Friday, January 30, 2015

Using the Passthrough Command in IVI drivers

Hi everybody!

I was working on a customer question yesterday and I thought that it would make an enlightening blog post.  We have a feature for our IVI-COM and IVI-C drivers that allows you to directly send SCPI commands to your instrument.  This is really useful if you run into a situation where you think there is a function is missing from our driver or you run into something unexpected.  Overall, it is pretty easy to use once you know where to find it.

Let's look at the IVI-COM driver first.  You can find the passthrough in the Systems Interface under the IO property.  One you get to the IO level, you can just use the standard VISA-COM commands to send commands to the instrument and read responses back.  For my little example here I am just going to send a voltage measurement and read back the response:

In the code above, agDrvr is the name I gave to the instrument when I initialized it.

We also provide an IVI-C driver.  The IVI-C driver also has a passthrough command that is just a little more complicated than the IVI-COM version.. To send SCPI commands, you use the AgN67xx_SystemWrite function and to read data back you would use the AgN67xx_SystemRead function.  The same example as above would look like this:

In this IVI-C code:
status is where the IVI-C driver will report an error if there is an unsuccessful call 
session is the handle that I gave to this instrument when I initialized
32 was my best guess at the size of the response string.  I like to overestimate.
strResp is where I want the response to be stored
respSize is the actual size of the response and is returned by the program

All in all it's not too difficult and will definitely come in handy.  

That all I have for you this month.  As always, let us know if you have any questions.

Matt







Thursday, January 29, 2015

New Keysight Power Analyzer called IntegraVision

Back on June 23, 2014, I posted about the last Agilent power products to ever be announced. At that time, we had not yet officially changed our name from Agilent to Keysight. So the AC6800 AC sources we released on that date were released under the Agilent name, soon to be rebranded to Keysight. Well, today, I am announcing the first new Keysight power product: the Keysight Technologies IntegraVision Power Analyzer Model PA2201A.


A press release went out about these products earlier today: click here to view. We here at the Power & Energy Division of Keysight have been involved in power products for decades, and of course, Keysight has an oscilloscope division with commensurate experience producing scopes. I consider the new IntegraVision power analyzers to be a combination of the vast experience of our engineers from these two disciplines combining a power analyzer and an oscilloscope. The power analyzer will enable you to accurately measure parameters such as watts, VA, VAR, power factor, crest factor, efficiency, watt-hours, amp-hours, and harmonics while the oscilloscope will allow you to visualize in real time the voltage, current, and power waveforms that are important in your design.

I am very exciting about this new line of power measurement instruments! I have been working for HP/Agilent/Keysight for nearly 35 years now and have always worked with power products during my career. One of my favorite product families to support has been the older sophisticated 6800 AC Power Source/Analyzers (not to be confused with the newer basic AC6800 series mentioned in the first paragraph above). The older AC sources can produce sine waves, square waves, and arbitrary waveforms (for tests such as cycle dropout tests) as well as measure most of the power analyzer parameters mentioned above since they have a power analyzer built into the AC source. But now the new IntegraVision power analyzer goes well beyond the capabilities of the power measurements built into our AC sources. Adding time-based measurements like watt-hours and amp-hours opens up many more energy measurement application areas for this new product and the visual waveform measurements are a huge benefit when doing things like characterizing AC inrush current or product response to AC line disturbances. I am delighted with the performance of the touch-screen on this product – it will help you gain faster insight into your designs plus it just makes using the product fun! With 0.05% basic accuracy, 5 MSample/second 16-bit digitization, and inputs isolated to 1000 V, the IntegraVision power analyzer really is a superb product for power consumption and power conversion applications. Click here for the IntegraVision web page with links to the individual products.

So the next time you need a power analyzer with great accuracy and you also want to see the power waveforms related to your application, be sure to look at Keysight’s new IntegraVision Power Analyzer Model PA2201A. I’m sure you will not be disappointed! And look here for future posts about some of the interesting applications for this product, such as AC power line disturbance measurements and micro-inverter efficiency measurements. Suggest some of your own power measurements for me to make and I’ll see what I can do for a future post for you!

