[Test-Equipment] Question regarding antique LCR bridge

[Barry]

Bob,

After thinking about it, I think I misunderstood what you said.  Are you saying that since most resistors of the period contained more L and C than today's components, then using AC across the bridge for R testing would not work as well due to the added reactance and hence the need for DC on the bridge and the chopper?

Thanks,
Barry - N4BUQ


 On Tue 10/04/12  1:37 PM , rbethman at comcast.net sent:
> Barry,
> 
> As you noted, 'Designed in 1960'.
> 
> Therefore, it was built 'that' way to account for the wire-wound
> resistors of its time.  All of those, with the exception of
> non-inductive, i.e., Globar types, had both a capacitive *and*
> inductive component.
> 
> Now you are trying to measure today's components, whether carbon
> film, metal film, or others, even carbon composition types.
> Your bridge doesn't understand that those are what is currently used
> and purchased.
> 
> Make sense?
> 
> Bob - N0DGN
> -------------------------
> FROM: "Barry" 
> TO: test-equipment at mailman.qth.net [2]
> SENT: Tuesday, April 10, 2012 10:06:49 AM
> SUBJECT: [Test-Equipment] Question regarding antique LCR bridge
> 
> 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 necessery 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
> 
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