Service Manuals, User Guides, Schematic Diagrams or docs for : Agilent 5991-3855EN English _ 2014-01-30 _ PDF 827 KB c20140728 [9]

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Keysight Technologies
EMC Compliance Testing:
Improve Throughput with
Time Domain Scanning




                          Application Note
Introduction

     Electromagnetic compatibility (EMC) testing requires detailed and exacting methodologies
     to ensure that all emissions are accurately measured. Long test times impact test facility
     availability and reduce the number of devices that can be certified--capping the amount
     of revenue a testing service can generate or the number of new devices a company with
     internal test capabilities can introduce without the cost of third party testing.

     To grow revenue without adding the considerable expense of new test site installations,
     companies must streamline the EMC product test cycle--which includes setup, scan,
     turntable rotation, and antenna height adjustment time--to maximize the throughput of
     their existing compliance facilities. Time domain scan is a technology that can reduce
     receiver scan time significantly, shortening overall test time to help increase revenue and
     introduce more products to market faster.

     This application note will provide you with an overview of time domain scan, discuss the
     test scenarios in which it provides the greatest time savings, and assess the trade-offs
     between time domain speed and receiver overload protection.
Time Domain Scan Reduces Overall Test Tim
Both commercial and military testing standards require specific amounts of
measurement time, also known as dwell time, for each signal in order to ensure
that impulsive signals are appropriately characterized. Time domain scan reduces
receiver scan time while maintaining required dwell times.

CISPR-based commercial testing can require dwell times up to 1 second for
pre-scans and, in the case of emissions with time-varying amplitudes, 15 seconds
or more for final measurements. MIL-STD-461 specifies dwell times of between
15 ms and 150 ms per measurement, depending on the frequency range. These
dwell times add up when using receivers that employ frequency domain scanning
based on stepped or swept local oscillators to collect data in individual resolution
bandwidths.

Time domain scanning became acceptable for prescans in CISPR 16-1-1:2010 and is
acceptable for final measurement in those CISPR standards specifically calling out
the use of this version of CISPR 16-1-1. MIL-STD-461 allows the use of any type of
measuring device that meets the requirements of the document.


How Time Domain Scan Works
Time domain scan reduces receiver scan time through the use of high-overlap fast
Fourier transforms (FFT) to collect emissions data simultaneously over a frequency
span that includes multiple resolution bandwidths (Figure 1). By contrast, in the
frequency domain, data is collected in individual resolution bandwidths. The FFT
acquisition bandwidths used for time domain scan can be in the range of 1 to 10
MHz or greater, significantly wider than the required CISPR and MIL resolution
bandwidths. The receiver collects the data in the wider acquisition bandwidth and
processes it into the appropriate regulatory bandwidths to ensure that the measure-
ments meet regulatory requirements. Time domain scan saves measurement time
because the appropriate regulatory dwell time is applied only once for all data in
a given FFT acquisition bandwidth, in comparison to frequency domain scanning
which requires that the receiver dwell for each measurement made.

                  Dwell for each                         Dwell for each FFT bandwidth
               resolution bandwidth                     (multiple resolution bandwidth)
           Receiver resolution bandwidth                     FFT acquisition bandwidth
   Amplitude




                                            Amplitude




                                Frequency                                                Frequency
  Swept or stepped frequency domain scan                        Time domain scan

Figure 1. Comparison of resolution and FFT acquisition bandwidths

                         3
An additional time savings is achieved with time domain scan because the wider
acquisition bandwidths require fewer frequency steps, compared to stepped
frequency domain scanning, to cover an entire band of interest. Each frequency step
requires the local oscillator to change frequencies--the fewer the number of steps,
the lower the total LO relock time.

Time domain scan measurements must comply with CISPR 16-1-1:2010 and
MIL-STD-461 amplitude accuracy requirements. In order to achieve the required
amplitude accuracy, designers use a very high level of overlap (~ 90%) when
calculating the FFTs. In addition, the EMI receiver must maintain a high level of
amplitude distortion performance over the wider IF acquisition bandwidths.

The high degree of FFT overlap in the time domain ensures that impulsive signals
are captured and measured accurately. Figure 2a displays an impulsive signal in
the time domain when using contiguous or low-overlapped FFTs. If an input signal
occurs outside of an FFT period, the reported signal amplitude could be low or
completely missing. Figure 2b displays the same signal in the time domain when
using highly-overlapped FFTs. In this situation, there is a much higher probability
of capturing the signal and reporting the correct peak amplitude.




Figure 2a. Traditional critically-sampled FFTs with contiguous windows has potential for
missing input impulsive signal




Figure 2b. Measurements made with FFTs highly overlapped in the time domain increase
probability of intercept and minimize amplitude measurement scalloping


                     4
Time domain scan acquisition bandwidths must also take RF and microwave prese-
lector bandwidths into account. Preselector filters band-limit the RF energy that can
reach the receiver's first mixer, improving available dynamic range when measuring
impulsive signals. Two ways in which time domain scan accounts for preselector
filters to ensure FFT amplitude accuracy are by:

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