Detailed review of Keysight SMU/Power source B2962A - Page 1
Forum section: electronics / general

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 10, 2020, 06:05:39 am
content: Some time ago I got my hands on a (used) B2962A Keysight power source. It is similar to a B2900 series SMU. There haven't been too many in-depths reviews of SMUs here. (There are a couple of review videos on EEVblog and SignalPath, but as most such videos they just demonstrate basic features and if Western brand - complement construction else if Chinese brand - criticize construction, usually without critical testing of the specs, end rant). So I thought I would do an in-depth review since my actual application pushes the capability of the instrument anyway.B2961A/B2962A are one/two channel power sources with up to 210V and up to 3A DC output. Compared with B2901A/B2902A/B2911A/B2912A SMUs they have slightly better noise specifications, slightly better AC waveform capabilities, and only 4.5 digit measurement resolution. As most specifications are identical I suspect the hardware is virtually the same in all these units.First test: turn the unit on, hook up to oscilloscope, we see spikes 40-80 mVpp:It is a switching power supply and not a particularly quiet one. The spec is 3 mVrms, which is roughly true on some ranges.Next test: Monitor 10V DC output for a while, we see this:The spec is 150 uV p-p. I suspect many DC power supplies can do better in terms of short-term stability, even my analog supply was better:https://www.eevblog.com/forum/testgear/power-supply-drift-battle-keysight-b2962a-vs-mpja-9623ps/Here are results for DC current, with comparable performance:Next, test AC voltage performance, which is more impressive, for example one can output 80Vpp sine at 1 kHz with a few ppm stability:I have already compared AC amplitude stability of B2962 with various function generators here:https://www.eevblog.com/forum/metrology/stability-of-ac-voltage-from-function-generators/It is generally significantly better than all function generators I tested and maybe comparable to AC calibrators. There are however, significant limitations on the slew rate, only 10 mA range has relatively high slew rate. For higher current the best one can do at 1 kHz is 14Vpp on 1.5A range. AC voltage output does not depend very much on the amount of current that is actually drawn.Next we look at AC current performance, which is a little worse than AC voltage. Also note the absolute value of the current is quite far off.Here, however, the bandwidth specs are given only for very low load resistance, 0.1 to 1 Ohm. For higher load resistance the AC current drops significantly with frequency:These types of measurements can be converted to a current source differential resistance as a function of frequency. For 2 Ohm load resistance and 0.9A current amplitude here is the differential resistance as a function of frequency:100 Hz 920 Ohm200 Hz 230 Ohm500 Hz 36 Ohm1 kHz 9.5 Ohm2 kHz 2.6 OhmIdeal current source should have infinite source resistance. One can see that at 1 kHz the performance is no better than a 9.5 Ohm resistor in series with a voltage source. This makes the current sensitive to small changes in contact resistance as well as changes of the load resistance with temperature, significantly degrading current stability.The SMU can also source very small currents, I have not investigated this in detail, but it seems to work pretty well:The offset is about 1 pA and noise is much less than 1 pA, significantly better than the spec. One bug I found is that the two lowest current ranges (10 nA and 100 nA) can only be accessed using manual range setting.As a two-channel unit, B2962 can output two signals at the same time. However, it has a major flaw in that the timebase of the two channels is not synchronized and differ by several ppm. As a result at 1 kHz the channels start to get out of phase after a few seconds. I contacted Keysight regarding this and they offered a solution consisting of re-triggering two channels from a common timer every 5 cycles. This however introduced additional small gaps in the waveform and requires changing several settings every time the frequency is changed. I found a more reliable solution is to define two arbitrary waveforms which are then output using a common timer. This also allows one to set a specific phase between the two channels, which otherwise does not seem to be possible. However, it negates the built-in waveform capabilities of B2962 compared with B2912. The waveforms are easy to upload using an ASCII file on a USB stickThe user interface of the SMU reminds me of old mainframes:One uses arrows to move cursor to desired field, press enter, input value, press enter again. Most settings of the instrument cannot be changed while the output is on. The interface appears to be fairly stable and crashed only once during my use.The instrument comes with two free software options for remote control, Quick IV Measurement Software and a web interface, however neither worked for me. Quick IV software simply would not start on my Windows 7 computer without giving any errors. The Web interface uses Java but after going through the steps of enabling Java and testing it on other websites, the Java part of the web server did not work. Regular VISA commands worked fine. I did not test the measurement capabilities of the device. It does not have waveform measurement tools build-in, so any measurement analysis requires downloading the waveform to the computer.Keysight offers low-pass filters for these units to remove switching power supply noise for $1-2k! This is clearly the most brilliant part of the product (marketing). They claim 1 nV/Hz^(1/2) noise, but only at 10 kHz and only for DC output. Low frequency (mediocre) DC performance of course cannot be improved by a low-pass filter. Here is the noise spectrum I obtained with a home-built filter:At 1 Hz the noise level is about 10 uV/Hz^(1/2), about 3 times higher than is claimed on a plot in the spec sheet. To filter out all high frequency spikes one needs inductors with high resonance frequency, I usedhttps://www.digikey.com/en/products/detail/tdk-electronics-inc/B82111B0000C019/4245889https://www.digikey.com/en/products/detail/tdk-electronics-inc/B82111E0000C024/4245909The filter works pretty well for AC and DC voltage and DC current. For AC current the performance is poor because the differential resistance is low and one cannot use 4-wire sensing, so the current depends sensitively on the impedance of the filter elements. Keysight also does not offer a filter for AC current. I used two filter stages, both stages together work well to source voltage, while only the first stage makes sense for AC current. Here are the pictures and elements of the filters.1st filter: Two 13 uH inductors and 0.1 uF capacitor2nd filter: 1000 pF and 0.1 uF on isolated BNC input, two 56 uH inductors, 0.1uF, 3.3 uF and 10 uF capacitors on output.Conclusion:The instrument offers a broad range of capabilities but is not refined. Only AC source performance up to 1 kHz is competitive but with some significant limitations.

author: emtgrhu
author_rank: Newbie
timestamp: February 09, 2021, 08:39:59 am
content: How did you measure the spectral noise density?Could you please share the setup and technique that you used?Very nice and through review of the B2962A, thank you!

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: February 09, 2021, 11:48:59 am
content: I used AlphaLab LNA10 preamp going to an Owon XDS3062A oscilloscope. The pre-amp is either AC coupled or used in differential mode, relative either to a stable voltage source or comparing two channels of B2962A. All the methods gave same result.

author: emtgrhu
author_rank: Newbie
timestamp: February 11, 2021, 03:07:57 pm
content: Nice! Great idea with the LNA and the scope has also a decent resolution.The spectral noise density was then plotted using FFT?Did you use the FFT function of the Owon itself or did you take the samples from the oscilloscope and calculated it in some other tool (the plots have a matplotlib look to it)?If it's not too much to ask, which equation did you use to convert Volts to V/Hz^1/2?

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: February 11, 2021, 04:18:40 pm
content: Acquired data on Owon (up to 40M), transfer to PC, then used Matlab to analyze. I think this was done with pwelch function to calculate noise spectral density.