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The Four Ws of Adjacent Channel Power

Blog Post created by benz on Nov 30, 2017

  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.

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