Page 11

EETE OCT 2014

and DAC cannot directly capture the signal, downconversion and upconversion are required, which is why direct-to-digital conversion is so desirable. Benefits are equally applicable to terrestrial or satellite communications, test and measurement, spectrum monitoring, and virtually any system operating at microwave frequencies. Today, that includes wireless systems ranging from carrier-based networks to public safety, Wi-Fi, machine-to-machine communication, distributed antenna systems, and more. A Long Road The microwave industry has not completely embraced what is almost certainly the way microwave systems will be constructed in the future. One reason for inaction is the long standing tension between digital and analog designers, who have rarely crossed boundaries. Additionally, the microwave industry is historically slow to change (except semiconductor suppliers), and change that does occur has mostly originated from military customers. At present, manufacturers of microwave downconverters are relatively safe in designing products at frequencies above ~5GHz where the combination of sample rate, resolution, and dynamic range are sufficient to provide improved performance for most applications. Still, converter technology is continuously moving forward in sampling rate, instantaneous bandwidth, resolution, and dynamic range. Analog Devices’ AD9250 series (Figure 2) combines high sampling rate, resolution, and dynamic range for demanding systems. Great advances in sampling rate have potential to eliminate chunks of hardware. Microwave components may still be required between antenna and ADC, but quantities will be significantly reduced with an irresistible combination of savings in reaction time, space, weight, and total system cost. Figure 2. The Analog Devices AD9250 ADC has all the attributes required for precision conversion. A Complex Mixture It’s tempting to focus on sampling rate and instantaneous bandwidth as benchmarks defining converter performance but they are meaningless without including others like effective number of bits (ENOB) and spurious-free dynamic range. But design trade-offs are such that the highest sampling rate rarely occurs in the same device with the widest bandwidth and dynamic range. For example, converters have been demonstrated with sampling rates of a stunning 64 Gsamples/s, so instantaneous bandwidth might be half as much (32GHz.) Such a device, if producible, would be breathtaking and could effectively eliminate analog downconversion up to 32GHz, well into the millimeter-wave region. However, the stated ENOB would probably be 5.8 at 10GHz, so at higher frequencies it could drop off dramatically, and dynamic range could be about 43dB. As a laboratory device, these values are impressive. However, dynamic range and ENOB would need to increase before it could be useful, and they ultimately will. With all converters, bits of resolution largely influence the degree to which the device can accurately represent the input signal. ENOB is essentially a simple way to summarize the overall performance of an ADC and particularly its accuracy at a given frequency and sampling rate. Unfortunately, as frequency increases, ENOB decreases, because various noise and distortion components also follow this curve, reducing signal-to-noise ratio and thus ENOB (as these metrics are intrinsically related). The term ENOB can become confusing because it can refer to the bits of resolution achievable by the ADC and the total “effective” number of bits that it achieves when part of a complete system. Device ENOB is invariably higher than when it is included in a system. For this discussion, it makes sense to use the device-level number. In conversion devices dynamic range is the breadth of signal amplitudes in decibels that the device can resolve, making it an essential metric for communication, EW, radar, and other applications in which signal strength varies rapidly and often over a huge range. Higher numbers are better. Together, ENOB, dynamic range, sampling rate, and bandwidth tell much about performance. Performance at high input frequencies and in demanding applications determines whether a signal can be identified in noisy, dense spectral environments and if characteristics can be identified with precision. Summary Does direct-to-digital conversion put an end to microwave downconverters? It has already done away with them at lower microwave frequencies. However, as microwave frequencies rise it will probably take many years before direct conversion at, say, 40GHz will be possible while preserving key characteristics. And conversion is not the only consideration. High rates of conversion produce large amounts of data very quickly and would require “big-data” processing power, requiring more hardware, space, and power than is practical for average users. Someday these feats will be attainable. For more information, visit the section of the Mouser website Applications & Technologies. Come and meet us at Electronica in Munich in November, hall A5, stand 524. Discover the newest development kits, development tools & evaluation boards. Get the chance to win a development kit on www.mouser.com/electronica. Mouser Electronics European headquarters Elsenheimerstr. 11 80687 Munich Germany Local: +49 (0) 89 520 462 110 Fax: +49 (0) 89 520 462 120 munich@mouser.com www.electronics-eetimes.com Electronic Engineering Times Europe October 2014 11


EETE OCT 2014
To see the actual publication please follow the link above