[volt-nuts] Building a box to measure DC resistance of multiple RF loads.

[Stephen Grady]

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 at febo.com [mailto:volt-nuts-bounces at 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 at 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