I seem to be having a lot of luck with Rohde & Schwarz lately, as a RoadTester of their RTM3004 oscilloscope, I was awarded RoadTester of the Year. I also RoadTested their HMP4040 power supply, which led to an opportunity to review their HMC8043 power supply. It hasn’t been even a month since the latest review, but I’m back with more exciting Rohde & Schwarz news!
Today (20th August 2019) marks the official launch of Rohde & Schwarz’s latest specialty power supply product, the NGM-series including the single-channel NGM201 and dual-channel NGM202. This is a two-quadrant programmable DC power supply suitable for bench-top and rack-mount use, being a more featureful version of the NGL-series power supplies launched earlier in late-January. Such power supplies have appeal to users working with DC-to-DC converters, battery operated devices or evaluating rechargeable batteries thanks to its ability to both source and sink current on the same channel with variable internal impedance. It would also appeal to users who need to profile the power consumption of microcontrollers and radios which transition rapidly between different operational modes or have very low consumption in certain modes thanks to its fast transient-response performance (<30µs to within 20mV), fine 6.5-digit metering resolution and fast data-logging capabilities up to 500kS/s. This is just scratching the surface.
Thanks to Rohde & Schwarz, I’ve been fortunate enough to have been sent an NGM202 one-and-a-half months prior to its official launch to put it through a barrage of tests and report my findings. Find out about what Rohde & Schwarz’s latest offering is capable of and how it performs under my barrage of testing in this first-look review.
Market Survey & Feature Introduction
When we think about power supplies, we often think of the basic single-quadrant power supply that only sources current. However, specialty supplies exist which operate in two or four quadrants, acting as both a source and sink, and in the case of four-quadrant supplies for positive and negative voltages. This can be beneficial in applications where tight integration between sourcing and sinking simplifies connections, reduces measurement errors, improves measurement speeds and saves bench real-estate.
A two-quadrant supply is particularly useful for those working on battery-powered equipment for accurately simulating the behaviour of a battery under load or being charged, or for testing and characterising batteries, and even in testing and characterising DC to DC power converters.
Having a two-quadrant supply also produces an unexpected benefit, in being able to produce a supply with tighter transient response characteristics. This makes such specialty supplies especially useful to those working with batteries, battery-operated equipment and devices which transition quickly between operational modes such as FPGAs, microcontrollers and radio modules. Faster responses can also make for more rapid automated test sequences or running I-V curves.
In the relatively limited market of two and four-quadrant power supplies, the landscape is dominated by Rohde & Schwarz, GW Instek, Keithley and Keysight. Sorted by price, it seems that both GW Instek and Rohde & Schwarz compete in the value segment of the two-quadrant market. In this respect, the NGM200-series looks quite attractive as it is able to best the GW Instek products across the majority of specifications.
The Rohde & Schwarz NGM200-series is a close relative of the NGL200-series launched earlier in the year and shares very similar specifications with the exception of additional hardware measurement ranges for voltage and current for finer resolution and improved accuracy, FastLog capability and availability of DVM mode and battery simulator options. These two units make the Rohde & Schwarz HM8143 and Keysight 663xx series look a little dated by comparison, offering superior user and remote-control interfaces without any significant deficiencies in their capabilities.
From there, the prices jump two to four-fold as we enter the four-quadrant supply territory dominated by Keithley and Keysight. These units offer finer resolution and greater accuracy, but many have limited current sinking capabilities when derating is taken into account and inferior user interface features. Given the increased expense for these instruments, it is perhaps not justified to directly compare their specifications, but it does serve to illustrate why a two-quadrant supply may be a better fit depending on your needs and budget.
When compared across the board, the NGM202 has a number of key standout features including 60W per channel sink current capability at up to 3A; rapid rise and fall time, narrow programming resolution, high accuracy, low ripple/noise compared to peers within a similar price range; 5” 800×480 colour touch LCD display; constant resistance sink mode; comprehensive suite of built-in features; and a large number of options. As the NGL200-series shares many of these features, I think the biggest competitor to the NGM200-series is perhaps Rohde & Schwarz’s own product!
