[Test-Equipment] Question regarding antique LCR bridge

[rbethman at comcast.net]

Barry, 

As you noted, 'Designed in 1960'. 

Therefore, it was built 'that' way to account for the wire-wound resistors of its time. All of those, with the exception of non-inductive, i.e., Globar types, had both a capacitive *and* inductive component. 

Now you are trying to measure today's components, whether carbon film, metal film, or others, even carbon composition types. 

Your bridge doesn't understand that those are what is currently used and purchased. 

Make sense? 

Bob - N0DGN 

----- Original Message -----
From: "Barry" <n4buq@ knology.net > 
To: test-equipment@ mailman.qth.net 
Sent: Tuesday, April 10, 2012 10:06:49 AM 
Subject: [Test-Equipment] Question regarding antique LCR bridge 

I have a Marconi TF1313 LCR bridge (designed in 1960). When measuring L or C, the bridge is fed with an AC signal (1KHz or 10KHz). The output goest to a couple of amplifiers and the detected signal is fed to a meter. When the bridge approaches balance, the detected signal approaches zero and the meter indicates a null. Standard stuff for a bridge. 

When measuring R, though, a DC (full-wave rectified, unfiltered) signal is fed to the bridge and the output is connected to the input of the first amplifier through a chopper (old fashioned vibrator). Amplification and detection is the same process as for L and C. 

My question is why it is necessery to change the configuration of the bridge for R measurement? Why wouldn't it work with an AC input for R the same as for L and C and eliminate the chopper? Wouldn't a balanced bridge made entirely of R produce a minimum signal at the output and allow for null detection? 

I know the answer is probably simple but I don't see why this is done this way. 

Thanks, 
Barry - N4BUQ 

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