When measuring noise or low-level signals, make sure the analyzer has a pre-amplifier. Also, consider the analyzer measurement architecture. Analyzing low-level signals often means setting a narrow span. When comparing speed in narrow spans across a number of analyzers, swept-based analyzers slow down considerably, while analyzers that employ DSP techniques don’t suffer this type of degradation. Finally, noise measurement results should be expressed in terms of noise density within a certain bandwidth.

Intermodulation measurements determinesthe distortion that a device or system may create when stressed under specific signal conditions. Figure 2 shows a device stimulated with two CW carriers, or tones. The tones cause the device to generate distortion that appears in the frequency domain as two distortion products, left and right of the input tones. As the analyzer is also a receiver with active components in its signal path, there is a risk that the analyzer can also generate this type of distortion, making the measurement invalid.

One easy check to verify the signal’s integrity is to increase the attenuation setting of the analyzer. If the signal reduces in amplitude with an increase in attenuation, then the distortion products are generated by the analyzer. If changing the attenuation has no effect on the distortion products, then the measurement is valid. When the attenuator value is increased, the noise floor increases by the same decibel amount so that the amplitude of the carrier remains constant with different levels of attenuation. The increase in noise floor, however, could mean that the noise could mask the intermodulation product. Fine attenuator steps are important for achieving the best measurement results as coarse attenuation steps can move the noise floor by 10 dB, quickly masking the signals that need to be measured.

The ability to measure small signals in the presence of large signals is a key function of any type of spectrum analysis instrument and is defined as the dynamic range of an instrument. Dynamic range is often expressed as a combination of the analyzer’s third-order intermodulation performance (e.g., the two-tone measurement discussed above), the instrument’s noise floor performance, and its phase noise. It is often quite difficult to directly compare the dynamic range of an instrument, as different manufacturers can optimize the instrument for noise floor performance or distortion performance. An easy way to relatively compare the dynamic range of multiple analyzers is to examine the W-CDMA adjacent-channel power. This measurement takes into account all of the above parameters.

Figure 2. A signal with two carrier waves and the accompanying distortion.

When measuring modulated signals, the spectrum/signal analyzer should be able to measure not only the spectrum of the signal, but also the quality of the modulation.

Figure 3 shows a typical digitally modulated signal in the frequency domain. This signal uses a modulation scheme that does not have a constant power envelope, so its amplitude varies over time. A key measurement for an analyzer is the average power of this type of signal and is usually specified over a defined bandwidth. The intermodulation and phase noise distortion manifest themselves in the signal skirt. The adjacent-channel power feature of the analyzer helps quantify the intermodulation and phase noise performance of the device under test.

Figure 3. A typical digital modulated signal as it appears in the frequency domain.

The ability to demodulate the signal and express the quality of the signal in terms of a metric such as error vector magnitude (EVM) is a key requirement for modern analyzers. Key analyzer performance characteristics that enable this type of measurement are the instrument’s digitizing bandwidth and its corresponding frequency and phase response. For common modulation schemes such as GSM or W-CDMA, demodulation and quality metrics are often built into the analyzer. Some advanced analyzers can adapt to evolving communications technologies.

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