See R&S NGM202 In-Depth – Ch1: Market Survey & Feature Introduction for more details.
Unboxing
The NGM202 arrived safely in a rather long-shaped cardboard box with the traditional blue Rohde & Schwarz branding. As I received a pre-release NGM202, the packaging may vary slightly from retail units, but the unboxing process was straightforward.
The instrument was well-protected inside a static dissipative bag, with a transit protector on the LCD, suspended from all sides by foam end-pieces. Accessories and documentation were supplied inside their own box which kept them secure.
The power supply looks almost identical to the NGL-series as it shares the same heritage. The front panel has a clean and simple design, dominated by a generously sized 5” 800×480 touch LCD screen. This is flanked with a limited number of buttons, rotary knob and shrouded 4mm safety banana plug sockets. The rear panel is much busier by comparison, including optional GPIB, optional digital I/O, LAN, USB (host and device), detachable terminal block channel outputs (including sense and optional DVM), ground and fused power connections. Grilles along the side of the case intake fresh air for cooling, which is exhausted from the rear. Retractable flip-out front feet are provided.
Documentation supplied includes a pre-delivery report, calibration certificate, delivery note, service brochure and getting started guide. An appropriate power cable is included.
See R&S NGM202 In-Depth – Ch2: Unboxing for more details.
Standalone Operation
From the perspective of standalone operation, the Rohde & Schwarz NGM202 was a pleasure to use with a wealth of onboard features and options. A few rough areas were discovered, but as this is a launch-day review, it is not unexpected.
I really liked the simple, clean and modern front panel. The 5” 800×480 colour LCD capacitive touch screen was bright, crisp and easy to read and the menu interface would be intuitive to anyone who has ever used a smartphone. The interface is also fast, with the voltage and current read-outs updating at a brisk 10Hz. The limited number of buttons are well placed for easy identification by feel, with some keys backlit with coloured LEDs to convey channel status at a glance. The design of the home screen uses colours and icons to quickly convey the status of the channels. Expanding the channels shows further details about its operation. Well-placed shortcuts allow for more rapid navigation to the appropriate settings menu (e.g. tapping on the ranges brings you directly to the DVM/Ranges sub-menu). Direct-entry of values via the on-screen keypad allows for more rapid and accurate setting of values, while the presence of the knob caters for those who may prefer a more “analog” approach. I found the menu interface to be logical and navigable despite the use of nested sub-menus. For those who prefer not to use touch-screens, there is the provision for attaching a USB mouse and/or keyboard.
The downside of this design decision is the limited space on the front panel, which means that the NGM202 has only channel output banana jacks on the front, with the sense and DVM connections only available via the rear.
The rear panel was cluttered by comparison, with an optional GPIB (IEEE-488) interface, LAN, USB A (for memory, keyboard/mouse), USB B (for attaching to a host as USB-CDC/USB-TMC), digital trigger I/O (DA-15) option, fused IEC power inlet and two detachable gender-reversed 3.81mm terminal blocks for channel output, sense and DVM option connections. I appreciated that detachable terminal blocks were used and included, allowing for rapid connection and disconnection without being hunched near the rear panel. I also liked the additional USB port on the rear. The location of the Channel 1 output being just underneath the mains power input is probably not the most convenient location, however.
While in use, the power supply had no problems handling its full rated sourcing and sinking load over two-and-a-half hours at an ambient of 16-23°C, although the datasheet claims that sinking power derating to 90W would be necessary for ambient above 30°C. The fan control appeared to be smooth with no obvious steps in speed, being relatively quiet (e.g. like a laptop) at currents below 3A. When higher currents are demanded, the noise increases to something similar to a hairdryer on low, with a noticeable whine that is somewhat distracting. I suspect this is a trade-off that is necessary to deliver such high regulation performance and power ratings within a 2RU-compatible form factor. To its credit, it is not the loudest power supply I’ve tested.
