Faster EMC compliance testing with accelerated time domain scan

Today’s fast-paced business environment and development cycles make it imperative to provide electromagnetic compatibility (EMC) measurement facilities with rapid test turnaround, high throughput and accurate measurements. EMC tests must meet strict emission standards developed by EMC regulatory bodies such as the Comité International Spécial des Perturbations Radioélectriques (CISPR) and military standards such as MIL-STD-461.

EMC testing ensures that devices will perform as designed and not emit radiation that could degrade the performance of other equipment. EMC testing requires a detailed and exacting methodology to ensure the accurate measurement of all emissions. However, a long test time reduces the availability of the test facility, limiting the number of devices that can be certified and reducing the revenue a testing service can generate. Growing revenue without adding new test sites requires streamlining the EMC product testing cycle—setup, scanning, turntable rotation and antenna height adjustment time—to increase the capacity of the existing test facility.

TIME DOMAIN SCANNING REDUCES TEST TIME

To ensure impulsive signals are properly characterized, commercial and military testing standards require specific measurement or dwell times for each signal. When testing in the frequency domain, data must be collected in individual resolution bandwidths. Using the time domain scan (TDS) technique shortens the overall test cycle by reducing the receiver’s scan time while meeting the required dwell time: With TDS, fast Fourier transforms (FFT) simultaneously analyze emissions over a range of frequencies, covering multiple resolution bandwidths to reduce the receiver’s scan time (see Figure 1). FFT acquisition bandwidths for TDS can range from 1 to > 10 MHz, far exceeding the resolution bandwidths required by CISPR and military standards. As soon as the receiver acquires the data, it processes it into the appropriate regulatory bandwidths to ensure measurements meet regulatory requirements.

Comparing the two techniques, frequency domain scanning requires that a receiver dwell on the resolution bandwidth required for each measurement. TDS saves measurement time by applying the regulatory dwell time once for all data in the FFT acquisition bandwidth. Using TDS saves time, since the wider acquisition bandwidth requires fewer frequency steps to cover the entire band of interest, unlike stepped frequency domain scanning. The local oscillator (LO) frequency changes with each frequency step: the fewer the number of steps, the lower the LO relock time.

Calculating FFTs often requires high overlap levels of more than 90 percent to achieve accurate amplitude measurements. Unlike the frequency domain scanner, a TDS receiver has amplitude accuracy fluctuations with frequency due to intermediate frequency (IF) effects, that must remain within regulator requirements. A high degree of FFT overlap in the time domain ensures accurate measurement of impulsive signals.

TDS acquisition bandwidths must account for microwave and RF preselector bandwidths. Preselector filters improve the dynamic range at the receiver’s first mixer when measuring impulsive signals. TDS compensates for preselector edge-of-band response by adjusting the amplitude versus frequency response across the FFT acquisition bandwidth. Alternatively, the maximum FFT acquisition bandwidth can be reduced so the FFT amplitude versus frequency effect does not significantly alter the preselector’s amplitude versus frequency response.

ACCELERATED TIME DOMAIN SCANNING FURTHER BOOSTS THROUGHPUT

Advances in high performance electromagnetic interference (EMI) receivers have enabled bandwidths in a single FFT acquisition up to 59 MHz for standard TDS and 350 MHz for accelerated TDS (see Figure 2). The maximum bandwidth for each FFT acquisition can be determined based on the IF filters in the mixer output. For both standard and accelerated TDS, the IF can be implemented with selectable IF bandwidths up to 350 MHz. The high-performance analog-to-digital converter (ADC) enables wide bandwidth digitization for IF signals. The short time FFT (STFFT) engine can analyze up to 16,000 frequency points in a single acquisition to support wide FFT spans, which is much better than traditional EMI receivers.

Accelerated TDS accesses up to 350 MHz bandwidth in each FFT acquisition to significantly enhance test throughput. To illustrate, non-accelerated TDS typically requires about 25 FFT acquisitions to scan the CISPR band C/D between 30 MHz and 1 GHz, which is significantly faster than conventional stepped scanning. Accelerated TDS typically requires no more than three acquisitions with the same frequency range, and allows to scan 8× faster than traditional TDS.

Pre-scanning the radiated emissions in an EMC test lab takes a considerable amount of time before final compliance measurements can be made. Accelerated TDS reduces this pre-scanning measurement time for each device under test (DUT). For example, an EMI receiver with accelerated TDS can reduce the scan time to less than 30 s, which is more than 5000× faster than stepped scan times (see Table 1). Furthermore, accelerated TDS can increase the data capture bandwidth during real-time scan measurements more than conventional methods, analyzing the data spectrum up to 350 MHz in a single segment for accurate EMC diagnosis.