Bruce Devine: In late 2012, Cypress introduced its FX3 peripheral host controller chip. That chip made the USB 3.0 interface practical for use with test equipment that requires high-performance computing. Remotely controlling test instruments can be conveniently achieved with 1-Gigabit Ethernet (GE) connectivity. Combining a USB 3.0-powered test instrument with a small industrial computer can accomplish both tasks. All of these popular applications are driving the market. And, as with anything else, performance naturally increases as a new technology becomes more widely adopted. After all, there is more time and money to devote to research and development.
When designing a compact USB spectrum analyzer, what challenges must be overcome to maintain the power budget?
BD: In spectrum-analyzer design, a single USB 3.0 port is limited to 5 V and 900 mA for a total of 4.5 W. Clearly, the single-port power limit was inadequate for the high-performance design of our BB60C spectrum analyzer. A USB3.0 Y-cable was therefore required to provide more current. Our BB60C design draws 1.3 A or 6.5 W. That’s as low as we could bring the power budget without losing valuable performance. Staying within a 6.5-W power budget, without compromising performance, proved very challenging. We were able to find a great analog-to-digital converter (ADC) that had the desired combination of low power consumption and high performance.
We also wanted a high second-order and third-order intercept point (IP2 and IP3) and low residual responses, which are key to having great spectral purity. Typically, high IP2 and IP3 are achieved with a high local-oscillator (LO) drive to the mixers and power-hungry, high-linearity RF amplifiers. We used mixers with a lower LO drive to save power. That required much lower RF input to keep down the spurious.
The lower-power RF-mixer input led to the need for better shielding to maintain spurious-free dynamic range (SFDR). We used several one-piece, single-cavity shields to provide isolation and maintain performance without adding too much cost. We got around the power-hungry RF amplifiers by using push-pull amplifiers and adding more RF leveling control. This allowed us to keep the mixer RF-input levels in a range where second- and third-order products are at least 50 dB down over a wide range of input levels.
We also had to model distributed-element bandpass and lowpass filters, which we embedded into the printed-circuit-board (PCB) laminate inner layers to save shielding space and still benefit from their filtering. In the end, filtering and shielding were even more challenging than fighting the power budget because of the added constraints of size and cost. Designing for minimal size, cost, and power while maintaining high performance is not a trivial endeavor.
How have customers been embracing these new devices’ compact footprint and rich features?
BD: Although size is an important feature to many customers, price appears to trump everything. Customers were very willing to embrace USB-powered test equipment initially because the price was right. They became used to seeing other companies’ USB-powered spectrum analyzers, which had very questionable performance and thus weren’t taken very seriously. Then, in February 2010, we introduced the USB-SA44 spectrum analyzer.
People didn’t know what to think about the USB-SA44. Our price got them in the door, so they were brave enough to try it out. Upon using it, they were delighted to discover that it did what they needed and performed as advertised. I have taken numerous calls over the years in which someone asked me if our products really did what we said they could do. I finally had to resort to offering a 30-day money-back guarantee just to strengthen our credibility.
Now, almost five years later, enough momentum has gathered that the notion of serious USB-powered test equipment is no longer foreign. More and more customers are embracing USB-powered test equipment. With its compact footprint and rich features, it offers something that was unaffordable when one-box-solutions were the only realistic choice. USB-powered test equipment is gaining respect because the customer finally has a full range of serious instruments available.
What unique new markets have been enabled by these devices?
BD: Universities are now able to afford to stock their labs with real-time spectrum analyzers. This provides engineering students with access to both a curriculum and hands-on experience that were previously prohibitively expensive. In addition, automated test equipment (ATE) has become more affordable for manufacturers to use on their production lines.
One of our customers manufactures small wireless devices. They have designed-in more than 50 of our USB-SA44B spectrum analyzers for use in production-line testing. In many instances like theirs, a traditional spectrum analyzer is overkill and a waste of money. A USB-powered real-time spectrum analyzer’s persistence function is perfect for troubleshooting and finding intermittent signals.
With many of these devices within a hobbyist’s budget, what interest have they garnered in the hobby corner?
BD: Ham-radio operators favor our USB-SA44B/USB-TG44A combination of spectrum analyzer and tracking generator. It is economical and the pair works together to form a scalar network analyzer. It allows them to plot filters, calibrate step attenuators, perform AM/FM modulation measurements, and--with the addition of a directional coupler--make antenna and cable insertion-loss and return-loss measurements.
Beyond USB2.0/USB3.0/Ethernet, what interface technologies are being reviewed for use? And what factors need to be considered regarding interface technology?
BD: These interfaces provide enough data throughput for all but the most demanding applications. Real-time spectrum analyzers are using USB 3.0 because it has the highest throughput of the commonly available interfaces mentioned. There is nothing wrong with directly connecting to PCI Express for desktop PCs, but it is difficult if not completely impractical for use with laptops. Computer manufacturers will begin integrating 10 Gigabit Ethernet into their laptop peripheral controllers, so it becomes as commonly used as USB3.0. At that point, we will consider using it as the next stable and practical interface for test-equipment architectures that require high-performance computing.
U.S. export controls will also have to be revised, as test equipment using 10 Gigabit Ethernet is currently restricted for export due to the technology being too powerful. Once U.S. commerce controls allow the unrestricted export of test equipment with a 10 Gigabit Ethernet connection, you will start to see affordable real-time spectrum analyzers with 100 MHz of instantaneous streaming bandwidth. But for now, USB 3.0 is king.
As computers become more compact, what creative opportunities are arising with PC-driven test instruments?
BD: Portability. For example, teaming a USB 2.0-equipped quad-core tablet PC with a USB test instrument produces a very effective field instrument. We’re already doing it at our factory. Once we release our rewritten SA/TG series software this January, customers will be able to make the same pairing. The SA/TG rewrite will be closely followed by the introduction of a touchscreen user interface. Users will be able to Velcro the USB-SA44B to the back of a tablet PC and have a spectrum analyzer for field work. The same possibilities exist for other manufacturers that offer USB-powered test equipment, such as oscilloscopes and signal generators.