FEATURE TEST & MEASUREMENT
BEST PRACTICES FOR ACCURATE SPECTRUM MEASUREMENT
Giovanni D’Amore from Keysight Technologies offers some tips on improving measurement accuracy and suggests ways to avoid making inaccurate, if not fundamentally wrong, spectrum measurements
T
he primary use of a spectrum analyser is to measure and display
amplitude versus frequency of known and unknown RF and Microwave signals. With the advent of digital technology and signal processing, modern spectrum analysers are equipped with more capabilities. By digitising the signal, phase as well as amplitude is preserved and can be included as part of the information displayed. Figure 1 shows the distinction between absolute and relative measurements, where they are respectively made with a single marker and delta markers. For example, measuring the frequency or power level of a carrier (the left peak) is an absolute measurement. Measuring the amplitude of the second harmonic distortion (right peak), relative to the carrier would be a relative measurement. All modern spectrum analysers have a
built-in calibration source, which provides a known reference signal of specified amplitude and frequency. Absolute amplitude measurement is in fact a measurement made relative to this reference signal. To translate the absolute calibration to other frequencies and amplitudes, we rely upon the relative accuracy of the analyser. Many spectrum analysers use a 50MHz reference signal. At this frequency, the specified absolute amplitude accuracy is extremely good. For instance, the high performance X-Series signal analyser gives the best absolute amplitude accuracy of ± 0.24dB at the reference frequency. When we make relative measurements
on an incoming signal, absolute values do not come into play. For example, we are only interested in how the harmonic differs in amplitude from the fundamental. Worst-case would be when the fundamental occurs at the highest point and the harmonic at the lowest point of the frequency response. If the relative frequency response specification is ±0.5dB, the total uncertainty would be twice that value or ±1.0dB. Absolute frequency uncertainty is often described under the frequency readout accuracy specification and refers to centre frequency, start, stop and marker
18 MARCH 2018 | ELECTRONICS
correction function in conjunction with a signal source and a power meter. This process shifts measurement reference plane from the analyser front panel to the DUT.
Connector care including proper torque
frequencies. Span accuracy comes into play when you make relative measurements. You can compute frequency accuracy
from the sum of sources of errors which can be found in the analyser’s datasheet. Sources of errors include frequency reference error, span error, and resolution bandwidth (RBW) centre-frequency error. Modern analysers can measure frequencies with an accuracy of <0.1%, which is ideal for wireless communication applications.
HOW TO IMPROVE MEASUREMENT ACCURACY? Before we begin any measurement, we can step through it to see if any controls, like RF attenuator setting, resolution bandwidth or reference level, can be left unchanged. If so, any uncertainties associated with changing these controls drop out. We may also trade off one uncertainty for another, e.g. reference level accuracy against display fidelity, using whichever gives better accuracy. It is always worthwhile to pay careful attention to the elements of the Device Under Test (DUT)/analyser connection, including the length, type and quality of cables and connectors. The signal-delivery network that connects the DUT to the analyser, as in Figure 2, may degrade or alter the signal of interest. These unwanted effects can be removed using the analyser’s built-in amplitude
Figure 1:
Absolute vs relative measurement
ensure minimum loss, good impedance match and repeatability especially at higher frequencies. From the mismatch uncertainty equation, improving the match or reflection coefficient of either source or analyser reduces the mismatch uncertainty. Setting the analyser’s input attenuator to 0dB should be avoided if possible as it gives the worst-case mismatch. For best amplitude accuracy, use an input attenuator ≥10dB. To measure low-level signals, you can
improve the analyser’s sensitivity by minimising input attenuation, narrowing RBW, and using a preamplifier. These techniques lower displayed average noise level (DANL), separate small signals from noise and enable accurate measurements. To measure modulated signals, it is
Figure 2:
The quality of the DUT/analyser connection can have a significant effect on measurement accuracy and repeatability, which increases with frequency
important to set the resolution bandwidth wide enough to include the sidebands. Otherwise, the power measured will be inaccurate unless an integrated band power measurement is made. Integrating power from many points measured with a narrow resolution bandwidth is often the most practical technique for wideband digitally modulated signals that are closely spaced. Making a measurement is not enough, you need to make an accurate measurement. No measurement instrument in the world can measure an absolute value, as the measurement will always include an uncertainty. The smaller the uncertainty is, the better the instrument. Acknowledging the existence of an uncertainty and being able to quantify it are an important part of the measurement process. Combine good measurement practices and useful analyser features to help you mitigate errors and shorten test time.
Keysight Technologies
www.keysight.com T: 0800 0260637
/ ELECTRONICS
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