This is the fourth blog post (and the last one) in a series that has covered my on-going roadtesting work of the Rohde & Schwarz NGU401 SMU instrument. I had to post this update as a separate blog because when I tried to append it to the previous updates I have increased the maximum number of images that can be posted in a blog – see screenshot below:
The previous updates 1 to 3 are located here: https://www.element14.com/community/groups/roadtest/blog/2021/06/20/on-going-work-roadtesting-the-rhode-schwarz-ngu401-smu
6. More Measurements and Experiments
Thermal Effects
In this experiment I wanted to evaluate the thermal effects when sweeping the current through a resistor. Ideally, if the resistor has a constant value over temperature then the voltage measured on the resistor should follow Ohm’s Law V=IR for any value of I. To evaluate this in a real case I have setup the NGU401 SMU to perform three current sweeps: 0-1A, 0-2A, and 0-3A and for each sweep I recorded the measured voltage. The assumption here is that a sweep at higher currents will increase the resistor temperature more than a sweep at low currents values. If the resistor value depends on temperature then the calculated resistance will have different values depending on the sweep current limits. The resistor nominal value is 2 Ohms.
The sweep results are shown in the figure below. Since it is hard to see any small variation from the ideal expected curves, I have saved these results in csv file format and I have processed them in Microsoft Excel.
The NGU401 SMU sweep software provides a convenient way to save the trace by right-clicking on the graph and selecting save trace. There are two options for saving the trace: .txt and .csv files. In my case I have chosen .csv to easily open the files in Excel. The figure below shows the computed resistance from the NGU401 measured I-V curves. In an ideal case we expect to see a perfect horizontal line at a value close to 2 Ohms (within +/-5% the tolerance from the nominal value of the resistor). If the resistor value changes due to thermal effects or if the NGU401 SMU force and measure values are dependent on current values and/or temperature then the computed resistance graph deviates from the ideal horizontal line.
In this figure Series 1 = sweep 0 – 1A in 1000 points and Series 2 = sweep 0 – 2A in 1000 points. We can notice that there is some deviation from a horizontal line at low currents in both cases but more significantly in the 0-1A sweep. I think this variation may be caused by the resistor value being dependent on temperature, which intuitively should increase with the value of the current; however there can be also some variation caused by the NGU401. I am thinking of this because the variations for 0-1A and 0-2A sweeps have opposite directions at lower current values. It is not easy to differentiate without running some more complex debugging experiments.
I have then processed the higher sweep range 0 – 3A in 1000 points, shown as Series 3 in the figure below:
The 0-3A range shows also the resistor variation at lower currents in the sweep but it also shows some strange non monotonic behavior that I could not explain from the information I have so far about NGU401 SMU. It could be something in the resistor or resistor contact but I cannot say that for sure.
Thermocouple Effect
The thermocouple effect happens in an electric circuit that contains a junction between two different metals. When this junction is heated a voltage is created across the junction, which may affect the measurements if not taking care of. In this experiment I have inserted a thermocouple junction in series in a circuit that contains also a resistor, and I have connected this circuit as device under test for the NGU401 SMU. My thermocouple had two variants that differ only by the direction of the current through the thermocouple junction, which then reflects in the polarity of the thermocouple effect voltage generated across the junction. The figure below shows the I-V curves traced with the NGU401 SMU on these two thermocouple test fixtures.
We can notice a small difference in the ending point of the I_V curves: the trace in the left graph ends at a voltage a little higher than 700mV while the right side trace ends at a voltage significantly lower than 700mV. To further process this difference I have saved the two traces as .csv files (right click on the graph and select save trace and choose csv file format) and I have imported them in Excel to plot the difference. Here is the voltage difference that is caused by the thermoelectric effect:
So the thermoelectric effect generates a voltage that gradually increases with current during the sweep and reaches close to 25mV/2 (divide by 2 since the measurement above is differential) at the max current of the sweep. This voltage increase can be explained as the result of junction temperature increase with the current increase during the I-V curve tracing sweep.
Measuring Very Low Resistance
Another experiment I wanted to setup is the measurement of very low resistance values. This measurement cannot be done with regular DMMs because the resistance of the DMM leads ads to the measured resistance degrading the accuracy of the measurement. The common method for measuring low value resistances is to use a four point measurement where two wires provide a circuit path for a current to flow through the DUT and the other two wires are used to sense the voltage drop on the DUT. The current flowing through the sense wires is negligible and does not degrade the accuracy of the measurement. So in this experiment I have created a setup like this for measuring the plane resistance of a copper PCB layer, as shown in the figure below:
The NGU401 SMU provides the force and sense terminals connected to the DUT fixture as marked on the PCB copper plane shown in the figure.
