[Test-Equipment] Question regarding antique LCR bridge

[Brooke Clarke]

Hi Barry:

Do you have a schematic for the TF1313?  Is the vibrator wired so that there's synchronous detection?

That's strange.  The problem with using DC as a test signal is that you then get errors due to dissimilar metals, see:
http://www.prc68.com/I/Kelvin.shtml
Even when using a 4-wire Kelvin setup there's a real limit on low Ohms measurements because of this problem.

My TF2700 has provision to use external DC bias but makes the measurements using AC signals.
http://www.prc68.com/I/TF2700.shtml

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Clarke4Congress.html


Barry wrote:
> 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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