Friday, January 23, 2015

Significance of the RC time constant for super capacitors

When we think about an RC time constant, also known as tau, the thing that usually comes to mind is its relevance to filters. But when it comes to super capacitors it has a related, but somewhat different connotation. Let me explain.

One of the things we learn about early on in electrical engineering about the RC time constant is its relevance to the frequency response of first-order low pass RC filter, as depicted in Figure 1.
  


Figure 1: First order low pass RC filter

The cut off frequency, fc, is the point where the AC signal amplitude is down by 3 dB, as shown in Figure 2. Correspondingly the power is down by 6 dB, which is also referred to as the half-power point. 
  


Figure 2: First order low pass RC filter response

Here the cutoff frequency is related to the RC time constant by the expression:



However, another aspect to consider about an RC time constant is its relevance to the time it takes for the capacitor to be charged and discharged.  This is significant for power and energy applications using capacitors for energy storage. The capacitor’s voltage response for charging is given by the expression:



For t = RC (one time constant) the capacitor is charged up to 63.2% of its final voltage. Similarly, when discharging, the capacitor is discharged to 36.8% of its final voltage.

So, how is this of significance with a super capacitor? The fastest limiting case for charging and discharging the capacitor is when any external resistance is set to zero. Then the only limiting resistance is the internal equivalent series resistance (ESR) of the capacitor. For conventional capacitors the RC time constant will typically be on the order of microseconds to tens of microseconds. However, super capacitors, with capacitance in Farads to hundreds of Farads (or greater) and ESR of milliohms (or less) have RC time constants on the order of roughly a second.  The fastest they can be charged and discharged for power applications is on the order of seconds or slower. This may sound slow compared to more conventional capacitors but this is not what you really want to compare them to. Because of their extremely large capacitance they are useful for energy storage applications. And when you compare them against other means of storing reasonable amounts of energy, like batteries for example, they are then extremely fast. This makes them ideal for an application needing to store and return quick surges of energy, such as is the case of regenerative braking.


So now when you see the RC time constant being used in reference to super capacitors you will know it’s meant as a figure of merit for how quickly they can be charged and discharged!

Wednesday, January 7, 2015

A new current measurement methodology: It’s all about counting the electrons going by!

One thing near and dear to us here at the Power and Energy Division is making accurate current measurements. What exactly is current? It’s basically the flow of electric charge per unit of time. In a conductor it’s the flow of electrons through it per unit of time. 

The ampere is the fundamental unit of current in coulombs per second, which equates to 6.241x1018 electrons per second. Accurate current measurement is one of the core values of virtually all of our products. Some of the precision SMU products can measure down to femtoamp (fA) levels (10-15 amps). This is where we tend to muse that we’re getting down to the levels where we’re virtually counting the individual electrons going by.

While there are a few different ways of measuring current, by far the most common is to measure the voltage drop across a resistive shunt. With careful design this provides the most accurate means of current measurement. There are a lot of non-obvious factors that can introduce unexpected errors that many are not aware of, leading them to believe they have better accuracy than what it really is. A good discussion of what it takes to truly make accurate current measurements was covered in a previous posting “How to make more accurate current measurements”(click here to review). We go through great pains in addressing these things in our products in order to provide accurate and repeatable measurements.

Unlike the volt and the ohm, which have quantum standards for their electrical units, the ampere instead relies on the standards for the volt and ohm for measurement, as a quantum standard for the ampere that directly relates it back to charge is still lacking. However, that may change in the not too distant future. A group of scientists were awarded the Helmholtz Prize in metrology for realization of the measurement of the ampere based on fundamental constants. Basically they’ve created an electron charge pump that moves a small, fixed quantity of electrons under control by a clock. You can say they’re literally “counting the electrons as they go by”. This could become the new SI standard reference for current measurement. To me this is very fascinating to find out about. More can be learned on this from the following link to the press release “Helmholtz Prize for the “new” ampere”(click here to review).  I am curious to see how this all plays out in the long run. Maybe it will lead to yet another, and better, way to make more accurate current measurements in products we all use today in our work in electronics!