I have then performed three measurements at three different forced voltage values: 1mV, 3mV, and 5mV. The corresponding measured voltage and current values as well as calculated resistance are shown in the figures below:
NGU401 SMU in force 1mV voltage measure current mode:
Applying Ohm’s law the resistance of the copper PCB plane is 0.42mV/493.73mA = 850.66 uOhms. In this calculation I have used the average values displayed on the right side of the screen.
If I use the “real time” displayed voltages and currents the calculated resistance value is: 0.422mV/492.313mA = 857.17 uOhms.
NGU401 SMU in force 3mV voltage measure current mode:
Applying Ohm’s law the resistance of the copper PCB plane is 2.37mV/2.6416A = 897 uOhms. If I use the “real time” displayed voltages and currents the calculated resistance value is: 2.361mV/2.634261A = 896.266 uOhms
NGU401 SMU in force 5mV voltage measure current mode:
Applying Ohm’s law the resistance of the copper PCB plane is 4.32mV/4.80207A = 899.6 uOhms. If I use the “real time” displayed voltages and currents the calculated resistance value is: 4.315mV/4.80069A = 898.83 uOhms
So the measured values at higher currents are closer to each other while the measured value at low current is about 6% lower. This result suggests that it is more accurate to measure low resistance values using higher currents.
The Trigger Function
The Digital I/O Trigger function enables the synchronization of multiple instruments of a test bench measurement setup. The connection among multiple instruments is done through electrical signals of the “open-drain” type asserted when pulled low and which share a common electrical interconnect wire. Multiple signals of this type are included in a 15 pin digital I/O connector located on the back panel of the NGU401 SMU instrument. This trigger function comes as an option to the standard features of the NGU401 instrument. The 15 pin connector pins description is shown in the table below:
To evaluate the trigger and remote synchronization function of the NGU401 SMU I have setup three experiments as summarized below.
First experiment will enable the trigger function on the NGU401 instrument and then it will check the signal “Digital Output Out1” accessible on pin 4 of the I/O connector while activating and deactivating the NGU401 output channel. The second experiment will check the signal “Digital Output Fault” accessible on pin 11 of the I/O connector. The third experiment will apply a trigger signal on the input “Ext. Trigger Ch1” on pin 2 of the I/O connector and it will check how the NGU401 SMU turns ON and OFF the output channel.
To setup these experiments first I had to build a connector fixture that would fit in the digital I/O connector on the back panel of the NGU401. Here is a picture of the connector that I have built:
Next I have enabled the trigger mode in the settings menu of the NGU401. There are a few options for trigger, as described in the user manual and summarized in the diagram below:
For my experiment I have selected as trigger source the “Digital I/O pin” on the “Digital I/O” source option, as shown in the screenshot below:
Experiment 1: Checking the digital output Out1 pin4:
This experiment checks the signal “Digital Output Out1” accessible on pin 4 of the I/O connector while activating and deactivating the NGU401 output channel. The experiment setup consists of an oscilloscope connected to probe pin 4 of the digital I/O connector while I turn on/off the NGU410 output from the front panel button. The measurement results are summarized in the figure below:
The output signal (yellow trace) changes the logic state on the NGU output On to OFF transition. This behavior is not what I was expecting after reading the manual:
It may be that I did not setup something correctly or the functionality that I measured is correct but the description in the manual is a bit confusing to me.
Experiment #2: Next I have looked at the digital output fault, pin11 with the same experiment setup:
The output signal at pin11 did not change logic states while I was toggling the NGU401 output button. This was expected based on the functionality description since there was no faulty condition in the NGU401 functionality. Here is the description in the pins table:
So if my understanding is correct, if I create a faulty condition this pin has to toggle. So I have reduced the max current clamp level to be below the expected current so that the NGU401 SMU goes into current limit mode. I have then repeated the experiment of tuning on/off the output from the front panel and now the signal at pin11 toggles as I have expected:
When the NGU401 output is on the instrument enters into the current limiting mode, which is the faulty condition, and it pulls low the “Digital Output Fault” signal at pin11.
