Combining Talents

Axcera test engineers Ken Alderson (front) and Michael Kustra are responsible for the quality of products ranging from individual printed-circuit boards to complete systems.

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The company was founded in 1982 by three RCA broadcast engineers who saw opportunities they believed RCA wasn't in a position to address, according to Rich Schwartz, director of marketing and product management at Axcera. Originally named ITS Corp., Axcera acquired its current name after a period of ownership by ADC Telecommunications in the mid-'90s. The company's first products were solid-state exciter retrofits that replaced aging exciters in RCA transmitters; the founders also noted the emergence of the MMDS (Multichannel Multipoint Distribution Service, a wireless implementation analogous to cable TV) market and followed up the exciter retrofits with solid-state MMDS transmitter systems. They applied their RF expertise up the power curve and by 1987 had introduced the first 10-kW TV transmitter.

Key to Axcera's founders' approach has been customer service, said Schwartz. "They wanted to build relationships that they perceived big, corporate RCA wouldn't or couldn't. Their belief was, 'if you just took care of the customer, you could get a lot more business,' and sure enough, they were right."

Customer service is particularly critical in the broadcast TV business because the physics of each broadcast channel is different, as is the terrain of the area served by each installation. Each channel has unique requirements with respect to waveguides and RF channel filters, and, added Schwartz, "you might successfully install a system on one channel at one location, but find that the same system at another installation on a different channel performs much differently. Customers completely understand that, but they also expect you to support them throughout the installation and afterwards. If site-related issues are discovered at turn-on, that's OK, that's expected, but you have to persevere until the customer is pleased."

Axcera currently offers the Innovator Series low-, medium-, and high-power solid-state television transmitters; Visionary Series high-power inductive-output-tube (IOT) transmitters that deliver up to 180-kW digital and 480-kW analog performance; and Axity3G broadband wireless-access RF products. The company also provides on-channel digital boosters, channel translators, and MMDS transmitters, channel combiners, and management systems. In addition, the company still does a handful of exciter retrofits each year.

"There are still a fair number of old RCA and other old transmitters in operation," Schwartz said. "The tube manufacturers support the output tubes, but RCA is no longer in the business, and the other manufacturers want to sell you a new transmitter—they don't want you to fix the old one. Well, we'll offer a new unit or fix the old one."

The breadth of product offerings adds up to a high-mix, low-volume production schedule that presents significant test challenges, said test engineer Ken Alderson, who, along with test engineer Michael Kustra, is responsible for the test of printed-circuit boards, subassemblies, and complete systems at the Axcera production facility here. The low volumes of highly specialized boards that the company produces preclude the automated inspection and in-circuit test techniques that high-volume manufacturers typically employ.

Indeed, even a complete functional test at the board level may not be practical. Said Alderson, "As soon as the boards are assembled, we try to do as much test as possible, but many [boards] are so complicated that they need to be connected to other boards to provide for a complete test. I take a board and evaluate how much we can do initially and write a test process, which our test technician can view over the network on his monitor. The process tells him what test fixtures and equipment he'll need—such as a spectrum analyzer, vector network analyzer, or oscilloscope." Alderson noted that the production test primarily uses Tektronix instruments for video test and Agilent equipment for RF test, adding that many technicians aren't fond of the menu-driven front panels on many newer instruments. "They have to push a lot of buttons just to change frequency."

"The first time a technician tests a board he may need a little bit of guidance, but for the most part it's easy to do," said Alderson. "In fact, the board-test stage might require only a visual inspection and a voltage check, to make sure there are no shorts."

When the MMDS market was hot, Axcera did find some application for test automation and employed a LabView-based approach to test the relatively high quantities of boards destined for MMDS systems, Alderson said. Axcera still enjoys some MMDS business, said Schwartz, but he added that the technology couldn't compete with incumbent cable systems. There are some successful MMDS systems operating in the US, but they were built before there was an established cable system in the area, he explained. With volumes now relatively low, it hasn't been cost effective to sustain the automated-test capability.

Kustra noted that even at low volumes, automation could provide benefits such as permitting remote access and minimizing operator error. But currently, he said, it's just not cost effective to pursue automated approaches for the majority of products that Axcera makes.