Inbuilt features include a full suite of protections, such as OVP, OCP, OPP and safety limits, with the power supply internally managing OTP. The OCP feature is improved over previous instruments, featuring independent delays for power-on and fuse blowing. Fuse linking is also implemented, although I would like to be able to set the OCP level and current limit independently. Safety Limits can be used to ensure the power supply cannot be set to values which might endanger the device under test. The unit also has the QuickArb feature, allowing for arbitrary waveforms of 4096-points to be generated with a dwell time as low as 1ms per point. The ramp function also helps to limit the rise time of a channel that has been turned on, linearly ramping the output over an adjustable period of 10ms to 10s. These functions appeared to operate as expected.
Various data recording features are included in the NGM202, including front and rear USB connectors to which USB mass storage devices can be attached to record screenshots, instrument settings and log voltage/current data. Screenshots are stored as .png files with a header including date, time and instrument details. Instrument settings are stored as binary .rds files. Ordinary logging (<10Hz) records directly to .csv with configurable delimiter, decimal separator, error designator and line-end marker. FastLog (specific to the NGM200-series) allows for much faster data logging at fixed rates of 15, 30, 61, 122, 244, 488, 976, 1953, 3906, 7812, 15625, 31250, 62500, 125000, 250000 and 500000 Sa/s. This mode records first into a binary .raw file with associated metadata in a .meta file which can be converted to .csv afterwards using the Binary to CSV Converter menu option.
While the data recording features are quite capable, I encountered a number of issues with lost data and filesystem errors during testing despite using different known-good USB mass storage devices, some with very high-performance capabilities. I hope that Rohde & Schwarz can improve the stability of this, as my current workaround is to record to internal memory first and then transfer it to an external device using the File Manager, the latter of which does not always succeed the first time and is not possible with FastLog.
A big differentiating feature between the NGM200-series and the NGL200-series is the availability of the NGM-K106 Battery Simulator feature. This option adds the capability for the NGM202 to simulate up to two independent virtual batteries at the same time, one on each channel. The battery models are defined by a .csv file containing capacity, initial state, current limits, and state of charge, open circuit voltage and internal resistance values at each point. The power supply then either sources (discharging battery) or sinks (charging battery) to the connected device, simulating the voltage curve and internal resistance effects of a battery. This allows for designs to be tested with a “repeatable” battery, allow designs to be developed in the absence of a battery, allow for evaluation of charger behaviour and peak current draw effects with the ability to log results all from the one unit.
Using the demo battery model files supplied, testing with a DC electronic load produced roughly the expected curves with some deviations likely due to limitations in the battery models in accurately describing behaviour at the test current draw. There was also a capacity discrepancy of about 5% while testing Ni-MH and Ni-Cd models. The key to making effective use of this feature is to develop a battery model which adequately describes your intended battery under the range of loads it is expected to experience. A discrepancy in how the simulation handles <0% SoC and >100% SoC was also identified and reported to Rohde & Schwarz for improvement.
The documentation for the NGM202 seems to be mostly complete, comprehensive and easy to read with only a number of minor errors and inconsistencies. There were some gaps when it comes to certain SCPI commands in the programming section where more detail would have been appreciated. The NGM200-series does not appear to have any model-specific software, but instrument driver support for IVI.NET, LabVIEW, LabWindows/CVI and VXIpnp is expected, consistent with the NGL200-series. While dealing with Rohde & Schwarz, I have found their staff to be highly knowledgeable about the technical details of their products. With service and sales locations around the world, those in Australia are especially lucky as we have a calibration, service and sales centre in North Sydney to provide local service. Aside from that, Rohde & Schwarz gear generally has a good reputation – the NGM202 is backed by a three-year warranty which can be extended by up to two years with, or without, (accredited) calibration coverage.
See R&S NGM202 In-Depth – Ch3: Standalone Operation for more details.
