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2017

  In praise of the humble power sensor

It’s always nice to get a reality check from a fellow RF engineer, and Keysight’s Eric Breakenridge recently delivered one in the form of an explanation of the capabilities of modern RF power sensors.

I guess I’ve become a bit of a measurement snob, having spent many years working with vector signal analyzers (VSAs). When we developed and introduced these products 25 years ago, we were really enthusiastic about a new tool that would tell us virtually anything about the most complex RF signals.

Measurements with the VSAs weren’t just comprehensive, they also had unprecedented accuracy, including RF power. They were significantly more precise than the spectrum analyzers of the time, especially on time-varying signals or those with complex modulation.

Looking back, however, I remember the RF engineers who were developing VSAs also had power meters and power sensors on their benches, and used them frequently. Those power meters and sensors were the benchmarks for our nascent VSAs, and the new analyzers would never have achieved such exceptional accuracy without them.

Power sensors—whether they’re connected to power meters or to PCs via USB or LAN—are relatively inexpensive and have advantages that ensure the ultimate in power accuracy. For one, you can attach the sensors directly to the DUT, eliminating cabling and adapters. Also, many sensors are designed for specific frequency ranges, letting them cover the frequencies in question with excellent impedance match and accuracy—and that accuracy is highly traceable.

The sensors, as I learned from Eric, can also make great measurements of power versus time. Here’s his example, a measurement of the time to switch the gain state of an amplifier.

RF power vs. time measurements using power sensor.  Power shown on log (dBm) and linear (Watts) scales

Two measurements from the U2042XA X-Series power sensor show the time to switch the gain state of an amplifier. Power is shown in watts (top) and decibels (bottom). The default 10 percent and 90 percent reference points have been adjusted to better reflect the time for the gain to reach its final value.

The USB and LAN power sensors can be connected to PCs and used with the power meter application in Keysight's BenchVue software. That application provides both graphical results and compiled tabular data such as this pulse analysis table.

Power meter application program on PC creates pulse statistics table from data acquired from RF power sensor via LAN or USB

When connected to an X-Series power sensor, the Power Meter Application assembles a series of measurements and creates a complete summary of pulse characteristics.

In addition to benchtop configurations, the LAN models (via power-over-Ethernet) are useful for remote monitoring, placing the sensor right at the DUT. Multiple sensors can be used with a single PC.

The power sensors have a wide dynamic range of –70 dBm to +26 dBm and sample rates as short as 50 ns. While they can’t match the speed, sensitivity, or selectivity of signal analyzers, their performance is a good fit for many applications, and the combination of low cost and measurement accuracy can help you make better use of the more-expensive signal analyzers in your lab. A power sensor demonstration guide shows some example measurements and configurations.

I don’t suppose anything will dull my esteem for VSAs, but my recent exposure to power sensors and the sophisticated power tools in my previous post have made me a little less of a measurement elitist. Whatever gets the job done the best!

  Understand, anticipate, and respect your power limitations

Are IoT and other smart/connected devices the biggest wireless opportunity right now, or the biggest source of hype? I suppose they can be both at the same time, and it’s clear that lots of devices will be designed and sold before we know the true magnitude of this wireless segment.

A crucial element of many wireless devices is operation on battery power. In recent years, this has often meant lithium ion batteries that are recharged once every day or two. These days, however, lots of design effort is transitioning to devices that use primary batteries, ranging from traditional alkaline cells to buttons and lithium-coin cells. These power sources are expected to last months, if not a year or more, despite their small size.

Meeting these power demands will require careful engineering—both RF power and DC power—and a holistic approach, to give you confidence that you’ll get the needed combination of performance and real-world functionality. This is a field with lots of investment and competition, meaning you may not have a second chance to fix a design failure or a development delay.

Exceptional power efficiency doesn’t happen by accident, and it isn’t a result of some tuning or tweaking at the end of the design process. Instead, it starts when devices are being designed, and overall success stems from a sustained process of measuring and optimizing. Two aspects of test & measurement are worth special note in designing for very low power: 

  • Using a power source with realistic limitations
  • Precisely measuring power consumption in all modes of operation, and during transitions

When powering a device or circuit, using a benchtop power supply can actually hide problems from you. Primary cells, especially when they’re very gradually going flat, can be highly imperfect power sources, and their imperfections can change with aging and temperature. Some precision power sources are now available to emulate real-world cells.

 

Diagram shows voltage-current emulation capabilities of Keysight B2961A/62A low-noise power sources

Keysight’s B2961A/62A low-noise power sources can emulate the DC voltage/current output characteristics of many different power sources, providing insight into real-world behavior in limited power conditions.

