[Test-Equipment] Question regarding antique LCR bridge

[Barry]

It wasn't an attachment - just a text reply.  The server kept rejecting the message due to some weird address errors but I finally got it to send and it chopped my reply out of the top.  Weird.


 On Fri 13/04/12  3:06 AM , "Howie" esrman at ameritech.net sent:
> Most groups don`t allow attachments.You`ll have to send it direct to
> him.----- Original Message ----- 
> From: "Barry" n4buq at knolo
> gy.net>To: test-equipment at mailman.qth.net>Sent: Friday, April 13, 2012 12:01 AM
> Subject: Re: [Test-Equipment] Question regarding antique LCR bridge
> 
> 
> > 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 knolo
> gy.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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