Remote-Control Interfaces
The Rohde & Schwarz NGM202 gives you lots of options when it comes to remote-control connectivity. The unit offers USB (CDC and TMC) and LAN (LXI/VXI and HiSLIP) out of the box, with options for GPIB (NGM-B105) and 2.4GHz Wireless LAN 802.11n connectivity (NGM-K102). Digital Trigger I/O option (NGM-K103) is also available. Interface configuration can be done via the menu interface using the touch-panel. Command processing speed depended on the interface used, but with a simple four-command loop, command processing times as low as 1.69ms were recorded, faster than the claimed <6ms. The NGM202 was also stable in handling week-long remote-control experiments. One does have to be careful around range changes, as the instrument will return NaN values. Repeated back-to-back MEAS:CURR? queries return the same value as the instrument is so fast in processing commands, so introducing delay may be necessary.
Unfortunately, during testing a number of incompatibilities which caused the USB interface to be temperamental with my AMD Ryzen-based machine and the wireless LAN interface to suffer delays and packet loss with my main access point. In both cases, however, changing to a different computer or access point appeared to resolve the issue. There was also an interesting discovery that the *RST command would reset LAN static IP configuration, resulting in disconnection after an instrument power cycle. These issues have been reported to Rohde & Schwarz, so hopefully they will be improved in the near future.
The biggest disappointment would have to be the web interface, which was limited to basic network configuration tasks and had a password security scheme that sent passwords in plain text. The lack of a SCPI terminal or remote front-panel capabilities is unfortunate.
The remote-control interfaces were very useful for various experiments including LED I-V curve tracing, automated lithium-ion battery cycle testing and verification of the constant resistance and source-impedance modes of operation. I also used it extensively for the instrument performance tests in the next chapter. The instrument was very reliable under remote-control, enduring multi-day testing marathons with aplomb.
Owing to a lack of time, I did not evaluate the Digital Trigger I/O option (NGM-K103), but will revisit this in a future posting.
See R&S NGM202 In-Depth – Ch4: Remote-Control Interfaces for more details.
Instrument Performance Testing
I attempted to verify the NGM202’s performance as a hobbyist with some time, equipment and knowledge in a bedroom running some experiments to the best of my abilities and in good faith – testing a pre-release single-sample under a “real world” scenario with all its imperfections. Variations are to be expected due to differences in testing environment, but I hoped to glean some insight into how accurate the datasheet is.
In all, I think that the NGM202 demonstrated its voltage and current programming and readback accuracies to be quite conservative, with actual instrument performance mostly beating the specified accuracy levels, inspiring confidence in the results obtained from the instrument despite the sub-optimal operating environment. Current sourcing performance appeared to be better than sinking performance, but all were rather impressive. Some minor sawtooth patterns were visible in voltage readback which were well within-spec. There were, however, transient glitches in the current read-out under very specific conditions which I would like to see resolved.
Testing of constant current overshoot was also interesting, as the results were probably slightly counterintuitive. As the NGM202 has such a fast rise time, the power supply always reached the set voltage before turning back, with a transient glitch at 3ms and 6ms, settling by about 8.5ms which is not terribly fast nor slow. This result may not actually reflect the NGM202’s true performance, but may be due to the use of a wire-wound resistor load that has an inductive characteristic causing the current to lag the voltage and interacting with the control loop. I did perform a test with an LED, setting the current limit to 20mA and the voltage to 20V prior to activating the channel. In every case, the LED failed immediately, suggesting that perhaps the supply’s fast rise time may indeed be quicker than the regulation loop.
With regards to transient response, difficulties in testing were encountered due to oscillatory behaviour at medium slew rates with the electronic load. At high slew rates, a transient response time of about 50-70µs was measured, which is plenty fast. It is slower than the claimed 30µs, but this could be down to differences in test setup and probing, as the passive probes are perhaps not the best for this type of measurement.
