New Technology Helps Drive Better Business Performance
Electromagnetic compatibility (EMC) testing requires detailed and exacting methodologies to ensure accurate measurement of all emissions. Unfortunately, long test times affect test facility availability and reduce the number of devices that can be certified. This caps the amount of revenue a testing service can generate and it limits the number of new devices a company with internal test capabilities can introduce without the cost of third-party testing.
Time-domain scan is a technology that can reduce receiver scan time significantly, thereby shortening overall test time. This methodology became acceptable for prescans in CISPR 16-1-1:2010 and is acceptable for final measurement in those CISPR standards that specifically call out the use of the 2010 version. The newly released draft version of MIL-STD-461 also permits the use of time-domain scanning.
Reducing Overall Test Time
Commercial and military testing standards require specific amounts of measurement time, also known as dwell time, for each signal 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 s for prescans and, for 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. With either standard, dwell times add up quickly when using receivers that collect data in individual resolution bandwidths during frequency-domain scanning based on stepped or swept local oscillators.
Understanding How Time-Domain Scan Works
Time-domain scan saves time by using high-overlap fast Fourier transforms (FFT) to collect emissions data simultaneously over a frequency span that includes multiple resolution bandwidths (see Figure 1). Typical FFT acquisition bandwidths range from 1 to 10 MHz, sometimes greater, which is significantly wider than the standards-mandated resolution bandwidths.
and FFT-acquisition bandwidths.
The receiver collects data in the wider acquisition bandwidth and processes it into the appropriate regulatory bandwidths to ensure standard-compliant measurements. Time-domain scan is faster because the appropriate regulatory dwell time is applied just once for all data in a given FFT acquisition bandwidth. In contrast, frequency-domain scanning requires that the receiver dwell for each measurement.
Time-domain scan saves additional time because the wider acquisition bandwidths cover an entire band of interest in fewer frequency steps than is necessary in stepped frequency-domain scanning. Each frequency step requires the local oscillator (LO) to change frequencies, and fewer steps mean lower total LO relock time.
Time-domain scan measurements must comply with CISPR 16-1-1:2010 and MIL-STD-461 amplitude accuracy requirements. To achieve the required accuracy, measurements use a high level of overlap ( ~ 90 percent) when calculating the FFTs. The EMI receiver must also maintain a high level of amplitude distortion performance over the wider IF acquisition bandwidths.
Substantial overlap ensures that impulsive signals are captured and measured accurately. Figure 2a shows 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 shows the same signal in the time domain when using highly overlapped FFTs. As illustrated, there is a much higher probability of capturing the signal and reporting the correct peak amplitude with time domain.
Time-domain scan acquisition bandwidths must also account for RF and microwave preselector bandwidths. Preselector filters band-limit the RF energy that can reach the receiver’s first mixer, improving available dynamic range when measuring impulsive signals. To ensure FFT amplitude accuracy, time-domain scan
accounts for preselector filters in two ways:
Adjusting the amplitude vs. frequency response across the FFT acquisition bandwidth to compensate for the preselector edge-of-band response
Reducing the maximum FFT acquisition bandwidth so that FFT amplitude vs. frequency effects do not significantly add to preselector amplitude vs. frequency response
Reducing Prescan from Hours to Minutes
Three main elements of compliance testing consume facility time:
- Setup and teardown of equipment under test (EUT)
- Prescan to identify suspect frequencies prior to final measurement, including time for antenna and turntable movement and receiver scans
- Final measurement, including time for antenna and turntable movement and single-frequency receiver measurements
Setup and teardown times vary greatly with EUT type and can range from less than one hour to more than a day. Antenna movement time varies depending on the manufacturer, but a typical value is 5 seconds for each antenna position. Turntable movement time also varies with manufacturer, but typical values are 5 seconds per 15-degree rotation. Final measurement time can vary broadly, depending on the number of frequencies in the suspect list and the amount of dwell time required at each frequency.
