[Test-Equipment] Question regarding antique LCR bridge

[Howie]

Most groups don`t allow attachments.You`ll have to send it direct to him.
----- Original Message ----- 
From: "Barry" <n4buq at knology.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 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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