Transmit Spectrum Mask

Definition

The transmit spectrum mask is the power contained in a specified frequency bandwidth at certain offsets, relative to the total carrier power.

Standards

• IEEE Std 802.11b-1999 Paragraph 18.4.7.3 Transmit spectrum mask
• IEEE Std 802.11a-1999 Paragraph 17.3.9.2 Transmit spectrum mask
• ANSI/IEEE 802.11 Std 802.11 First edition 1999-00-00 Paragraph 15.4.7.4 Transmit spectrum mask
• FCC Title 47 Code of Federal Regulations Part 15 (47 CFR 15)

Measurement Subtleties

This test ensures that multiple WLAN devices do not unduly interfere with each other. The spectrum test can be a good indicator of deteriorating performance. Yet, this test is not a direct measure, such as packet error rate.

The transmit spectrum mask measurement includes the in-band and out-of-band spurious emissions. It may be expressed as a ratio of power spectral densities (PSD) between the carrier and the specified offset frequency band. For WLAN, the reference power is taken as the peak PSD in the signal. All offset results are also the peak PSD in that offset, as opposed to the integrated power. (See the figure for an example measurement.) 

The IEEE 802.11 standards do not specify a transmit modulation filter function. Yet, filtering is implied in the spectrum mask. The IEEE 802.11 standard does not specify the type of measurement detector for the test. Empirically, the shape of the spectrum (for an unframed signal) is similar on a swept spectrum analyzer or a vector signal analyzer.

The IEEE 802.11b spectrum mask is that of a continuous signal. The drawback with configuring a transmitter for continuous output is that the final result may not be representative of actual performance under burst conditions. Some standards are explicit in the need to use framed signals.

Test Tools

You can use the 89600 Series vector signal analyzer standalone, in conjunction with the ESA-E and PSA Series spectrum analyzers, and with several of Keysight's Infiniium family of oscilloscopes.

You can use the ESA or PSA Series spectrum analyzers as down-converting front ends for the 89600 (with specific options). This provides vector signal analysis capabilities up to 50 GHz. You can also use 89600 as standalone. This provides vector signal analysis up to 6 GHz, with one or two channel inputs, for IQ, IF, and RF measurements.

The 89607A WLAN test suite software comes pre-programmed with the IEEE802.11a/b/g transmitter performance specifications for automated control of the 89600 Series vector signal analyzer.

The PSA and ESA spectrum analyzers offer built-in 802.11 spectral emission mask (SEM) measurements.

Related Measurements

• Receiver Adjacent Channel Rejection
• Non-Adjacent Channel Rejection
• Packet Error Rate (PER)