Understanding Channel Power Measurements

Channel power is defined as the sum of all the power in a given channel, within the defined channel bandwidth.

Channel power is an absolute power measurement with results expressed in units like dBm. In some cases, signal’s power may extend or leak beyond the edges of a channel. The measurement of this leakage, or energy outside of the channel, is called ACLR or adjacent channel leakage ratio.

Channel power is also a very common measurement, particularly for signals used in wireless communications services. There are three main ways in which channel power can be measured.

• Using an RF power sensor
• Spectrum analyzer using the zero span method
• Spectrum analyzer using the integrated bandwidth method

Measuring channel power using an RF power sensor

An RF power sensor can be used to measure channel power, and this method has several advantages. It’s a very simple measurement, power sensors are relatively inexpensive. They are also portable, which is important because this measurement is often made in the field. Depending on the power sensor, it also yields very accurate results. No special configuration is needed to measure channel power with a power sensor: just connect the sensor to the signal and read the result.

The power sensor method only works if, the signal which is measured is the only signal present at the sensor’s RF input. If other signals are within the power sensor’s bandwidth, these have to be removed, otherwise the sum of all power in the sensor’s bandwidth will be measured, not just the power of the channel ot interest.

Spectrum analyzer using the zero span method

Another way to measure channel power is using the zero span mode on a spectrum analyzer. Instead of sweeping across a span, or range of frequencies, the analyzer is parked on the center frequency of the channel with span set to zero. The analyzer measures the power within the resolution bandwidth, and channel power appears as a line on the display.

Power can be read off manually or using a marker. In zero span mode the RMS detector needs to be used as power is measured.

When making channel power measurements in zero span mode, a resolution bandwidth slightly wider than the channel bandwidth should be chosen. For example, if the channel is 1.4 MHz wide, a 3 MHz resolution bandwidth would be appropriate. Anyhow, this can be problematic measuring wider channels, because the spectrum analyzer may not have a resolution bandwidth setting wide enough to cover the entire signal.

There should not be any other signals present within the resolution bandwidth as well. Increasing the sweep time in zero span mode tends to yield better measurement results.

Spectrum analyzer using the integrated bandwidth method

Modern spectrum analyzers can automatically calculate channel power given the channel bandwidth and frequency. This measurement is done by integrating over the channel bandwidth, so it’s sometimes called the integrated bandwidth method. Like the zero span method, this method also requires the use of the RMS detector.