Today’s technologies continue to reach higher frequencies and wider bandwidths. Take, for instance, 5G in the U.S.—it will utilize 28- and 39-GHz frequency bands with 1.2 GHz of bandwidth. The IEEE 802.11ay standard makes use of frequencies between 60 and 70 GHz with 2-GHz bandwidths. Satellite-communication (satcom) systems routinely extend beyond 70 GHz with bandwidths greater than 5 GHz. The list of these emerging high-frequency and high-bandwidth technologies grows every year.
While it’s exciting to think about the possibilities surrounding millimeter-wave (mmWave) frequencies and above, advances such as these stress the overall capabilities of today’s test-and-measurement equipment. To truly show that the technology is working, one must measure it. Test-and-measurement instrument classes such as spectrum analyzers have limitations above 50 GHz, which necessitates finding another option to test at these high frequencies.
One class of instrument that’s gaining traction is the real-time oscilloscope. While rarely considered in the past, real-time oscilloscopes continue to make major strides in terms of both bandwidth and signal integrity performance, and thus could be an excellent choice for emerging markets among RF instruments.
The Easy Specification to Consider: Bandwidth
The first and most important specification centers around having the bandwidth to make measurements at very high frequencies (i.e., above 50 GHz). As recently as 2010, oscilloscope bandwidths were limited to just 30 GHz. Of course, that bandwidth spans dc to 30 GHz, so even as early as 2010 one could measure very wide bands with an oscilloscope. Unfortunately, reaching frequencies greater than 30 GHz required a downconverter. Extra calibration was also needed to remove loss and maintain a flat magnitude.
Over the last decade, though, things have changed. Around 2015, both 70- and 100-GHz oscilloscopes were introduced. And in 2018, Keysight introduced the 110-GHz bandwidth UXR-Series oscilloscope (Fig. 1). With the UXR-Series, it’s possible to directly digitize frequencies as high as 110 GHz. One notable point concerning the UXR-Series is that it’s the first real-time oscilloscope to employ full-bandwidth pre-amplifier and sampling chip designs, which is attributed to Keysight’s proprietary indium-phosphide (InP) technology. Other oscilloscopes must use a form of time interleaving to achieve high bandwidths, but this comes with signal-integrity tradeoffs.
1. Keysight’s UXR0134A Infiniium UXR-Series oscilloscope is a four-channel model with a bandwidth of 13 GHz.
The Growing Importance of MIMO
Consumers are demanding seamless integration between mobile networks and Wi-Fi networks, with an insatiable demand for more bandwidth and faster throughput. Consumers want better performance in crowds and densely populated centers. To address these needs, technologies such as multiple-input, multiple-output (MIMO) and beamforming are making their way to the market in an accelerated fashion. MIMO, by definition, means more than one input and more than one output. The latest technology news has mentioned channel densities as high as 64 channels. However, the need for a four-channel density exists right now.
Suddenly, the real-time oscilloscope becomes important: One of its major advantages is that it naturally comes with more than one channel. Real-time oscilloscopes are typically found as two- or four-channel models. Even more crucial than the channel count is the fact that real-time oscilloscopes offer channel-to-channel calibrations as standard features. As an example, UXR-Series instruments attain channel-to-channel skew inside the box of less than 75 fs. This capability contrasts with tying multiple individual instruments together using a local oscillator (LO).
Other instruments employ a modular form factor, offering the ability to tie as many as 32 channels together. The UXR-Series oscilloscopes offer the same capability—but in a more standard box format. Thus, oscilloscopes can now meet the needs for high-frequency coverage, wide bandwidths, and multiple channels in a single box.
That’s Great—But What About Spurious and Other Key Measurements?
Real-time oscilloscopes have always been a natural fit when it comes to bandwidths and channel count. However, their problem is linked to RF performance, which has never been good enough. Specifications like error vector magnitude (EVM), spurious-free dynamic range (SFDR), and effective number of bits (ENOB) have paled in comparison to other RF instruments. However, real-time oscilloscopes continue to improve in terms of RF performance, which begins with the analog-to-digital converters (ADCs) they employ.
For the last 20 years or so, 8-bit ADCs have dominated the real-time oscilloscope world. This limits the overall dynamic range of the scope in comparison to the 12- and 14-bit ADCs exploited by other RF instruments. In 2014, Keysight introduced the S-Series oscilloscope family, which marked the first time the company employed a 10-bit ADC in a real-time scope. Unfortunately, the scope was limited in bandwidth to only 8 GHz. However, the company had a 10-bit platform that could be leveraged above 8 GHz.
The recently introduced UXR-Series oscilloscopes now take that 10-bit ADC technology to 110-GHz bandwidths and 256-Gsample/s sampling rates. While 10 bits is better than eight bits, it still doesn’t reach the performance of historical RF instrumentation. Enter oversampling to the mix.
Today’s real-time oscilloscopes have sampling rates as high as 256 Gsamples/s. Typical RF frequency bands are less than 5 GHz, meaning that the oscilloscope can look at the specific band of interest and massively oversample the data, lowering the effective noise of the instrument. The oversampling allows the real-time oscilloscope to look like a 12- or 14-bit instrument at the targeted bandwidth.
2. Shown is a UXR1104A Infiniium UXR-Series oscilloscope performing a 32-QAM optical signal analysis.
When the oversampling is coupled with higher-performance ADCs, SFDR measurements range somewhere in the 60s for dBc. Historically, the same measurements have fallen in the mid-30 dBc range. Add in the fact that real-time oscilloscopes have always been extremely flat in both magnitude and phase, it’s clear that these instruments have now closed the measurement performance gap (Fig. 2).
Quickly performing RF measurements is the final real hurdle that real-time instruments must overcome. Typically, oscilloscopes use non-native software to analyze high-frequency signals, which means that the software must import all of the data into the software to analyze it. Even if a measurement, such as a fast Fourier transform (FFT), is completed on the scope, it’s completed in software and can be very slow. Moving forward, as oscilloscope field-programmable-gate-array (FPGA) and custom application-specific-integrated-circuit (ASIC) performance continues to increase in capability, one can expect the speed of an oscilloscope to increase, and that limit will be addressed.
The world’s data demands are moving today’s data communications to higher frequencies, wider bandwidths, and multiple channels. For developers of these new technologies, finding the right instrument to perform measurements is becoming an increasingly difficult challenge.
Fortunately, today’s test-and-measurement vendors continue to upgrade their instrumentation. This is especially evident in the improvements made to real-time oscilloscope technology. Real-time oscilloscopes now offer up to 110 GHz of bandwidth, multiple phase-coherent channels, and specification performance that can be good enough to meet the testing challenges of today’s markets.
Brig Asay is Director Strategic Planning, Internet Infrastructure Group at Keysight Technologies.