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The Four Ws of Spurious Emissions

Blog Post created by benz on Apr 30, 2017

  Refining your design and creating happy customers

 

Note from Ben Zarlingo: I invite you to read this guest post by Nick Ben, a Keysight engineer. He discusses the what, why, when and where of spurious emissions before delving into the importance of identifying them in your device’s transmitting signal and thereby improving your product design.

 

If you’re reading this, you’re probably an engineer. That means you may be looking for ways to improve your present and future designs. If yours includes a transmitter, one key to success is checking for spurious emissions that can interfere with signals outside your device’s designated bandwidth. Characterizing spurious behavior can save money and help you create happy and loyal customers.

But wait, you say: WHAT, WHY, WHEN and WHERE can I save money and create happy customers by measuring spurious emissions? I’m glad you asked. Let’s take a look.

What: A Quick Reminder

Ben Z has covered this: Spurs are unwanted stray frequency content that can appear both outside and within the device under test’s (DUT’s) operating bandwidth. Think of a spur as the oddball signal you weren’t expecting to be emanating from your device—but there it is. Just like you wouldn’t want your family’s phones to overlap with one another in the same band, they shouldn’t interfere with that drone hovering outside your window (made you look). If you refer to the traces below, the left side presents a device’s transmitted signal without a spur and the right side reveals an unwanted spurious signal, usually indicating a design flaw.  

 

Two GSM spectrum measurements are compared.  The one on the right contains a spurious signal.

In these side-by-side views of a 1 MHz span at 935 MHz, the presence of an unwanted spur is visible in the trace on the right. Further investigation should identify the cause.

Why and When

In densely populated regions of the frequency spectrum, excessive spurs and harmonics are more likely to be both troublesome and noticed. To prevent interference with devices operating on nearby bands, we need to measure our own spurious emissions.

Where (& How)

Spur measurements are usually performed post-R&D, during the design validation and manufacturing phases. Using a spectrum or signal analyzer, these measurements are presented on a frequency vs. amplitude plot to reveal any undesirable signals. Spur characterization is done over a frequency span of interest using a narrow resolution bandwidth (RBW) filter and auto-coupled sweep-time rules. The sweep normally begins with the fundamental and reaches all the way up to the tenth harmonic.

While current spur-search methods are good for the design validation phase, they aren’t great because measurements are too slow for pass/fail testing of thousands of devices on the production line. These tests are often based on published standards (perhaps from the FCC) that may be described in terms of a spectrum emission mask (SEM). Fortunately, SEM capability is available in Keysight X-Series signal analyzers.

To tackle the issue of slow sweep times and enable faster testing, today’s signal analyzers use digital technologies, especially DSP, to improve measurement speed and performance (see one of Ben’s earlier posts). Ultimately, you can achieve faster sweep times—as much as 49x faster than older analyzers—when chasing low-level signals at wide bandwidths.

Wrapping Up

If you’d like to learn more, a recent application note titled Accelerating Spurious Emission Measurements using Fast-Sweep Techniques includes detailed explanations, techniques, and resources. You’ll find it in the growing collection on our signal analysis fundamentals page.

I hope my first installment of The Four Ws of X provided some information you can use. Please post any comments—positive, constructive, or otherwise—and let me know what you think. If it was useful, please give it a like and, of course, feel free to share.

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