Rohde & Schwarz NGU401 Source Measure Unit - Review

Table of contents

RoadTest: Rohde & Schwarz NGU401 Source Measure Unit

Author: neuromodulator

Creation date:

Evaluation Type: Independent Products

Did you receive all parts the manufacturer stated would be included in the package?: True

What other parts do you consider comparable to this product?:

What were the biggest problems encountered?:

Detailed Review:

1. Introduction


Source measure units (SMUs) are instruments that can supply voltage and current while simultaneously measuring the voltage and current. They can act as sources (i.e, as a power supply) or as sinks (i.e, as an electronic load), have high dynamic range sourcing/sinking and measuring capabilities. SMUs usually operate in 4 quadrants of the I/V plane. where quadrant I & II are sourcing quadrants, while quadrant II & IV are sinking quadrants.





2. Rhode & Schwarz NGU401


The 4 quadrant NGU401 and its 2 quadrant sibling NGU201 are two new Rhode & Schwarz modern touchscreen-based SMUs released in the early 2021. These half-rack 2U SMUs use the same chassis and GUI previously used in the half-rack NGM200 and NGL200. The main differences between the NGU401 and NU201, is that the latter covers only Quadrant I & II, supports variable output impedance and can simulate batteries. The NGU401 on the other side is a more general purpose SMU.





2.1 Overview


2.1.1 Package Content


The NGU401 came with:

  • 4 power cables with different AC plugs.
  • A jumper connector to connect the Force Low to ground
  • A Calibration certificate
  • A "Getting started" manual
  • ROHS documentation
  • A Trifold brochure with generic Rhode & Schwarz information





2.1.2 First Impressions


The instrument is relatively long (436 mm) and heavy (7.1 kg) and feels robustly built. Its 800x480 glossy screen has good contrast and brightness. The GUI uses big fonts for the V/I measured values, medium size fonts for the V/I settings and limits, and smaller fonts for the state of the instrument, the stats and the current ranges. The chassis feel familiar and is similar to what other manufacturers are building for their touch-screen based DC instruments. The GUI has a classic hierarchical menu feeling more than a mobile phone GUI feeling.






2.2 Electrical Characteristics


2.2.1 Electrical Specifications


The NGU401 is a 1 channel 60 W SMU able to source/sink from -20 V to +20V and from -8 A to 8 A. For voltages up to 6 V it can source/sink 8 A, while above 6 V it can source only up to 3 A. The programming and readback ranges, their resolution and accuracy is summed in the following table:




Two interesting features of the SMU are its arbitrary function generator (AFG) and its high speed data acquisition (FastLog). The AFG supports up to 2048 points, and each of these can have a minimum duration of 1 ms. The data acquisition feature supports rates of up to 500 ksps. These two features in addition to the fast regulation of the instrument can be used to perform pulse-tests with reduced perturbation of the DUT temperature.


Another great feature of the SMU is that it can operate as an analog amplifier. In this mode of operation the instrument generates an output that is proportional to the multiplication of its input (at the MOD terminals) by the set gain value (0.5, 1 or 2).



2.2.2 Analog Connectivity


The instrument has 6 front banana connectors, 1 x Ground, 1 x Force High, 2 x Force Low, 1 x Sense High and 1 x Sense Low. Sourcing/sinking can be done with only the Force terminals, but when making measurements of high currents or of low DUT resistances, using the Sense connectors to make a Kelvin connection is recommended. The duplicated Force Low is used to jumper it to ground, which is recommended to make small voltage and current measurements.


The instrument also provides these terminals in the rear panel in the form of screw terminals and additionally provides the terminals used for the modulation mode (MOD+, MOD-).



2.2.3 Digital Connectivity


Remote control ports


The instrument can be controlled through Ethernet, GPIB (IEEE-488) and USB connectivity. They are all more or less equivalent, but USB tends to have the highest throughput followed by Ethernet and GPIB. In terms of latency GPIB has the lowest, followed by USB and Ethernet. Ethernet on the other side is very inexpensive to setup (in contrast to GPIB), very flexible as it easily makes it possible to control the instrument from anywhere in the network, and easier to setup (in contrast to USB).



