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DD

Dr. David Kirkby (Kirkby Microwave Ltd)

Sat, Aug 16, 2014 7:03 AM

I am looking to measure as accurately as possible the DC resistance of a
number of types of RF loads - all around 50 Ohms. Types of interest include
SMA, 3.5 mm, N, APC7, 7-16 & BNC. I want to see if the difference between
the actual resistance at DC and 50 Ohms correlates with the performance at
RF as measured on a vector network analyzer. I want to do both sexes, with
the exception of the APC7.

I would expect RF performance to correlate well at low frequencies, but as
the frequency is increased I would expect it not to correlate.

I am looking for advice on the best way to do this. I have a HP 3457A (6.5
digit DVM with 4-wire resistance capability), but will consider purchasing
a meter designed for low resistance measurements which I believe uses AC to
avoid thermal EMF issues. But if possible I would rather use the 3457A.

I was thinking of a plastic box with the RF connectors & 4 banana plugs. I
suspect running the sense and drive wires back to the banana plugs would
work. No signicant current would flow into the unterminated connectors as
they would have just the dielectric.

Ideally I would like to measure just the load and contact resistance and
not the resistance of the plug or socket I connect it to. That might be
next to impossible.

Can anyone offer any recommendations of how to wire it up, type of
connector (e.g. banana plug or similar)

PS I once see someone selling a dummy load on eBay with an SO 239 socket
and 47 Ohm wire wound resistor! Apparently the inductance made it 50 Ohms.

Dave

I am looking to measure as accurately as possible the DC resistance of a number of types of RF loads - all around 50 Ohms. Types of interest include SMA, 3.5 mm, N, APC7, 7-16 & BNC. I want to see if the difference between the actual resistance at DC and 50 Ohms correlates with the performance at RF as measured on a vector network analyzer. I want to do both sexes, with the exception of the APC7. I would expect RF performance to correlate well at low frequencies, but as the frequency is increased I would expect it not to correlate. I am looking for advice on the best way to do this. I have a HP 3457A (6.5 digit DVM with 4-wire resistance capability), but will consider purchasing a meter designed for low resistance measurements which I believe uses AC to avoid thermal EMF issues. But if possible I would rather use the 3457A. I was thinking of a plastic box with the RF connectors & 4 banana plugs. I suspect running the sense and drive wires back to the banana plugs would work. No signicant current would flow into the unterminated connectors as they would have just the dielectric. Ideally I would like to measure just the load and contact resistance and not the resistance of the plug or socket I connect it to. That might be next to impossible. Can anyone offer any recommendations of how to wire it up, type of connector (e.g. banana plug or similar) PS I once see someone selling a dummy load on eBay with an SO 239 socket and 47 Ohm wire wound resistor! Apparently the inductance made it 50 Ohms. Dave

SG

Stephen Grady

Sat, Aug 16, 2014 9:00 AM

David,

In order to measure DC resistance of an RF load to an accuracy lower than
the accuracy of a network analyser (say a reflection coefficient of 0.0001)
this equates to a DC resistance accuracy of 0.02% or 200 ppm and this is
well within the capability of most 6.5 DMM's in 4-wire measurement mode.

When I worked in precision RF Metrology we measured our loads to 1 mOhm
pushing it into the uOhm range has no advantage as the uncertainty due to
the mating connector centre pin is in the order of a couple to mOhms anyway.
When you measure the DC resistance insure that the meter has Offset
Compensation and Auto Zero ON. Allow the load the thermally stabilise after
handling. Also if your meter has a low voltage setting (or low power ohms
such as an Agilent 34420A or Fluke 8508A) try this setting and see if the
resistance changes as the test current of the meter may affect the
resistance of the load.

Your idea of the test box is a good one but I would suggest soldering (or
connecting) the sense terminal as close as possible to the back of the
mating connector, to reduce the connection uncertainty.

I hope this helps.

Kind Regards,

Steve Grady
Sydney Australia

-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On
Behalf Of Dr. David Kirkby (Kirkby Microwave Ltd)
Sent: Saturday, 16 August 2014 5:04 PM
To: Discussion of precise voltage measurement
Subject: [volt-nuts] Building a box to measure DC resistance of multiple RF
loads.

I am looking to measure as accurately as possible the DC resistance of a
number of types of RF loads - all around 50 Ohms. Types of interest include
SMA, 3.5 mm, N, APC7, 7-16 & BNC. I want to see if the difference between
the actual resistance at DC and 50 Ohms correlates with the performance at
RF as measured on a vector network analyzer. I want to do both sexes, with
the exception of the APC7.

