[Test-Equipment] Question regarding antique LCR bridge

[Barry]

Hi Ed,

I have an HP-410C and have seen the photochopper in it.  I didn't realize it had two, though.  I think thats a common failure point in these meters and I think mine needs to be addressed.  Another project for another day...

Just for fun, I disconnected the bridge from the first amp.  I wanted to see if the bridge was indeed nulling when in R mode.  It appears it is.  When in C mode, using my Fluke DMM, I can get the bridge to balance to less than 0.1mV (the minimum resolution of my meter).  When in R mode, I can get it down to about 2.4mV; however, that's not a sine wave so that's just a reference measurement.  I only have 10x scope probes so my TEK 465B will only go down to 50mV/div (which pretty much shows as a flat line for that signal).

With the bridge disconnected and in C mode, the null indicator meter goes to nearly zero (perhaps one click off of zero); however, in R mode, the null meter is about 1/3 scale (from the zero end).  I'm wondering if 1) this is normal (I don't think so) and 2) if this is possibly caused by noise from the mechanical chopper.

In C mode, the bridge couples to the first amp via a 470pF cap.  In R mode, the bridge is coupled through an RC network and the 470pF is grounded.  I'm thinking this is to help eliminate noise from the chopper(?).

I'm going to try this with the chopper disconnected and see if it makes any difference.  If it does, then perhaps something isn't filtering out the chopper contact noise.

Anyway thanks so much for the reply!

Barry - N4BUQ


 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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