FEAT RE FEA ATURE


TEST &TEST & MEASUREMENT BESTPRA GiovGiova A vanniD’A’Amorefro acc racy RAC


NT


BEST PRACTICES FOR ACCURATE SPECTRUM MEA USAEASUREMENT


ACTICES FORAC REMENT


ovanni D’Amore from eysight Te accuracy and how


romKey ccuracyandhowtoavo


spectrum analyser or signal analyser is an essential and


fundamental measurement tool for an RF engineer. At the most basic level, it can be described as a frequency- selective, peak-responding voltmeter calibrated to display the Root Mean Square (RMS) value of a sine wave.


WH SPEC RUM MEASUREMEN measurement strength. It is especially


WHY SPECTRUM MEASUREMENT? The frequency domain has its


useful for determining the harmonic content of a signal. Figure 1 shows the waveform of a complex signal in the time domain on the left and frequency domain on the right. The time domain representation only shows that the signal is not a pure sinusoid and provides no further indication why. In the frequency domain, we observe that the waveform comprises of two sinewaves with corresponding frequency and amplitude.


The frequency domain is extremely important for those involved in wireless communications to evaluate devices, systems and signal performance to ensure key feature specifications such as linearity, distortion, noise, and spurious emissions are met. When we make relative


measurements on an incoming signal, absolute values do not come into play.


how the harmonic differ s For example, we are only


from the fundamental. W would be when the funda


in amplitude interested in


mental occurs orst-case


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 frequenc y, start, stop and marker 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


14 14 DECEMBER JANUAR 201 ECEMBER/JANUARY 2018 | ELEC RO ELECTRONICS CS


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.


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 fidelit y, using whichever gives better accuracy. Some tips and best practices for accurate spectra measur ements are given below :


SIGNAL-DELIVER NETWORK It is always wort attention to the


SIGNAL L-DELIVERY NETWORK


elements of the Device hwhile to pay careful


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, may degrade or alter the signal of interest. These unwanted effects can be removed using the


analyser’s built-in amplitude 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. You can save the correction values for different


combinations of cables and adapters. Figure 2: Figure 2:


Block diagram of a classic spectrum analyser


Block diagram of a classic spectrum analyser


Figure 1: Figure 1:


Relationship between time and frequency domain


Relationship between time and frequency domain


Technologiesoff ffe ACCURA RAT ow to avoid making i accurate,, ifnot fundamentallywong, spectrum


Keysight Technologies offers som tpsonhow voidmakinginaccurate


, fers some tiips on howtoi prov not fundamentaly wrong, spectec rummeasurements


ow to improve m asuremen t re


roveovemeasurement easurements


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. Use a preamplifier with low noise and high gain to achieve max imum sensitivity .


MODULAT MODULA ED SIGNAL ATED SIGNAL


To measure modulated signals, it is important to set the bandwidth wide eno .


,


ugh to include the resolution


sidebands Otherwise the power measured will be inaccurate unless an integrated band power measurement is made. Integrating power from many pointsmeasured with a narrow resolution bandwidth is a practical technique for wideband digitally modulated signals that are closely spaced.


The indicated frequency of the marker is a function of the frequency


calibration on the display, the location of the marker on th e display, and th e number of display points selected. Narrowing the span and resolution bandwidth minimise these effects and make it easier to place the market at the peak of the response. Use a more accurate calibrator or one closer to the frequency of interest if available, instead of the built-in calibrator. No measurement instrument in the world can measure an absolute value, as the measurement will always include an uncertainty Acknowledging th e existence of an uncertainty and being able to quantify it are an important part of the measurement process.


.


Keysight Technologies www.keysight.com


www.keysight.com T: 0800 0260637


/ ELECTRONICS ELECTRONICS ATE SPECTRU M


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56