Testing of output synchronicity resulted in values of about 10-13µs, faster than the claimed <25µs typical. Likewise, with overcurrent protection, trip times were measured at 722µs and 1.026ms which are both faster than the claimed <1.5ms. On unexpected power down, it seems that regulation is lost and the output (when unloaded) can fly back up to about 3.4V, which is perhaps not too severe but something to be mindful of in case of using the hardware power switch with outputs still activated.
The unit’s output was quite stable even from cold, with a drift of about 0.2mV registered which could be due to the measurement equipment. Power consumption at standby is non-existent due to the hardware power switch, but idling with channels disabled was 20.3W and with the channels enabled but unloaded was 21.9W. Both are quite respectable given the linear architecture of the supply and the sophisticated touch-screen interface.
See R&S NGM202 In-Depth – Ch5: Instrument Performance Testing for more details.
Teardown
Taking a look under the hood of the NGM202 was rather impressive and in some ways, overwhelming. Rohde & Schwarz have managed to pack a toroidal transformer with multiple voltage windings and independent windings for each channel into the slim case, meeting the promise of linear design while ensuring better efficiency and lower heat generation. The unit was constructed in a sturdy aluminium chassis inside a steel shell ensuring good shielding and heat dissipation.
The mainboard was populated with a large number of high-end integrated circuits from Analog Devices, Silicon Labs and Texas Instruments for regulation and power control, while also housing on-board fuses from LittelFuse to ensure safety. The design of the board has a symmetry around the centre rear portion where a tunnel heatsink and a high-static pressure fan provide cooling. The man semiconductors appear to be from Vishay and On Semiconductor. Input power goes through various stages of filtering and transient suppression.
The brains of the unit sit behind the front panel, controlled by a Freescale iMX6 SoloX SoC with Micron Technology memory, while channels are managed by an Altera Cyclone IV FPGA. Capacitors used in the unit are high-quality branded aluminium electrolytic capacitors from Nichicon, Matsushita (Panasonic) and Vishay BC Components and solid electrolytic surface mount capacitors. Relays all appear to be Omron branded. In all, it seems that only good quality branded components are utilised in the design.
The build quality seemed good and in line with expectations, although the board did have some Kynar wiring and the soldering of the cable for the front outputs seemed to leave some flux residue on the board. The LAN interface relies on an internal Ethernet cable to bring the connection from the rear to the front panel, whereas the wireless LAN interface relies on a PCB printed trace antenna on a Microchip/Atmel module mounted just behind the front panel, which may explain the apparent lack of Wi-Fi sensitivity.
See R&S NGM202 In-Depth – Ch6: Teardown for more details.
Conclusion
The NGM202 feels like a modern, up-to-date, easy-to-use high-performance instrument which is a bigger brother to the NGL200-series launched earlier in the year. Sharing an identical platform, the NGM202 improves upon the earlier NGL200-series with additional voltage and current ranges, FastLog, DVM-mode option and Battery Simulator option which increase its utility, especially for those who are working with battery-operated equipment, battery chargers, DC-to-DC converters and studying power consumption of microprocessors, FPGAs and radio modules. While I am not aware of its final pricing, considering the NGL200-series’ competitive pricing within the small two-quadrant power supply market, I expect the NGM200-series to occupy a more premium position but still be great value.
Using the unit in standalone mode was quite enjoyable with its intuitive, easy to use and easy to navigate interface that would be familiar to anyone who has ever used a smartphone. The 5” screen is large, clear and sharp, using various icons and colours to quickly convey supply status at a glance. Screen updates are swift, with readings appearing at a brisk 10Hz rate. The unit also accommodates users who would rather use a keyboard and mouse, or the input knob on the front panel. The limited set of buttons are also well positioned for reach by feel. The only thing that seemed to be lacking was a graphing display mode – something that should be coming in the medium-term according to Rohde & Schwarz.
