Products > Test Equipment

Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)

<< < (30/91) > >>

SilverSolder:

--- Quote from: garrettm on February 05, 2021, 08:15:01 pm ---
--- Quote from: SilverSolder on February 05, 2021, 05:13:21 pm ---[...]
Nice workaround, I'll see if I can characterize my problem boards this way. 

Perhaps it is just a hardware offset adjustment that is the problem.

--- End quote ---

Hopefully all you'll need is to play with some trimmers, but if not, I have a spare Active Filter module (716316) on hand, though shipping might be more than its worth.

If needed, I could unsolder the j-fets and mail a few off. I recently found out my Current Shunt module has weird offsets that come and go, so I think it needs a few j-fets replaced too. Parting out the spare filter might be able to fix both our problems.

I also have a spare GPIB controller if anyone wants it.

--- End quote ---

Thank you for your kind offer, I'll keep that in mind if I lose traction completely with this one!

garrettm:

--- Quote from: SilverSolder on February 05, 2021, 07:53:16 pm ---Perhaps just turning off the Filter will have the desired (noise) effect?

The filter will suppress any signal over 7.5Hz or 75Hz, respectively, so it is unclear how much effect adding a signal would have unless sample periods get very long?

The noise may need to be added later in the chain, perhaps in the R2 converter itself...

--- End quote ---

Good point.

One interesting feature of the R2 ADC design is that each reading from the ADC is 6.5 digits. So with S0 sampling, that’s 250 6.5 digit readings per second. To see if there really are missing codes and how the filters might affect the results, it could be worthwhile to do the averaging remotely on a PC than at the 8505/6A. Simply wait for 1024 samples before computing the mean and place that value into a new time series (which would be equivalent to an average mode reading direct from the DMM). This way we eliminate any limitations with the precision of the internal data types and can quantify the noise of the ADC from the raw samples. This also lets us compute 7.5 digits with any of the three filter settings.

Unlike integrating ADCs, it makes more sense to use the fastest sample rate possible from a R2 ADC and perform the appropriate conditioning at the PC, rather than on the DMM. At least for these ancient boat anchors. With an auto zero circuit and more advanced controller and display, the 8506A could have been a much more impressive instrument. The "analyze" features on the DMM4050 (trend plot, stats and histogram) are be perfectly suited for the 8505/6A and its high speed, high resolution sampling. And with finer sample size control, speed vs noise could be better optimized for a given measurement. Then there is the possibility of using digital filtering to avoid the need for analog filters.

bdunham7:

--- Quote from: garrettm on February 05, 2021, 09:56:24 pm ---With an auto zero circuit and more advanced controller and display, the 8506A could have been a much more impressive instrument. The "analyze" features on the DMM4050 (trend plot, stats and histogram) are be perfectly suited for the 8505/6A and its high speed, high resolution sampling. And with finer sample size control, speed vs noise could be better optimized for a given measurement. Then there is the possibility of using digital filtering to avoid the need for analog filters.

--- End quote ---

Although true, the 'advanced controller' would have been tough to come by when the 850x was being developed.  There's a lot of performance difference between an 8080 and the twin Cyclones of the 8446A/DMM4050--over 10,000 times!  Averaging samples was about as sophisticated a digital filter as was practical.

joeqsmith:

--- Quote from: SilverSolder on February 05, 2021, 06:36:43 pm ---
--- Quote from: joeqsmith on February 05, 2021, 06:29:41 pm ---Outside of a short note about the there being a fast (Y) and slow (Z) filter and them being a 3-pole Bessel with different cutoffs, they seem to provide very little detail about them.  Maybe the schematic IS the document.   

With Dave's UEI meter, I would feed a chirp into it and record the filters response.   Seems like you could do something like this to test out the hardware filters.

--- End quote ---

I ran the filters from the diagram in LTSpice and came up with the figures in red, in the Active Filter circuit diagram I posted earlier  (i.e. 7.5Hz and 75Hz, 18dB/octave for the slow and fast sections respectively).

I think those numbers are unrelated to the 50ms and 550ms "timeouts" as they are called in the manual.  My theory is that when one of the "timeouts" is activated, the meter simply waits that number of ms after a trigger before performing the reading (to guarantee the filter has settled).  Should be easy to verify on GPIB (still not set up here!)

--- End quote ---

Odd they wouldn't have documented the filters better and that you had to run SPICE to sort it out.     

Log sweep from 100mHz to 10Hz, attempting to collect the data as fast as the meter will allow it (about 20Hz).  Showing the rolloff with the slow filter enabled.   
 

joeqsmith:
Log Sweep 1Hz to 100Hz with fast filter enabled.   Obviously aliasing on the PC side but we can still see the roll off. 


Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod