[Test-Equipment] Question regarding antique LCR bridge

[Barry]

 On Thu 12/04/12  8:07 PM , "k4pf at juno.com" k4pf at juno.com sent:
> 
> 
> > "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 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
> 
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