[Test-Equipment] Question regarding antique LCR bridge

[Barry]

Not sure what happened.  I posted a long reply and it all got chopped out.  I'll try again later.
Thanks,
Barry


 On Thu 12/04/12 10:24 PM , Barry n4buq at knology.net sent:
> 
> 
> 
> 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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