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What bandwidth does your signal occupy?
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Transcript:
Determining whether or not your W-CDMA signal is distorted in any way can give you qualitative insight into your device’s transmitter operation. Today we’ll discuss a quick way to identify whether or not your transmitter is operating as expected.

Hi, I’m Ally, and welcome to Ready, Set, Measure. In this video, we’ll be discussing occupied bandwidth.

In an earlier video, we discussed measuring channel power. A similar but slightly different measurement is occupied bandwidth.

With channel power, we’re looking at signal power in a predetermined channel. With occupied bandwidth, we’re starting with a signal and looking at its bandwidth.

Doing so gives us a better picture of whether or not our transmitting signal is behaving as expected.

Normally, any distortion such as harmonics and intermodulation produces power outside of a specified bandwidth. However, by looking at the bandwidth occupied by the majority of our signal’s power, we can check to see if there are ‘shoulders’ on the side of the spectrum or ‘rounding or sloping’ on the top of the spectrum. These indicate intermodulation or filter- shape problems that we need to address during the design phase.

It’s important to measure the occupied bandwidth, which is typically where 99% of your device’s signal lies. Doing this manually can be cumbersome and could slow down your device design and manufacturing.

So, instead of doing this manually, we can use a signal analyzer with a built-in occupied bandwidth- or OBW- tool to make this measurement automatically. Here we can calculate and display the total power of our signal, the occupied bandwidth, and the transmit frequency error.

We need no prior knowledge of the signal other than its nominal carrier frequency.

The occupied bandwidth measurement integrates the power of the displayed spectrum. It also adds markers to show us the occupied bandwidth. The measurement defaults to 99% but this can be changed if required.

Additionally, you often want to know if your modulation is symmetrical. To do this, you can look at the transmit frequency error, which is the difference between the intended center frequency and actual center frequency.

So, the occupied bandwidth measurement is a quick way to get qualitative and quantitative insights into your transmitter’s operation.

With any transmitter, you have an intended bandwidth you’re trying to stay within. This occupied bandwidth measurement gives you a quantitative idea of how well reality lines up with your expectation.