Wednesday, 18 December 2013

A tale of two scopes and a horrible signal

I hade the chance yesterday to get my hands on a Agilent DSOX3024A in the school lab.
Usually these devices are locked away from the students because they are way to expensive to withstand the daily abuse from not so much caring students. It's a shame but reasonable,...

On the other hand there is the Rhode&Schwartz HMO2024 for which I got a great package deal including all options.

They both are  200 Mhz scopes with 4 channels and have about the same feature set.
The Aglient though is a bit pricier.

Since I own the Hameg of course I am biased for that model so please keep that in mind although I will do my best to stay objective.

The second I sat before the Agilent I must confess it has a huge wow factor.
It is bigger than the  R&S but it has about the same weight.

I love the display it is so huge and has a nice resolution and best of all a great sampling depth.
According to Agilent it also has a lighting fast update rate so it should be a joy to catch glitches.

Unfortunately I could not play with the logic analyser because it is an option which was not installed.
I don't understand why Agilent does not sell a full blown scope to a school or university.
This would have been the best advertisement money can buy. Once the students get used to the features they don't want to miss them any more.And since the scopes are not used in a commercial environment why not add something for free ? 

I really love that every channel had its dedicated voltage divider knob, a feature I miss on the R&S scope. Speaking of the knobs there is one fact I hated on the scope, the layout.
I am used to the Textronix layout which is voltage divider knobs beside the timebase knob.
This layout makes sense because that are the most important knobs and it became an industry standard.Why changing something that works so well and people are used to ? I even did not find the seconds divider knob for quite a while and had to use autoset which I don't like that much because I prefer to be in full control of the displayed data.

I also could not get a trigger on an I2C data signal the data acquisition had to be stopped before the picture stood still. But I assume that was my fault in fact it had to be. Since the I2C signal is irregular according to frequency the displayed waves were all over the screen and the irregularities were interpreted as glitches I assume.
Btw the R&S scope also had a hard time to trigger correctly and I had to tweak the trigger settings quite a bit (like setting a trigger hold off) so in fact the Agilent did quite good.

I was not impressed with the wave auto analysing tough. The auto measure function is somewhat greedy and does not nearly display as much info as the R&S scope does but then again that could also be an operator fault.
To get my hands on this scope was a quick settling opportunity I had not planned for. If I were better prepared I would have inspected the scope way more closely.

The test setup:
I used an LM75 on a breadboard driven by an 500kHz I2C bus.
So the signal should work but it also should look horrible.
The Msp430 was also powered by usb which also should inject quite some noise.

The pictures:

Pretty much the same, huge over/under shooting on the clock channel and massive cross talk

The detail view is interesting, while on the Agilent side the ringing is visible on the Data line the R&S shows the ringing on the clock line.
Which one is correct ?
I tend to believe the R&S version more because it makes more sense imho. Since the clock line over/under- shoot visible on both scopes has to degrade somehow (because there are capacitive and inductive components involved) the dampened oscillation makes more sense on the clock line, while the data line has no comparable over/under shoot so why should it ring ? Any ideas from the reader which might shine some light on the issue is highly appreciated since I am somehow puzzled.

The automeasure function on the Agilent is somehow greedy compared to the R&S but that might be my fault, I will study the manual to check if there is a possibility to get more measurements.

The final verdict:

I could fall for the Agilent, the big screen is a winner no question.
The update speed is insane.

I don't like the knob layout but I think after a while I would get used to it.

The feature set on a first glance is pretty much the same.
But the devil is in the details.

Is the scope worth the higher price tag  compared to the R&S Hameg line, I would say yes.
I only had  a few hours playtime on the Agilent scope so I only scratched the surface.
It is an impressive scope but what else would you have expected from such a great company ?

Wednesday, 11 December 2013

Agilent 34461A Miniseries Part 1 Histogram and Cdf

Today I received a Agiltent 34461A Truevolt multimeter from the Austrian Agilent distributor.
I plan to write some articles about this  multimeter and especially about features which imho separates it from it's competition.

I am a Hewlett Packard / Agilent fan but I will try hard to be as unbiased as possible,...

A very interesting feature for me is the histogram display. I enjoyed the online demo's about this feature and after unpacking the meter I had to try this feature immeadetly.

If you wanted to measure the deviation of a source before you had to collect data via a serial usb or gpib bus and then create an excel sheet to visualize it. Therefore you would do such a cumbersome task only if you really need statistical data badly.

With the new Agilent 34461A you can watch the deviation instantly by pressing a button.
To test this feature I connected a 5V reference and let the instrument visualize the data.

After some 4500+ measurements you get a nice picture with an expected Gaussian distribution form.
You can also see that the Reference drifts about ±10 uV and that the reference is about 283 uV off (not as bad as it looks).

The reference and meter grosso modo  work as expected, since I have not trimmed the reference against a calibrator yet.

The  reference I used is based on the Max6350 chip with an ±0.02% initial accuracy, which translates to ±1mV. That considered it is doing great.It is awfully hard to  get your hands on a reference which is a match to the precision of a modern 6.5 digit meter, unfortunately.

Beside the histogram which is relatively easy to interpret, the meter also offers to show the cumulative distribution function (in short CDF). The CDF is represented by the green line on the picture above.
The y axis represents the percentage, the x axis the voltage.

So what is the CDF good for ? You can use the CDF to determine what percentage of the data falls between two points. The gradient of the function is also interesting.The first 10% have a considerably smaller gradient than the rest of the function, which means that the negative outliers in that sector are more rare than the other ones.
You can also see f.e. that 10% of the voltages  are of in the range of  5.000272 and 5.000279 V.
The position where the function crosses the x axis is the 50% point which in this case is slightly of the origin point on the positive side.
All in all the CDF is a great addition to an already useful statistical feature.

Test setup:

Many thanks go to the Austrian Agilent Distributer for making this test possible.

More mathematical insight on  Gaussian Deviaton and the cumulative distribution function can be found here.