Experiment #3: In this experiment I wanted to see the NGU401 SMU response to an external trigger signal on pin 2, which is described in the user manual as:
So I have connected a push button that pulls low the signal at pin 2, as shown in the picture below:
When I push the button the signal at pin 2 is pulled low and the NGU401 SMU receives a trigger signal that turns on the output. When I release the button the NGU output remains on, which is expected for an “edge” type trigger mode. These results are summarized in the figure below:
Remote Control and Programming the NGU401 SMU
In this experiment I am evaluating some of the remote control commands that can control the NGU401 SMU from a computer through a USB connection and also through a LAN or GPIB connection. In my experiment I have used a USB connection. The user manual contains detailed information with examples that has helped me get up to speed quickly with remote controlling the NGU401 instrument.
So first I have connected the NGU401 to the computer (USB connection in my experiment) and I have started the HMExplorer application (available to download on the Rohde & Schwarz web site):
The application has found the NGU401 instrument automatically and it has shown it as connected on COM3 port. Next I have opened a SCPI terminal part of the HMExplorer application:
In the SCPI terminal I selected the NGU401 under the “device” tab and then I have sent my first command – a command that sets the max voltage limit to 10V “VOLT:ALIM 10” as shown in the screenshot below.
Next I have sent some subsequent commands to set the lowest voltage limit and the min/max current limits:
I was now ready to set a voltage value to be forced through the output channel, and to do this I have sent a command to set the force voltage value to 1V “VOLT 1” as shown in the screenshot below – I have played a bit there with lower case and upper case choices of sending commands:
These settings could be now seen on the NGU401 display:
Notice the SCPI highlighted in red at the top of the screen, which means that the NGU401 is connected to a computer and controlled from an SCPI terminal (or application). Now I was ready to turn on the NGU401 output channel. To do this I have typed the command “OUTP:SEL 1” and then I typed a few commands to read the average, min, and max current values:
These are the corresponding measurements on the NGU401 screen:
Notice that the min value corresponds to the circuit current before turning on the channel. This happens on both the NGU401 screen and on the SCPI remote read measurement. I have found this a bit confusing since as an user I have initiated the measure command after I have turned on the channel, so I expect the min/max/avg to represent the sample values collected after I turned on the channel. I know I can manually reset the data sampling by touch pressing the button on the screen under the displayed numbers; however, if I were to manufacture this instrument I would have implemented a feature that would automatically reset the sample collection every time I turn on or off the output channel. This way I would eliminate any potential confusion – now it is obvious since we know the current is expected to be zero when the output is off (passive DUT), but this may not be always the case for active DUTs with energy storage components.
Script Editing and Programming
There is a “Script” feature in the HMExplorer application and I wanted to explore if I can write a script with a few commands and then save it and execute it. So I have selected the script screen, which has opened a default example script on the right side of the screen (see screenshot below). On this default script I have added the commands that I tried above and I was able to save the file in a script format. I was also able to load the script file in the HMExplorer; however, I haven’t been able to run the script because the buttons on the script form “Play”, “Pause”, and “Stop” were inactive and shaded at all times.
I have “poked around” on the HMExplorer application but I haven’t been able to activate these three buttons, so at some point I gave up the scripting idea. I am sure it is possible to implement a script with the remote commands and run it even though it did not work for me in the HMExplorer. It probably works in some other tools/software.
Conclusion
This update #4 concludes the work I have done for roadtesting the Rohde & Schwarz NGU401 SMU instrument. Over the past few weeks I had the opportunity to roadtest the Rohde & Schwarz NGU401 SMU instrument. I enjoyed this activity and I want to thank Rohde & Schwarz and Element 14 for selecting me as one of the road testers for this instrument. During these weeks I have run multiple experiments and I have performed multiple measurements with the NGU401 SMU. From all this work I am very impressed with the quality and performance of the NGU401 SMU. The front panel display is crisply sharp and clear, and the instrument control is easy and intuitively straight forward. The instrument functions and remote control commands are clearly described in the user guide and in multiple application notes. There were a few things that I would like to see them done differently and some suggestions that I have described above. My overall conclusion is that the NGU401 SMU is a great instrument made so that it gives me the impression of a solid, high quality, and reliable test equipment. I feel fortunate for having the opportunity to explore the functionality and performance of the R&S NGU401 SMU through this roadtest.
Best Wishes,
Cosmin
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