal, I would assume current does as well. Couplers have
a built in cross talk parameter.


It's much more difficult than most people realize to make accurate RF
measurements. Any measurements have numerous potential sources of
error. If you need accurate results, you should always measure a
number of known impedances approximately equal to the measurement
being questioned to verify that the system is sound. Measuring
antennas brings the additional problems of coupling among the antenna,
test equipment, and you; disturbances from other signals being
received by the antenna; common mode currents; and properties of
connecting feedlines.

Roy Lewallen, W7EL

I'm going to take slight issue with what Roy wrote about measurement
using a directional coupler. Though it's true that the receivers could
be considered to be measuring voltage, they are measuring it across a
well known load (very close to 50 ohms). When we set up a system to
test the performance of units we build, it's done very carefully to
calibrate them for input _power_, not voltage. The usual directional
coupler in a VNA is indeed a resistive bridge, with some care taken to
make the known legs and the source driving it and the load observing
the imbalance all 50 ohms (in a 50 ohm system). Generally a bridge
whose impedance is near the impedance you want to measure is a good
choice for a starting point for making accurate measurements, but it's
for sure not easy to build a 10k ohm bridge at 10GHz, for example...

I certainly agree with Roy that an understanding of the measurement
system and the error sources is critical to getting consistently good
measurements. In fact, I have several old ap notes from HP about such
measurements; those ap notes may still be available through the Agilent
web site, but I couldn't guarantee it. NIST (the US National Institute
of Standards and Technology) has done a lot of work in characterizing
errors in measurement, and they are also a source of good (but usually
mathematically deeper) info. Rohde und Schwarz is another source of
VNA and impedance measurement ap notes. It's such a complex subject
that I really wouldn't want to even try to get into it here. And why,
since it's already been dealt with nicely in freely available pubs?

Cheers,
Tom

I gladly defer to Tom, who has a great deal more experience with and
knowledge of measurement devices and techniques than I. My point was
that you need to know magnitude and phase of the measured quantity if
you're going to determine impedance. If all you were measuring was
power, you wouldn't have this information. So you must measure the
voltage (or current).

Roy Lewallen, W7EL

K7ITM wrote:
I'm going to take slight issue with what Roy wrote about measurement
using a directional coupler. Though it's true that the receivers could
be considered to be measuring voltage, they are measuring it across a
well known load (very close to 50 ohms). When we set up a system to
test the performance of units we build, it's done very carefully to
calibrate them for input _power_, not voltage. The usual directional
coupler in a VNA is indeed a resistive bridge, with some care taken to
make the known legs and the source driving it and the load observing
the imbalance all 50 ohms (in a 50 ohm system). Generally a bridge
whose impedance is near the impedance you want to measure is a good
choice for a starting point for making accurate measurements, but it's
for sure not easy to build a 10k ohm bridge at 10GHz, for example...
. . .

On Thu, 04 May 2006 11:49:37 -0700, Roy Lewallen
wrote:

Thanks for the clarification.

The HP4805a isn't an impedance meter. It's a vector voltmeter, which
measures voltage amplitude and phase. As you know, it requires external
directional couplers to measure impedance. (Or you could use current
measurements using current probes in addition to the voltage measurements.)

Whoa, hold on, wait a minute or as we are now required to say in
Southern Arizona, Alto Amigo!

I think that Dan made a typo and meant to say "HP-8415", which is a
vector impedance meter that does use a constant current r-f source and
a voltage measurement to determine impedance.

It's a handy instrument for probing circuits, but it has a limited
higher frequency limit of 108 MHz.

There is one he

http://cgi.ebay.com/HP-4815A-RF-VECT...cmdZVi ewItem

but without the probe, which makes it totally useless.

[snip]

It must have been something in the way that Roy worded it the first
time, but with that concise rewording, I absolutely agree! You gotta
measure phase as well as amplitude if you want to determine a complex
impedance. In no way should my previous post in this thread be
interpreted as going against that.

Cheers,
Tom

Wes Stewart wrote:
On Thu, 04 May 2006 11:49:37 -0700, Roy Lewallen
wrote:


Thanks for the clarification.

The HP4805a isn't an impedance meter. It's a vector voltmeter, which
measures voltage amplitude and phase. As you know, it requires external
directional couplers to measure impedance. (Or you could use current
measurements using current probes in addition to the voltage measurements.)


Whoa, hold on, wait a minute or as we are now required to say in
Southern Arizona, Alto Amigo!

I think that Dan made a typo and meant to say "HP-8415", which is a
vector impedance meter that does use a constant current r-f source and
a voltage measurement to determine impedance.

It's a handy instrument for probing circuits, but it has a limited
higher frequency limit of 108 MHz.

There is one he

http://cgi.ebay.com/HP-4815A-RF-VECT...cmdZVi ewItem

but without the probe, which makes it totally useless.

[snip]


I think the OP really means 8405 which is a VVM. I think he swapped the
first two numbers.

He can tell us if I'm wrong.

Cheers,
John

John - KD5YI wrote:
Wes Stewart wrote:
On Thu, 04 May 2006 11:49:37 -0700, Roy Lewallen
wrote:
. . .
The HP4805a isn't an impedance meter. It's a vector voltmeter, which
measures voltage amplitude and phase. . .

I think the OP really means 8405 which is a VVM. I think he swapped the
first two numbers.

He can tell us if I'm wrong.

Can't speak for the OP, but I definitely meant HP8405a, the vector
voltmeter. I apologize for the transposition, and also to the OP if I
made the wrong assumption about what instrument he meant.

Roy Lewallen, W7EL