The front panel outputs are perhaps slightly inconvenient as the sense connections are only available on the rear on the NGM202, thus I found myself using the rear removable terminal block connections most of the time. The rear panel is slightly cluttered with the variety of interfaces and the exhaust vent. The fan was slightly distracting at high loads, sounding like a hairdryer on low, but no difficulties were encountered running at full rated power for extended periods. Sourcing and sinking were tested with no difficulties, including the internal impedance and constant resistance modes. However, compared to a dedicated electronic load, the lack of a constant power sink mode seemed slightly lacking.
The biggest positive was the onboard features which include QuickArb, Ramp, OVP/OCP/OPP/Safety Limits, and Battery Simulator (option) which pack a lot of user-friendly capability out of the box. The integrated data capture functionality including Logging, FastLog, Screenshot and Settings Save work well to capture results, all without a computer required. A small issue with SoC limits in the battery simulator and some instability with USB mass storage devices was experienced during testing and has been reported for improvement.
The NGM202 has a range of remote-control interfaces including USB-CDC, USB-TMC and LAN as standard. Optionally, the unit can also accommodate GPIB, Wireless LAN and Digital Trigger I/O connections. Remote control capabilities were used extensively during this review to run experiments and verify instrument performance. The interfaces were mostly trouble-free, although the web browser interface was perhaps the main disappointment due to its lack of functionality. Some minor USB and wireless LAN incompatibilities were perhaps the main issues encountered.
When it comes to instrument performance, the NGM202 made a good showing overall with a number of key specifications such as voltage/current programming/readback accuracy, OCP trip time and channel synchronicity being conservative, easily beaten even under non-ideal circumstances. Transient response times measured were not quite as per the datasheet, but likely due to differences in set-up, probing or interaction with the electronic load. The datasheet figures remain plausible, however. The only real issue seemed to be very infrequent erroneous current readouts under very specific circumstances.
Looking under the covers reveals the impressive feat that is the NGM202. In the 2RU-compatible form factor, it has a small toroidal transformer with many independent windings for each channel and voltage range to ensure efficiency, symmetrical design sharing a single tunnel heatsink outfitted with semiconductors from Vishay and On Semiconductor, regulation and power control using integrated circuits from Analog Devices, Silicon Labs and Texas Instruments, relays from Omron, protection fuses from LittelFuse, with power filtered through capacitors from brands such as Nichicon, Matsushita (Panasonic) and Vishay BC Components. The whole operation is orchestrated by a Fresscale iMX6 SoloX SoC supported by Micron Technology RAM/Flash and Microchip/Atmel Wi-Fi module, with channels controlled by an Altera Cyclone IV FPGA. While the result is greater than the sum of its parts, it’s good to see that the parts are a parade of high-quality brand-name components, assembled with care into a sturdy aluminium chassis, providing us confidence that the unit will perform well in the long haul. The instrument is backed by a three-year warranty as standard, which can be extended up to five years with or without accredited calibration coverage.
It’s important to remember that this was a pre-release unit, thus it is anticipated that minor issues would arise and be resolved through future firmware updates. To this end, I am in communication with Rohde & Schwarz, reporting my findings to hopefully ensure any issues are ironed out as soon as possible. That being said, none of the issues seem to be show-stoppers and the instrument has excellent performance characteristics even if it doesn’t quite have the most polished firmware from day one. But things should only improve from here …
So, if you’re thinking about a two-quadrant power supply, keep an eye out for the Rohde & Schwarz NGM200-series at your preferred test-equipment distributor or Rohde & Schwarz office – it should be showing up any time soon! Perhaps if the NGM200-series doesn’t quite fit the budget, the NGL200-series is worth considering as it is priced competitively compared with other two-quadrant supplies and shares a majority of features.
Further posts about the NGM202 are expected to be made as improvements are made to the firmware and further experiments are conducted focusing on aspects which I didn’t have time to cover prior to launch.
Updates
- R&S NGM202 Update: F/W V2.005 Changes, Digital Trigger I/O, FastLog Fun & Battery Cycle Testing published 7th October 2019