That could change, as the increasing popularity of the company's Innovator LX low- and medium-power solid-state transmitter systems is generating higher volumes of LX boards. That prompted Alderson to modify a production backplane board to serve as a test fixture for modulator and upconverter IF boards and control/power-supply boards for LX systems. "I took the backplane board and had drafting lay it out in the form of a single-board test fixture for a modulator, an IF stage, and so forth," providing board-level test capabilities that go beyond visual inspection and power-up.

Axcera's MMDS expertise has also offered the company opportunities to pursue related high-volume product lines—if not MMDS wireless cable products, then in broadband wireless access technologies that make up the Axity3G line, which uses the same 2.5- to 2.7-GHz frequency ranges of the MMDS systems to provide 3GPP-compliant metropolitan-area Internet access. The Axity3G product lineup includes desktop and PC Card modems as well as node B wireless access points.

System engineer John F. Pierce puts an Axity3G broadband wireless-access core network through its paces. Node B access points (right) provide wireless communications with PC Card and desktop wireless modems.

Before an Axcera product gets to the production floor where Alderson and Kustra can get their hands on it, it naturally undergoes extensive design and test. That's currently the life-cycle stage of a replacement due out this month for the company's DT2B digital TV exciter. In its current version, the DT2B offers continuous linear and nonlinear adaptive correction, a scheme that takes a transmitter's output signal and adds digital precorrection to compensate for distortions in the transmitter system to get the most linear output possible. "We were the first to have continuously adaptive precorrection for both linear and nonlinear distortions," said Schwartz.

The exciters are available with Axcera's built-in DTVision signal-analysis system, which displays digital signals to help diagnose transmitter problems. "Our recommendation is that with the DTVision option the customer also purchase a rack of independent test equipment," including a Rohde & Schwarz EFA 53 ATSC measurement set (which measures such DTV parameters as peak to average ratio and linear and nonlinear distortion and which can generate constellation and eye diagrams) plus a Tektronix MTM 400 transport-stream monitor, a Sencore IRD 3385 receiver/decoder, a Wohler ATSC-3 5.1 audio monitor, a Leader LV5152 HDTV analog waveform monitor, and a Sony PVM 14L5 multiformat 14-in. monitor.

"The functions you get in the complete lineup of rack-mounted test equipment are valuable, but DTVision is nice if a customer has multiple stations and doesn't want to keep a complete set of equipment at each one or continually move a set around. With DTVision, you get all of the critical performance information—for example, you can measure SNR or display the eye diagram of a digital signal, which can help you see how hard your equalizers are working. It gives you an idea if the transmitter is out of tune, which you might not realize because your adaptive function is precorrecting for it—if the adaptive equalizer is working too hard, then you know you have to go in and take a look at your system."

During my visit, digital design engineer Jason Hutchinson described the new exciter project by phone from Axcera's Richardson, TX, facility. "The new exciter will have much faster processors, and the adaptive correction will be able to react more quickly," providing, for example, full correction within a minute of power-up, vs. 10 to 15 min for the current version. "It will also have a nicer, more Web-based, user interface."

The exciter, Hutchinson commented, is based "almost entirely on digital signal processing, from the input side until the output DAC, taking advantage of the latest generation FPGA hardware and more accurate and advanced DSP algorithms, which use longer-bit-length processing."

Despite the overwhelming digital content of the product, he said, "Surprisingly, one of the best ways to do analysis—even on a pure digital box like this—is in the analog domain. We do a lot of testing with traditional spectrum analyzers. We can quickly see problems that sometimes would take a little longer to find with something like a logic analyzer—for example, a digital filter that's clipping. That shows up right away on a spectrum analyzer, whereas with a logic analyzer you might need to rifle through tons and tons of data to see this little digital glitch." Hutchinson does still have a logic analyzer on his bench, though, as well as a Rohde & Schwarz EFA 53 ATSC measurement set. For software debugging, he relies on the tools that come with the Xilinx FPGAs he uses to implement the DSP algorithms.

Asked what new instruments he would like to see vendors develop, Hutchinson said, "better tools to analyze the SMPTE 310 MPEG data stream format as it is coming into the transmitter. We can measure what's coming out of our transmitter, but the quality of the input signal is somewhat unknown. This SMPTE 310 stream was never designed to be transmitted across long distances out to a transmitter that might be 30 miles from a studio, and a lot of real-world problems can happen to that signal as it gets transmitted. We need a simple way to demonstrate whether the problem is in the transmitter or the SMPTE 310 input."

