Thursday, May 31, 2012

*OPC and You

Today I have a guest-blogger, one of my Agilent colleagues (and friends), Matt Carolan. Matt has experience with programming our power products, so I asked him if he had anything he wanted to share with our audience. He has many things to talk about and will most likely contribute to future posts, but decided to start with the *OPC command, which is the “Operation Complete” command. Here is Matt’s post:

Hi, my name is Matt and I am an Application Support Engineer at Agilent Technologies. I have had 12 years of experience programming our power products and I wanted to write about a small but powerful command, *OPC.

*OPC is a standard IEEE-488.2 command that allows you to synchronize your power supply with your program. *OPC lets you know when all pending operations are complete. A pending operation is something such as the voltage being set or the output turning on. I worked as a test engineer for a few years and we always used the *OPC command in our calibration routines. We would send a calibration command (such as CALibration:LEVel:P1) followed by a *OPC? query. This allowed us to ensure that the calibration command had finished executing and that the power supply should be outputting the correct level before we took any measurements with our test system.
There are two ways to use *OPC. There is a standard *OPC command and the *OPC? query. The *OPC command will set bit 0 of the Standard Event Status register when all pending operations are complete. You can then use a *ESR? Command to poll the Standard Event Status Register. When this returns a 1, all pending operations are complete. When you use this command, it only works for any pending commands that were sent BEFORE you sent the *OPC command. It will not work for commands sent after the *OPC command. You can send another *OPC command to start the cycle again.

The other way is to use it as a query. When you send a *OPC? query, it will put a 1 in the output buffer when all pending operations are complete. The main drawback of this method is that if there is an operation that takes a long time to complete, your *OPC? query will timeout. You would need to have a long timeout set in your IO library to avoid this. You cannot send any commands after the query without getting an error so this will hold up your program until all pending operations are complete.

If you have any questions please comment here or on the Agilent forum at:

Friday, May 25, 2012

Battery-killing cell phone apps?

Two days ago, I came across an article entitled “Do Android Security Apps Kill Your Batteries?” The article talks about mobile device users avoiding security apps because they think the apps run down their batteries too quickly. A member of the Anti-Malware Testing Standards Organization (AMTSO) is using Agilent’s N6705B DC Power Analyzer to evaluate just how much the security apps affect battery run time. While the results are not yet complete, the researchers are planning to measure power usage with no security app running, with the app running in the background, and with the app actively working. Their full report is due out by the end of July. Here is a link to the article, written by Neil Rubenking in his SecurityWatch blog for PC Magazine Digital Edition:

I was pleased to see the N6705B DC Power Analyzer used in this way – this product has power modules and software that are specifically designed to do exactly this type of evaluation!

If you have to evaluate a mobile device’s battery run time for any reason, here is a link to “10 Tips to Optimize a Mobile Device’s Battery Life” written by our own Ed Brorein (contributor extraordinaire to this blog):

And here is a link to Ed’s post from a few months ago on “Using Current Drain Measurements to Optimize Battery Run-time of Mobile Devices”:

When the researchers complete and publish their evaluation on how security apps affect your cell phone battery run time, we’ll be sure to follow-up with another post! In the mean time, protect your phone in whatever way you like, and keep charging ahead by charging your batteries!

Wednesday, May 16, 2012

What Is Old is New Again: Soft-Switching and Synchronous Rectification in Vintage Automobile Radios

I have to admit I am a bit of a vintage electronics technologist.  One of many pass times includes bringing vintage vacuum tube automobile radios back to life. In working with modern DC sources I’ve seen innovations come about in the past decade for efficient power conversion, including soft switching and synchronous rectification. A funny thing however, for those who have been around long enough, or into vintage technologies like me, is that these issues and somewhat comparable solutions existed up to 70 years ago for automobile radios and other related electronic equipment. What is old is new again!

As we know, vacuum tubes (or valves to many) were to electronics back then as what semiconductors are to electronics today. The problem for portable and mobile equipment was that the vacuum tubes needed typically 100 or more volts DC to operate. They did have high voltage batteries for portable equipment but for automobiles the radio really needed to run off the 6 or 12 volts DC available from the electrical system. The solution: A DC/DC boost converter!

Up until the mid 1950’s most all automobile radios used vacuum tubes biased with high voltage generated from a rather primitive but clever DC/DC boost converter design. The inherent technological challenge was semiconductors did not yet exist to chop up the low-voltage, high-current DC to convert it to high-voltage, low-current DC. Of course if the semiconductors did exist this would all be a moot point! Making use of what was available the DC/DC boost converters employed what were called vibrators, which are a form of a continuously buzzing relay, to chop up the low-voltage DC for conversion. Maybe some of you are familiar with the soft humming sound heard when an original vintage automobile radio is turned on, prior to the vacuum tubes finally warming up and the audio taking over? That humming is the vibrator, the “heart” of the DC/DC boost converter in the radio.

Figure 1 below is an example circuit of vibrator-based DC/DC boost converter in a vintage automobile radio. This is just one of quite variety of different implementations created back then. Two pairs of contacts in the vibrator act in a push-pull fashion to convert the low-voltage DC into a low-voltage AC square wave. This in turn is converted to a high-voltage square wave by the transformer. Because the vibrator is an electro-mechanical device, it is limited in how fast it can switch. Switching frequencies are typically about 100 to 120 Hz. The transformers used are naturally the steel-laminated affairs similar in nature to the transformers used to convert household line voltage in home appliances. Very possibly some radio manufacturers used off- the-shelf appliance transformers in reverse to step up the voltage!  Often a small rectifier vacuum tube, such as a 6X4 (relatively modern, by vacuum tube standards) would be used to convert the high voltage AC to high voltage DC, but in this particular example I am showing here another two pairs of contacts on the secondary side switch simultaneously with the first pairs of contacts to rectify the high voltage AC. Highly efficient synchronous rectification, up to 70 years ago!