I would expect RF performance to correlate well at low frequencies, but as
the frequency is increased I would expect it not to correlate.

I am looking for advice on the best way to do this. I have a HP 3457A (6.5
digit DVM with 4-wire resistance capability), but will consider purchasing a
meter designed for low resistance measurements which I believe uses AC to
avoid thermal EMF issues. But if possible I would rather use the 3457A.

I was thinking of a plastic box with the RF connectors & 4 banana plugs. I
suspect running the sense and drive wires back to the banana plugs would
work. No signicant current would flow into the unterminated connectors as
they would have just the dielectric.

Ideally I would like to measure just the load and contact resistance and not
the resistance of the plug or socket I connect it to. That might be next to
impossible.

Can anyone offer any recommendations of how to wire it up, type of connector
(e.g. banana plug or similar)

PS I once see someone selling a dummy load on eBay with an SO 239 socket and
47 Ohm wire wound resistor! Apparently the inductance made it 50 Ohms.

Dave

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David, In order to measure DC resistance of an RF load to an accuracy lower than the accuracy of a network analyser (say a reflection coefficient of 0.0001) this equates to a DC resistance accuracy of 0.02% or 200 ppm and this is well within the capability of most 6.5 DMM's in 4-wire measurement mode. When I worked in precision RF Metrology we measured our loads to 1 mOhm pushing it into the uOhm range has no advantage as the uncertainty due to the mating connector centre pin is in the order of a couple to mOhms anyway. When you measure the DC resistance insure that the meter has Offset Compensation and Auto Zero ON. Allow the load the thermally stabilise after handling. Also if your meter has a low voltage setting (or low power ohms such as an Agilent 34420A or Fluke 8508A) try this setting and see if the resistance changes as the test current of the meter may affect the resistance of the load. Your idea of the test box is a good one but I would suggest soldering (or connecting) the sense terminal as close as possible to the back of the mating connector, to reduce the connection uncertainty. I hope this helps. Kind Regards, Steve Grady Sydney Australia -----Original Message----- From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of Dr. David Kirkby (Kirkby Microwave Ltd) Sent: Saturday, 16 August 2014 5:04 PM To: Discussion of precise voltage measurement Subject: [volt-nuts] Building a box to measure DC resistance of multiple RF loads. I am looking to measure as accurately as possible the DC resistance of a number of types of RF loads - all around 50 Ohms. Types of interest include SMA, 3.5 mm, N, APC7, 7-16 & BNC. I want to see if the difference between the actual resistance at DC and 50 Ohms correlates with the performance at RF as measured on a vector network analyzer. I want to do both sexes, with the exception of the APC7. I would expect RF performance to correlate well at low frequencies, but as the frequency is increased I would expect it not to correlate. I am looking for advice on the best way to do this. I have a HP 3457A (6.5 digit DVM with 4-wire resistance capability), but will consider purchasing a meter designed for low resistance measurements which I believe uses AC to avoid thermal EMF issues. But if possible I would rather use the 3457A. I was thinking of a plastic box with the RF connectors & 4 banana plugs. I suspect running the sense and drive wires back to the banana plugs would work. No signicant current would flow into the unterminated connectors as they would have just the dielectric. Ideally I would like to measure just the load and contact resistance and not the resistance of the plug or socket I connect it to. That might be next to impossible. Can anyone offer any recommendations of how to wire it up, type of connector (e.g. banana plug or similar) PS I once see someone selling a dummy load on eBay with an SO 239 socket and 47 Ohm wire wound resistor! Apparently the inductance made it 50 Ohms. Dave _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com

BC

Brooke Clarke

Mon, Aug 18, 2014 8:37 PM

Hi David:

I would think to get the most accurate result you should use Kelvin connections where the V & I leads for each terminal
contacted the load under test.
So, rather than using a mating connector you would use something like pogo probes where two probes would contact the
ground and two would contact the center terminal. For a nominal 50 Ohm value Dc measurements will be OK, but I found
for measuring very low resistance values AC must be used.