USB host


The instrument has 2 USB host ports, one at the front and one at the back. They can be used to update the firmware, standard or fast log directly into the USB drive. For fast logging (FastLog) a USB drive faster than 10 mb/s is recommended.



Digital I/O


The instrument provides a female DB-15 port with 3 input and 2 output pins to synchronize it to other test equipment. To use the digital I/O port, the R&S NGU-K103 option must be enabled.


The triggering system of the instrument makes it possible to use the 2 digital inputs, the instrument operation mode, the output channel state, the USER button and the SCPI *TRG command as triggers. As response to the trigger the instrument can initiate the standard or fast logger, turn the instrument output on, off, or switch its state, start the AFG and generate a digital output.




2.3 Interfaces

2.3.1 Front Panel


The instrument is controlled through a combination of physical buttons, a rotary knob and the touchscreen. The home button is used to switch to the sourcing / measuring screen, the menu button opens the instrument main menu, the user button is a button that can be programmed to perform different tasks such as taking a screenshot or generating a trigger, the rotary knob can be used to navigate across the menu and also to increase/decrease digits of a number, the back button is used to move back or to cancel, and output turns on or off the SMU output.





When the instrument is not sourcing/sinking, no measurement values are shown as the instrument is relay-disconnected from the output. When the output is turned on, the numbers and the output button turn green if the instrument is acting as a voltage source, or red if its acting as a current source. The home screen also displays the programming and measuring ranges, the voltage, current and power stats, and general information of the current state of the instrument at the top of the screen.


To change the sourcing voltage or the min and max current limits, their values can be pressed and changed using either a touchscreen-based keypad or the rotary knob.





Pressing the menu button opens the device menu, and pressing the "channel 1" tab at the top right of the screen opens the channel specific options. These channel specific menu can also be opened by pressing the cogwheel icon at the home screen.





One interesting feature of the SMU is that it has a graphical view where voltage, current and power are plotted in a rolling mode.





The instrument comes with a few predefined functions for the AFG, but they can also be created directly with the "Arb editor". The editor makes it possible to set the mode to either voltage priority mode (VPM) or current priority mode (CPM), set the number of repetitions, the voltage/current, the minimum and maximum limits, the duration at each point, and if it should perform an interpolation between points.





2.3.2 Web Interface


The instrument hosts a webpage that provides general information, the ability to change settings such as the network configuration and the password, access to the "getting started" manual and a way to send SCPI commands and readback the SMU screen.






2.3.3 Software


Rhode & Schwarz provides 3 programs for the SMU: HMExplorer, NGx Sweep Tool and NGx Log & Chart.





HMExplorer (Hameg Explorer) is a general tool that support different type of instruments. For the NGU401, only "SCPI Terminal", "Screenshot" and "EasyArb" are available.




SCPI Terminal makes it possible to send  SCPI commands and also use a minimalistic scripting language to automate a few tasks.




Screenshot just does that, takes a screenshot from the current state of instrument screen.





EasyArb is used to create, upload, and output arbitrary functions to the instrument's AFG. Compared to the front panel, the tool shows graphically the waveforms, but also does not expose the complete functionality of the instrument (e.g, it can't set the minimum and maximum limits separately).




NGx Log & Chart

NGx Log & Chart is used to log data either in the standard or fast way. To use the tool, first the instrument output settings must be set (i.e., using the front panel), then when the instrument is outputting the desired output, the tool logging settings can be set and logging initiated. As the data gets streamed from the instrument to the tool, the tool saves it in real-time to the hard disk. When logging ends, the data can be visualized by pressing the Show Chart button.



The charting functionality not just shows the waveforms but also makes it possible to use cursors to make measurements.





NGx Sweep Tool


The NGx Sweep Tool is used to perform sweeps and generate I/V curves. After the parameters have been set, pressing Set prepares the SMU to perform the sweep, and Start initiates it.