In other respects, he said, test-and-measurement vendors over the last couple of years have addressed most issues of importance to transmitter design. For instance, Hutchinson said, "the R&S EFA 53 is based on receiver technology, so it acts a lot like an actual receiver a customer is going to be using instead of acting like a piece of test equipment." Additional problems he sees include instruments from different vendors providing different measurement results: "One instrument might be very sensitive to a pilot level and give you one reading, while another box less sensitive to that level might give you a different result." Understandably, he said, "Customers want to see a single number that summarizes performance, and test equipment manufacturers are slowly but surely rooting out the reasons why these readings differ."

Much of Axcera's focus today is smoothing its customers' transition to digital TV. Schwartz cited the company's Visionary series, a high-power system that can employ air-cooled, water-cooled, or oil-cooled IOTs and that can handle digital and analog in the same system. "Half of our business is still analog, and someone who wants an analog transmitter can buy our analog system and convert it directly to digital in the future. We are the only one of the three major manufacturers with an analog depressed-collector transmitter," he added, referring to an IOT configuration that offers from two to five depressed collectors for higher efficiencies—55% to 60% for digital signals vs. 35% to 45% for single-collector tubes.

"The depressed collector took a while to catch on, but it's picking up speed, and by end of 2005, I don't think we will be selling any more single-collector transmitters. Depressed collectors make for a little more expensive transmitter, but a lot of customers will see a payback in a couple of years in energy savings. But some are skeptical and reluctant to pay up front. We let the customer decide."

Figure 1.  Distributing DTV from WPSX’s Clearfield, PA, transmitter over the mountainous terrain of central Pennsylvania presents challenges; synchronous boosters at slave transmitters in nearby cities could result in picture-killing interference in zones reached by multiple signals. A distributed transmission adapter feeding separate studio to transmitter links (STL) helps to ensure trouble-free reception.

With this approach, problems could arise in interference areas, in which a channel 15 signal from one antenna (Johnstown, for instance) may essentially jam a stronger channel 15 signal from another (located in Altoona, for example). The potential for jamming depends on signal strength as well as delay. Because of the delay effect, the transmitter cannot act as a slave, deriving its signal from a master transmission from Clearfield. Further complicating the process is the fact that ATSC modulators are not deterministic devices; transmitted data is encoded as differences between successive symbols instead of absolute symbol values, and initial conditions related to trellis-coder and precoder states as well as field-synch insertion points determine what the output signal will look like (Ref. 1).

WPSX addressed this problem with a distributed transmission capability, implemented using Axcera's DTxA2B distributed transmission adapter, which will feed four separate transmitters. "The distributed transmission adapter happens to be located in Clearfield," explained Schwartz, "but it's not part of the Clearfield transmitter. So, what we have is a distributed transmission adapter that feeds three different microwave links—one going to each satellite—as well as a link to the Clearfield transmitter, which it just happens to be in the same room with.

"The point is, the Clearfield transmitter is not feeding the three satellite slave transmitter locations—all four transmitters are fed by DTxA and are locked to a GPS signal to eliminate varying delays in the studio to transmitter links. All operate as slave transmitters in order to synchronize the signals to prevent interference in signal overlap areas. Digital TV receivers—as long as a signal is within a certain window—are capable of looking at a signal and saying, 'oh that's just a reflection, and I'll reject the weaker signal.' The receiver is smart enough to know to reject the weaker and use the stronger, but the two signals must arrive coincidently. If one arrives after a 100-µs delay, it's going to be a jammer. The latest receivers can tolerate delays of 50 µs. Also, a receiver is smart enough to reject any signal down 15 dB or more."

As digital TV places new demands in the field and the factory, Axcera engineers remain equal to the tasks at hand. When asked if testing digital TV is more challenging than analog TV, Kustra said, "It's not more challenging—it's just different. The transmitter itself is still a transmitter, but it's a different ballgame altogether as to how you do your measurements. But as long as the product is designed for testability—and it's part of my job to see that it is—it's really not an issue for us that the actual parameters we need to measure are completely different. To make sure we can make the measurements, I spend anywhere from 10% to 75% of my time with design engineering, depending on where in its life cycle a product is. The last thing we want is a design that we can't test."