I've measured the real and imaginary components of various things with the HP 4395A 0 to 500 MHz combo box which is
essentially a measurement near DC.
Here's a page showing the the Zo of a transmission line is not constant across all frequencies. Scroll down for some
plots showing what amounts to the DC resistance.
http://www.prc68.com/I/Zo.shtml

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html

Dr. David Kirkby (Kirkby Microwave Ltd) wrote:

I am looking to measure as accurately as possible the DC resistance of a
number of types of RF loads - all around 50 Ohms. Types of interest include
SMA, 3.5 mm, N, APC7, 7-16 & BNC. I want to see if the difference between
the actual resistance at DC and 50 Ohms correlates with the performance at
RF as measured on a vector network analyzer. I want to do both sexes, with
the exception of the APC7.

I would expect RF performance to correlate well at low frequencies, but as
the frequency is increased I would expect it not to correlate.

I am looking for advice on the best way to do this. I have a HP 3457A (6.5
digit DVM with 4-wire resistance capability), but will consider purchasing
a meter designed for low resistance measurements which I believe uses AC to
avoid thermal EMF issues. But if possible I would rather use the 3457A.

I was thinking of a plastic box with the RF connectors & 4 banana plugs. I
suspect running the sense and drive wires back to the banana plugs would
work. No signicant current would flow into the unterminated connectors as
they would have just the dielectric.

Ideally I would like to measure just the load and contact resistance and
not the resistance of the plug or socket I connect it to. That might be
next to impossible.

Can anyone offer any recommendations of how to wire it up, type of
connector (e.g. banana plug or similar)

PS I once see someone selling a dummy load on eBay with an SO 239 socket
and 47 Ohm wire wound resistor! Apparently the inductance made it 50 Ohms.

Dave

volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

Hi David: I would think to get the most accurate result you should use Kelvin connections where the V & I leads for each terminal contacted the load under test. So, rather than using a mating connector you would use something like pogo probes where two probes would contact the ground and two would contact the center terminal. For a nominal 50 Ohm value Dc measurements will be OK, but I found for measuring very low resistance values AC must be used. I've measured the real and imaginary components of various things with the HP 4395A 0 to 500 MHz combo box which is essentially a measurement near DC. Here's a page showing the the Zo of a transmission line is not constant across all frequencies. Scroll down for some plots showing what amounts to the DC resistance. http://www.prc68.com/I/Zo.shtml Have Fun, Brooke Clarke http://www.PRC68.com http://www.end2partygovernment.com/2012Issues.html http://www.prc68.com/I/DietNutrition.html Dr. David Kirkby (Kirkby Microwave Ltd) wrote: > I am looking to measure as accurately as possible the DC resistance of a > number of types of RF loads - all around 50 Ohms. Types of interest include > SMA, 3.5 mm, N, APC7, 7-16 & BNC. I want to see if the difference between > the actual resistance at DC and 50 Ohms correlates with the performance at > RF as measured on a vector network analyzer. I want to do both sexes, with > the exception of the APC7. > > I would expect RF performance to correlate well at low frequencies, but as > the frequency is increased I would expect it not to correlate. > > I am looking for advice on the best way to do this. I have a HP 3457A (6.5 > digit DVM with 4-wire resistance capability), but will consider purchasing > a meter designed for low resistance measurements which I believe uses AC to > avoid thermal EMF issues. But if possible I would rather use the 3457A. > > I was thinking of a plastic box with the RF connectors & 4 banana plugs. I > suspect running the sense and drive wires back to the banana plugs would > work. No signicant current would flow into the unterminated connectors as > they would have just the dielectric. > > Ideally I would like to measure just the load and contact resistance and > not the resistance of the plug or socket I connect it to. That might be > next to impossible. > > Can anyone offer any recommendations of how to wire it up, type of > connector (e.g. banana plug or similar) > > PS I once see someone selling a dummy load on eBay with an SO 239 socket > and 47 Ohm wire wound resistor! Apparently the inductance made it 50 Ohms. > > Dave > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. >

BA

Bob Albert

Mon, Aug 18, 2014 9:00 PM

At least my HP3456A does it by compensating for thermal emf. It measures the emf with no current and subtracts that from the measurement so that you don't need to reverse the polarity and average.

Bob

On Monday, August 18, 2014 1:38 PM, Brooke Clarke brooke@pacific.net wrote:

Hi David:

I would think to get the most accurate result you should use Kelvin connections where the V & I leads for each terminal
contacted the load under test.
So, rather than using a mating connector you would use something like pogo probes where two probes would contact the
ground and two would contact the center terminal. For a nominal 50 Ohm value Dc measurements will be OK, but I found
for measuring very low resistance values AC must be used.