Figure 1: Representative DC/DC boost converter for a vintage automobile radio

The clever part of these DC/DC boost converters is making the vibrators last. Let’s see; 100 cycles/second, times 60 seconds/minute, times 60 minutes/hour, times ~2 hours/day, times 365 days/year; that’s 263 million cycles in one year! And while the vibrator was replaceable, it would often last for many years or more, which is quite remarkable. The trick was paying close attention to the switching as to not stress the vibrator‘s contacts. Referring to the waveforms in Figure 2, there is quite a bit of dead time between the non-overlapping switching of the contacts. This was by design. The capacitor across the secondary of the transformer in Figure 1 is carefully matched to ring with the transformer’s inductance such that the voltage is near zero across the alternate set of contacts is just as they’re closing, minimizing arcing and wear. Low-stress soft switching, again, up to 70 years ago! Ironically the cause for the vibrator failing was often due the capacitor degrading with stress and time. The capacitor was actually slightly larger than ideal value at the start to prevent overshoot and allow for aging. When resurrecting a vintage automobile radio frequently the vibrator will still work. Make certain to replace the capacitor first however or the vibrator is bound to have a very short second life.

Figure 2: Switching waveforms in a vibrator-based DC/DC boost converter

These vacuum tube automobile radios with vibrator-based DC/DC boost converters had quite a long run before being displaced, first for a very short period in the later 1950’s by hybrid radios using low voltage vacuum tubes and early germanium power transistors, and then finally overtaken by fully transistorized automobile radios in the early 1960’s.

So my hat’s off to the many design engineers of yesteryear who encountered such challenges, fully understood the principles, and just as creatively came up with solutions for them so long ago, based on what they had available. And again for those seasoned engineers who see such things come around yet once more as a new innovation, who humbly smile to themselves knowing that “what is old is new again”.

By chance are you a vintage electronics technologist?

Tuesday, May 8, 2012

Establishing Measurement Integration Time for Leakage Currents

The proliferation of mobile wireless devices drives a corresponding demand for components going into these devices. A key attribute of these components is the need to have low levels of leakage current during off and standby mode operation, to extend the battery run-time of the host device. I brought up the importance of making accurate leakage currents quickly in an earlier posting “Pay Attention to the Impact of the Bypass Capacitor on Leakage Current Value and Test Time”(click here to review). Another key aspect about making accurate leakage currents quickly is establishing the proper minimum required measurement integration time. I will go into factors that govern establishing this time here.

Assuming the leakage current being drawn by the DUT, as well as any bypass capacitors on the fixture, have fully stabilized, the key thing with selecting the correct measurement integration time is getting an acceptable level of measurement repeatability. Some experimentation is useful in determining the minimum required amount of time. The primary problem with leakage current measurement is one of AC noise sources present in the test set up. With DC leakage current being just a few micro amps or less these noises are significant. Higher level currents can be usually measured much more quickly as the AC noises are relatively negligible in comparison. There are a variety of potential noise sources, including radiated and conducted from external sources, including the AC line, and internal noise sources, such as the AC ripple voltage from the DC source’s output. This is illustrated in Figure 1 below. Noise currents directly add to the DC leakage current while noise voltages become corresponding noise currents related by the DUT and test fixture load impedance.

Figure 1: Some noise sources affecting DUT current measurement time

Using a longer measurement time integrates out the peak-to-peak random deviations in the DC leakage current to provide a consistently more repeatable DC measurement result, but at the expense of increasing overall device test time. Measurement repeatability should be based on a statistical confidence level, which I will do into more detail further on. Using a measurement integration time of exactly one power line cycle (1 PLC) of 20 milliseconds (for 50 Hz) or 16.7 milliseconds (for 60 Hz) cancels out AC line frequency noises. Many times a default time of 100 milliseconds is used as it is an integer multiple of both 20 and 16.7 milliseconds. This is fine if overall DUT test time is relatively long but generally not acceptable when total test time is just a couple of seconds, as is the case with most components. As a minimum, setting the measurement integration time to 1 PLC is usually the prudent thing to do when short overall DUT test time is paramount.

Reducing leakage current test time below 1 PLC means reducing any AC line frequency noises to a sufficiently low level such that they are relatively negligible compared to higher frequency noises, like possibly the DC source’s wideband output ripple noise voltage and current. Proper grounding, shielding, and cancellation techniques can greatly reduce noise pickup. Paying attention to the choice and size of bypass capacitors used on the test fixture is also important. A larger-than-necessary bypass capacitor can increase measured noise current when the measuring is taking place before the capacitor, which is many times the case. Establishing the requirement minimum integration time is done by setting a setting an acceptable statistical confidence level and then running a trial with a large number of measurements plotted in a histogram to assure that they fall within this confidence level for a given measurement integration time. If they did not then the measurement integration time would need to be increased. As an example I ran a series of trials to determine what the acceptable minimum required integration time was for achieving 10% repeatability with 95% confidence for a 2 micro amp leakage current. AC line noises were relatively negligible. As shown in Figure 2, when a large series of measurements were taken and plotted in a histogram, 95% of the values fell within +/- 9.5% of the mean for a measurement integration time of 1.06 milliseconds.

Figure 2: 2 Leakage current measurement repeatability histogram example

Leakage current measurements by nature take longer to measure due to their extremely low levels. Careful attention to minimizing noise and establishing the minimum required measurement integration time contributes toward improving the test throughput of components that take just seconds to test.