[Test-Equipment] Question regarding antique LCR bridge

[Richard Knoppow]

----- Original Message ----- 
From: "Barry" <n4buq at knology.net>
To: <test-equipment at mailman.qth.net>
Sent: Tuesday, April 10, 2012 7:06 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
>

     For many resistance measurements a low AC frequency 
would be fine but not for resistances that had much 
reactance such as a choke or capacitor or even a pair of old 
fashioned headphones.
     Using a chopper and AC amplifier to drive the null 
detector results in a more stable system than a  DC 
amplifier since certain kinds of drift in the amplifer 
become unimportant.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com