I've measured the real and imaginary components of various things with the HP 4395A 0 to 500 MHz combo box which is
essentially a measurement near DC.
Here's a page showing the the Zo of a transmission line is not constant across all frequencies. Scroll down for some
plots showing what amounts to the DC resistance.
http://www.prc68.com/I/Zo.shtml

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html

Dr. David Kirkby (Kirkby Microwave Ltd) wrote:

I am looking to measure as accurately as possible the DC resistance of a
number of types of RF loads - all around 50 Ohms. Types of interest include
SMA, 3.5 mm, N, APC7, 7-16 & BNC. I want to see if the difference between
the actual resistance at DC and 50 Ohms correlates with the performance at
RF as measured on a vector network analyzer. I want to do both sexes, with
the exception of the APC7.

I would expect RF performance to correlate well at low frequencies, but as
the frequency is increased I would expect it not to correlate.

I am looking for advice on the best way to do this. I have a HP 3457A (6.5
digit DVM with 4-wire resistance capability), but will consider purchasing
a meter designed for low resistance measurements which I believe uses AC to
avoid thermal EMF issues. But if possible I would rather use the 3457A.

I was thinking of a plastic box with the RF connectors & 4 banana plugs. I
suspect running the sense and drive wires back to the banana plugs would
work. No signicant current would flow into the unterminated connectors as
they would have just the dielectric.

Ideally I would like to measure just the load and contact resistance and
not the resistance of the plug or socket I connect it to. That might be
next to impossible.

Can anyone offer any recommendations of how to wire it up, type of
connector (e.g. banana plug or similar)

PS I once see someone selling a dummy load on eBay with an SO 239 socket
and 47 Ohm wire wound resistor! Apparently the inductance made it 50 Ohms.

Dave

volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

At least my HP3456A does it by compensating for thermal emf. It measures the emf with no current and subtracts that from the measurement so that you don't need to reverse the polarity and average. Bob On Monday, August 18, 2014 1:38 PM, Brooke Clarke <brooke@pacific.net> wrote: Hi David: I would think to get the most accurate result you should use Kelvin connections where the V & I leads for each terminal contacted the load under test. So, rather than using a mating connector you would use something like pogo probes where two probes would contact the ground and two would contact the center terminal. For a nominal 50 Ohm value Dc measurements will be OK, but I found for measuring very low resistance values AC must be used. I've measured the real and imaginary components of various things with the HP 4395A 0 to 500 MHz combo box which is essentially a measurement near DC. Here's a page showing the the Zo of a transmission line is not constant across all frequencies. Scroll down for some plots showing what amounts to the DC resistance. http://www.prc68.com/I/Zo.shtml Have Fun, Brooke Clarke http://www.PRC68.com http://www.end2partygovernment.com/2012Issues.html http://www.prc68.com/I/DietNutrition.html Dr. David Kirkby (Kirkby Microwave Ltd) wrote: > I am looking to measure as accurately as possible the DC resistance of a > number of types of RF loads - all around 50 Ohms. Types of interest include > SMA, 3.5 mm, N, APC7, 7-16 & BNC. I want to see if the difference between > the actual resistance at DC and 50 Ohms correlates with the performance at > RF as measured on a vector network analyzer. I want to do both sexes, with > the exception of the APC7. > > I would expect RF performance to correlate well at low frequencies, but as > the frequency is increased I would expect it not to correlate. > > I am looking for advice on the best way to do this. I have a HP 3457A (6.5 > digit DVM with 4-wire resistance capability), but will consider purchasing > a meter designed for low resistance measurements which I believe uses AC to > avoid thermal EMF issues. But if possible I would rather use the 3457A. > > I was thinking of a plastic box with the RF connectors & 4 banana plugs. I > suspect running the sense and drive wires back to the banana plugs would > work. No signicant current would flow into the unterminated connectors as > they would have just the dielectric. > > Ideally I would like to measure just the load and contact resistance and > not the resistance of the plug or socket I connect it to. That might be > next to impossible. > > Can anyone offer any recommendations of how to wire it up, type of > connector (e.g. banana plug or similar) > > PS I once see someone selling a dummy load on eBay with an SO 239 socket > and 47 Ohm wire wound resistor! Apparently the inductance made it 50 Ohms. > > Dave > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.

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Building a box to measure DC resistance of multiple RF loads.