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

[Bob Albert]

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 at 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
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