Sunday, June 27, 2010

Breakthrough Dynamic Current Measurement Technology for Low Power Optimization in Portable Electronics

On June 21st Agilent released two new 2-quadrant SMU modules, the N6781A and N6782A. The N6781A is optimized for battery drain analysis and the N6782A is optimized for functional test. These modules go into Agilent’s popular N6705A/B DC Power Analyzer and N6700 Modular Power System. These SMU modules provide current measurement capability down to 10s of nano amps, four different current measurement ranges, fast transient response, I and V arbitrary waveform capability, and two 200 KSa/s 18 bit measurement digitizers for capturing voltage and current in parallel. Now those are awesome features right there, but what really makes these SMUs special is a patent Agilent only technology that gives them the ability to seamlessly transition through current measurement ranges without any discontinuities or glitches in the output power being delivered to the DUT. This innovative feature is intended for engineers testing devices where low power optimization is critical for delivering maximum battery life such as cell phones, handheld radios, or portable medical devices.

The seamless measurement ranging ability means the N6781A and the N6782A can capture dynamic current ranging from milli or micro amps to amps with high accuracy and 18 bits of resolution over the whole range. For those out there who currently use current shunts to capture dynamic current this means no more worrying about shunt loading effects when the current goes to the high end of the range and no more worrying about poor accuracy and resolution when the current dips to the low end of the range. For those currently using high performance supplies to capture dynamic current this means no more running tests multiple times at different measurement ranges to get each piece of the current profile puzzle and then having to piece it together. The figure gives a visual picture of what the seamless ranging can do.

For more information on the new N6781A click here

For more information on the new N6782A click here

Friday, June 18, 2010

What is the Load Transient Recovery Specification Telling Me?

Whenever a load change occurs on the output of a power supply, that power supplies output voltage will momentarily change from its programmed value. The figure to the right is a screen shot from an scope showing the change of a supplies voltage level after its load went from 1 mA to 500 mA in about 30 us.

The specification for this power supply characteristic is called the Load Transient Recovery Time or Transient Response Time. It represents how long it takes a power supply to return to its set voltage level after a sudden change in load current.
The specification typically has three parts to it:
•Magnitude of the load change, such as from 50% of full load to 100% of full load. So if we had a power supply that was rated for a max current of 10 A the spec would be referring to a load change from 5 A to 10As
•Voltage settling band is how close the voltage level will settle to its original level before the load change. Note that after a load change the power supply’s never recovers to its original level. How close it gets to returning to its original level is dependent on the magnitude of the load change.
•The time it takes the power supplies voltage level to settle within the voltage settling band

Below is an example of a transient response specification for two of Agilent’s N6700B modular power system’s supplies.

N6751A & N6761A Transient Response
Magnitude of load change: 60% to 100% and from 100% to 60% of full load for models N6751A & N6761A
Voltage settling band: ± 75 mV
Time: < 100 μs

The example spec can be interpreted as the output of the N6751A will return to no more than 75 mV within its original value in less than 100 us when a load transient that is 60% to 100% or 100% to 60% of its full scale load current occurs.

The transient response time is highly dependent on the speed of the supplies internal voltage output monitoring loop. Speeding up this loop provides better transient response time, but the output becomes more susceptible to instability and oscillations. That is why supplies with short transient response times typically cost more because of the extra investment in engineering it to ensure good output stability.

If the load is changing too fast for the supplies transient response time to keep up the power supply will never be at its programmed voltage level. With that in mind, the transient response time of a power supply is strongly related to the output bandwidth of the supply. The reason you typically do not see an output bandwidth spec is because the bandwidth of the supply is dependent on its load and power supply manufacturers cannot predict what type of load their customers will connect to the output of their supplies.

Monday, June 14, 2010

The World's Fastest Real-Time Scope!

Although this maybe a bit out of the gpete realm I felt I had to mention that about a month ago Agilent released the Infiniium 90000 X-Series oscilloscope family, which is the fastest most accurate scope family ever. The X-series offers up to 32 GHz of true analog bandwidth in a scope!! This is truly a milestone in test and measurement engineering. I can remember back in '99 digital scopes were just starting to take over and 500 MHz of bandwidth was considered high! Technology is sure moving fast. I have not yet had a chance to play with one of these amazing instruments yet, but I did get a chance to speak to some of the people from the R&D team that worked on them. The engineering that went into the timing, digitizing, and signal conditioning is pretty amazing. Not only that the engineering that went into the probes (up to 30 GHz probing)is amazing. Seeing scopes with this much bandwidth and accuracy makes me think that eventually we may see the spectrum analyzer, signal analyzer, and scope merge into a single instrument (FFTs are a common feature on most scopes today). It is still years off but I can see it coming.

You can check out the world's fastest scope family by clicking on the link below.

Infiniium 90000 X-Series oscilloscope family

Friday, June 11, 2010

Eloads are great for outdoor Photovoltaic test but where is the MPPT capability?

Eloads have become a popular solution for outdoor testing of higher power photovoltaic (PV) devices, like PV panels and concentrated PV. The main reason for this is eloads can sink a lot of current at a low cost compared to 2 and 4 quadrant power supplies. The testing is usually of the design verification variety and one of the main roles of the eload is max power point tracking (MPPT) on the output of the PV device. One request of end users of eloads for this application is does the eload have MPPT capability or can you put built-in MPPT capability in the eload? Currently there are no general purpose eloads (that I know of) that have built-in MPPT capabilities. This means it is up to the test engineer to implement an MPPT algorithm in software. This adds time and complexity to the test engineer’s job. Also since the algorithm is in the software it has to deal with IO latency between the computer and the eload which lowers the test systems MPPT speed. To help test engineers out with this challenge I wrote an article entitled “A Photovoltaic MPPT Algorithm for DC Electronic Loads” that was published by Electronic Design and can be found at the link below. The article introduces an ideal algorithm for performing MPPT with an eload. It is ideal because of its low complexity and it keeps IO transactions to a minimum to reduce the affects of IO latency.

A Photovoltaic MPPT Algorithm for DC Electronic Loads

Friday, June 4, 2010

Why is the Voltage Double the Value I Set on the Function Generator??

One question that comes up a lot from junior engineers when measuring the output of a function/arbitrary waveform generator (FAWG) with a scope or DMM is "why is the measured voltage double the value I set on the FAWG?" The quick answer to that question is there is an impedance mismatch that is causing the discrepancy. Below is a link to a YouTube video created by a colleague of mine that quickly and clearly answers that question.

33220A: The voltage is double the value I set....WHY?