Once the sweep has finished a window with the I/V plot pop up. From this window the curve can be explored, a screenshot of the curve can be taken and the I/V data can be saved into a file.





2.3.4 Remote Control (SCPI)


The highest level of control of the instrument and flexibility is achieved through SCPI commands. SCPI remote control can be very simple, but also get quite complex depending on the required instrument features.


A simple task could be to set the SMU to source 2 V, turn on the output and readback the voltage and current. This is can be done with just 4 commands:


<- *RST
<- VOLT 2
<- OUTP 1
<- READ?
-> 1.998974E+00,4.989946E-01


These commands are all very self explanatory, but I will explain them just in case:

*RST resets the instrument to a well defined  state.

VOLT 2 sets the voltage output to 2 V.

turns on the output.

READ? reads back the voltage and current.


Since the output of the SMU was connected to a 4 Ohm resistor, I got a current of ~0.5 A.


While running the commands it is also possible to watch the state of the instrument by watching the screen.





3 Experiments


3.1 Regulation Tests


I performed a few tests to see how different settings and operation modes affect the instruments regulation. More specifically I tested the voltage priority mode (VPM), current priority mode (CPM) and the voltage modulation capability of the instrument.


NGU401: Regulation Tests



3.2 Solar Sell Characterization (using different workflows)


I connected a small 6 V solar panel to the instrument to show how the Isc, Voc, Pmax, Vmax, Imax parameters can be measured using different workflows.


R&S NGU401: Solar Cell Characterization



3.3 Peltier Module Characterization


I measured the I/V and R/V curves using short voltage pulses and continuous voltage (steady temperature) to see how temperature affected the curves. I also measured the efficiency of a 100Hz PWM driven Peltier to see how it compared to the efficiency when driven with continuous voltage.


R&S NGU401: Peltier Module Characterization



3.4 PI Control System


I used the voltage modulation feature to build a temperature control system that did not rely on SCPI commands but the analog output of an MCU.


NGU401: PI Control System



3.5 DC-DC Converter Characterization


I showed how to use the NGU401 with another SMU to characterize a step-down converter. I measured the Efficiency at different input voltages and output currents, and also the ability of the step-down converter to generate a steady voltage output. PWM, PFM and DCM modes were measured and compared against each other.


R&S NGU401: DC-DC Converter Characterization



3.6 MCU Energy Consumption Measurement


With the help of an oscilloscope, I took advantage of the high dynamic range and fast sampling rate of the NGU401 to evaluate the energy consumption of an ESP32 microcontroller running in different operation modes.


R&S NGU401: MCU Energy Consumption Measurement



4 Conclusions


After having spent many hours testing almost every feature of the instrument my overall opinion is that it is a very good instrument, even though it still has a few issues. Of these issues what I felt requires more attention is the computer software, followed by the firmware and the documentation. As an example of a software bug, I had an issue with NGx Sweep Tool where it tried to open multiple VISA connections to the instrument until it could not connect to the instrument anymore (This bug got fixed after telling R&S about it). An example of a firmware issue is that the "measure" bit of "STAT:OPER:INST:ISUM1:EVENT?" gets triggered even when there are no new measurements available (such as when the output is off). Some problems that I found in the documentation are that some parts need a more detailed explanation such as the status reporting system, and I also found a few errors in the figures (they appear to be from the NGM200 or NGL200 ipower supplies). I've been in contact with R&S so I expect these issues to get solved at some point, so far my experience with the customer service has been great.


Probably the two features that I liked the most were the AFG and the fast logger. These two worked together quite well and opened a wide variety of possibilities, some of which I explored while characterizing a Peltier module or measuring the energy consumption of an MCU. I also liked the front panel interface, it is functional and aesthetic. This may not seem like a great deal but it actually is, I have a couple of instruments with awful user interfaces that make me waste quite a bit time because of the difficulty to get to the desired options, not clearly showing all the relevant information on the screen, wasting useful screen space, etc. All in all I am very happy with the instrument and I think Rhode & Schwarz did a very good job designing the instrument.


Code and data generated to write this review can be found on GitHub.