These advanced power sources can give you early warning of DC power problems while there’s still time and flexibility to design around them. They can also emulate power sources such as solar cells, with their very non-battery characteristics.

As always, if you’re going to optimize something, you have to measure it. On the power measurement side, extended battery life may require the ability to measure small currents, and perhaps a form of power scheduling to avoid excessive demand from simultaneous digital and RF activities. Whether you’re using a real battery or an emulator, instruments such as a DC power analyzer can tell you how much power is being used, and just when it’s needed.

Short term and long term current measurements from Keysight N6075C DC power analyzer

On the Keysight N6705C, the dynamics of current consumption are shown over 30 ms in scope view (left) and over 30 seconds in data logger view (right). Measurements such as these provide a more complete understanding of the real-world power demands of a device or subsystem.

The use of periodic quiescent states is one proven technique for extended battery operation, and it presents its own measurement challenges. Extremely tiny currents must be assessed to understand cumulative consumption, and recent products, such as the Keysight CX3300A device current waveform analyzer, are meant for just that. These analyzers have both analog and digital inputs, and the ability to time-align measurements of both.

In this post I’ve drifted away from my usual focus on RF measurements but, of course, the core concern for us in these DC power issues is to ensure that RF matters are proceeding as they should, no matter the state of DC power. Fortunately, there are ways to use the new power analyzers to trigger RF signal analyzers and thus correlate DC power with RF power and modulation, and that’s a subject for a future post.

  Are you a good spectral citizen?

 

Note from Ben Zarlingo: This is the third in our series of guest posts by Nick Ben, a Keysight engineer. In this post he provides an overview of adjacent channel power, a measure of how well your products play with others.

 

In the previous edition of The Four Ws, I discussed the fundamentals of noise figure. This time I’m discussing the WHAT, WHY, WHEN and WHERE of adjacent channel power (ACP) measurements so that you can ensure your device is only transmitting within its assigned channel and doesn’t interfere with signals in undesignated adjacent channels.

 

What

A key requirement for every wireless transmitting device is that it should only transmit within its assigned channel. To make sure of this, adjacent channel power (ACP) measurements determine the average power or interference a transmitting device generates in the adjacent channels compared to the average power in its assigned channel. This ratio is known as the adjacent channel power ratio (ACPR). ACP measurements use a reference level of 0.00 dBm, and the desire is to have the ACP be as low as possible. A poor ACPR is an indication of spectral spreading or switching transients for the device under test (DUT), which are a big no-no.

General diagram of adjacent channel power measurements, including main transmitter channel and two adjacent channels. Hand-drawn diagram

A look at generated power in a transmitting device’s adjacent channels. Adjacent channels are located above and below the generated power in the transmitting device’s designated channel. The ratio of the two gives you your ACPR.

 

Why and When

ACP is key in ensuring we avoid interference with other signals in adjacent channels where your device has not been licensed (from a regulatory body or agency like the Federal Communications Commission) to transmit.

 

The measurement is made on digital traffic channels. However, it is especially important to make the ACP measurement when there may be more stringent requirements beyond regulatory licensing. For example, Bluetooth, LTE and W-CDMA have ACP as part of their physical layer requirements.

 

Where (& How)

When using a spectrum analyzer, the results of an ACP measurement are displayed as a bar graph or as spectrum data (or a combination), with data at specified offsets. They can also be displayed as a table that includes the actual power of the adjacent channels and your transmitting device’s channel in dBm. This is done in addition to the power relative to the carrier in dBc for both the upper and lower sidebands.

 

As described earlier, to determine ACPR you have to integrate the power in your assigned channel and the power in the adjacent channels, and find the ratios between the integrals.  Keysight’s ACP PowerSuite measurement simplifies this process to give you fast results without manual calculations. All you do is set the channel frequency, bandwidth, and channel offsets for your signal’s specifications. The ACP PowerSuite measurement on X-Series signal analyzers takes care of everything else.

Adjacent channel power ratio measurement screen and partial front panel of Keysight signal analzyer.

Keysight Technologies EXA Signal Analyzer displaying a transmitter output using the ACP measurement, which is one of nine power measurements in the X-Series PowerSuite. Ideally a good signal should not go outside the transmitted channel (purple) into adjacent channels (green). Channel power ratios are shown in the table below the spectrum/channel bar display.

 

Wrapping Up

If you’d like to learn more about making fundamental measurements and spectrum analysis concepts to ensure your transmitting device is behaving as you’d expect, refer to the application note Spectrum Analysis Basics. The application note collects the fundamental knowledge needed to ensure your continued development of a great product. I hope my third installment of The Four Ws of X has provided some worthwhile information that you can use. Please post any comments – positive, constructive, or otherwise – and let me know what you think. If this post was useful give it a like and, of course, feel free to share.