[Test-Equipment] Question regarding antique LCR bridge

[k4pf at juno.com]

> "Barry" <n4buq@ knology.net > 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 necessary 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 

Hi, Barry

They probably used a chopper-stabilized dc amplifier
for the resistance null circuit.  DC-coupled amplifiers
of the period tended to drift, very annoying if you are
trying to establish a null using high gain amplifiers.

One chopper-stabilized voltmeter that comes to mind
is in the HP-410C VTVM.  With this arrangement, the dc signal 
is modulated by a chopper, amplified using ac amplifiers,
and then demodulated back to dc by another chopper
at the same (chopper) frequency.  This way, you avoid dc amplifiers
and their drift problems.

Of course, you could measure R (actually magnitude of Z)
at 60 Hz, like any number of service-grade RC bridges did, 
using an eye-tube null. But they wanted to provide a measurement
of the true dc resistance for those people who had the need,
and were willing to spend the money for lab-grade test equipment.

73,
Ed Knobloch