Time-domain scanning saves a significant amount of time during prescan because the receiver must tune through the entire measurement band. For example, when collecting suspect frequencies according to methodologies required in CISPR 16-2-3:2010, ed. 3.1, section 7.6.6, a sweep should be made for every 15 degrees of turntable rotation and for both polarizations of the receive antenna (48 receiver scans in all). In addition, antenna-height scanning may be required: making measurements for three heights at each azimuth for each polarization requires 144 receiver scans.
To measure emissions in the 30 MHz to 1 GHz range, the suspect list is created by prescanning with a peak detector, four measurement points for every resolution bandwidth (e.g., every 30 kHz for a 120 kHz CISPR resolution bandwidth), and 10 ms dwell time for each point. In the frequency domain, commercially available receivers make this scan in approximately 250 seconds, netting a total prescan time of about 10 hours.
Using time-domain scan, a receiver can require as little as three seconds or less, reducing the total scan time to less than seven minutes. Note that in both scenarios, the total time for turntable and antenna movement is approximately 12 minutes to collect 144 scans.
Applying This Method Appropriately
Time-domain scanning also can be used to save time when using the CISPR-specified weighted detectors: quasi-peak, EMI average and RMS average. The respective weighted charge and discharge times lead to time-domain scans that are slower than those taken with a peak detector; however, these scan times are significantly faster than weighted frequency-domain scans.
This reduction in scan time has led some in the industry to suggest the use of time-domain weighted-detector prescan results in place of a final measurement. Rather than using the final measurement amplitude, this approach uses the weighted-detector prescan amplitude to determine if the suspect meets the emissions limit.
Unfortunately, this does not align with the recommended measurement methodology. CISPR 16-2-3:2010 edition 3.1, section 6.5, requires that the weighted amplitude of each final signal be monitored to ensure that it is steady. For unsteady signals, CISPR requires that the amplitude variation of the signal be monitored for 15 seconds. If the variation over that period is greater than 2 dB, then the signal must be monitored for a longer period to ensure that the maximum value of all signals has been captured in the scan. This increase in dwell time negates any potential reduction in test time.
Balancing Speed Versus Overload Protection
Receiver design can improve time-domain speed with wider acquisition bandwidths, collecting more measurement bandwidths in each acquisition. Taking advantage of wider acquisition bandwidths requires wider receiver preselector filter bandwidths. However, this reduces the available impulse-measurement dynamic range, lowering the impulsive overload threshold.
As noted earlier, RF and microwave preselector filters improve broadband overload levels and measurement dynamic range when measuring impulsive signals (see Figure 3). For a given impulse, the maximum signal level passing through the filter is proportional to the amplitude of the pulse (V), the duration of the pulse (T) and the impulse bandwidth (BWi) of the filter Vin max ∝ VT BWi.
Widening a preselector filter to increase time-domain scan speed increases the filter impulse bandwidth and effectively reduces the receiver overload level for impulsive signals. Additional input attenuation will restore the overload protection, but at the expense of measurement sensitivity. Because sensitivity is a key parameter for EMC testing, system designers need to consider whether they are willing to trade sensitivity for additional measurement speed. In many cases, the time savings will be a small percentage of the total measurement time and may not warrant a reduction
in sensitivity.
When evaluating a receiver, it is important to understand the tradeoffs between scan speed and overload protection. An effective way to determine relative overload protection for receivers with comparable distortion specifications—especially 1 dB compression and third-order intercept (TOI)—is to compare the ratio of the preselector 6 dB filter bandwidths at a given frequency. By calculating 20 log [(wider BW6 dB)/(narrower BW6 dB)], system designers can get a reasonable estimate of the additional input attenuation the receiver will acquire with the wider preselector bandwidth to avoid overload when measuring large impulsive signals.
Wrapping Up
Time-domain scanning can significantly improve EMC test lab throughput by reducing overall measurement test time. This can translate into additional revenue and new product introductions for capacity-constrained test facilities. Although test time savings vary with measurement requirements, time-domain scan can reduce testing to commercial standards by several hours. It provides the greatest benefit for testing methodologies that require turntable rotation and antenna-height variation during prescan while identifying a list of suspect frequencies for final measurement.
