Precision measurements for precision filters
Rick Nelson, Editorial Director- June 1, 2011
ANDECHS-FRIEDING, GERMANY. If you are in the market for precision filters, you could consider the offerings of Wainwright Instruments. The company specializes in filters, offering a variety of low-pass, high-pass, band-pass, band-reject, notch, and diplex filters, including L/C filters, filters with helical and cavity designs, tunable filters, and filters for high-power applications. Wolfgang Heine, technical director, said Wainwright’s filters find use in prototypes and in laboratories in a variety of application areas, mainly in telecommunications but also in automotive, medical, military, and aerospace (see “An abundance of filters”).
Wainwright Instruments focuses on quality, not quantity. Christel Wainwright, who until recently served as managing director, said Wainwright Instruments will make anywhere from one or two to a few thousand pieces. Christel Wainwright assumed management of the company after the death of her husband, Claire, but now Terry Wainwright, who studied international business rather than follow his father’s path to electrical engineering, has taken over the management role after pursuing international business investment opportunities for 16 years in several Asian countries.
Christel Wainwright described the filter business as exciting but with many challenges. Building filters has an addictive quality, she said, in which you experience successes and failures very quickly. That, she said, is unlike designing a complete instrument, which may take months—after which you learn whether or not your product is successful or whether your competition got to market first. But the test challenges are no less daunting: “With regard to test protocol, what a company has to do for a big instrument, we have to do for every filter,” Heine said.
The challenges involved in building filters range from economic and business ones to technical ones related to the design, manufacture, and test of the esoteric filters Wainwright Instruments offers. The company must, of course, comply with relevant standards and regulations: Wainwright Instruments is certified to ISO 9001 (for quality), OHSAS 18001 (for occupational health and safety), and ISO 14001 (for environmental management); all its products are manufactured in compliance with the RoHS directive. Other business challenges, Christel Wainwright said, include interpreting contracts from a worldwide base of customers. Furthermore, the strength of the euro presents difficulties. “This is a high-cost country,” she said, compared with countries in Asia, for example, where workers are paid less, “so we have to be always on our toes and come up with something special” that companies in countries with lower costs can’t make.
Wainwright Instruments’ personnel perform the calculations and do the machining and adjustment necessary to build filters that meet customer requirements in a process that can take from minutes to days, depending on the filter complexity. Except for the silver plating and powder coating, all the steps in the manufacture of the filters can take place at the Wainwright Instruments factory, Heine said.
Heine emphasized that producing the filters requires careful machining as well as painstaking, precision hand tuning. He said one of his key employees is more like a watchmaker than a typical electronics technician. Heine added that a key challenge in producing Wainwright’s line of products is to precisely tune the filters and see the results of that tuning in real time. To do that, the company deploys VNAs (vector network analyzers) ranging from 9 kHz to 40 GHz and including the Models ZVRE, ZVM, ZVK, and ZVB20 from Rohde & Schwarz.
When you want to test a wide range of products, you need a versatile instrument, Heine said, adding that technicians need to see measurement results immediately when they build filters with 10, 20, or more resonators. They need quick measurement results when adjusting parameters such as pass band, reject band, and return loss. He pointed to a VNA screen as a technician made an adjustment to a filter under test: “As you can see, the instrument is very fast.” The technician can make an adjustment and see the results immediately. Without that speed, Heine said, it would be nearly impossible to tune the filters in a reasonable amount of time.
American founds company
On the occasion of Claire Wainwright’s death in April 2007 at the age of 81, Jack Browne, writing in Microwaves & RF (Ref. 1), summarized Claire Wainwright’s work history, which included training in electronics in the US Navy; studying electrical engineering at the University of Illinois at Champaign/Urbana; and serving a stint at P.R. Mallory in Indianapolis, IN, where he designed sweep generators and took an interest in filter design. He founded his own American firm, Telonic Industries (which made sweep generators) in Indiana and later Telonic Engineering (which made filters) in California. It was on a business trip to Germany on behalf of Telonic that he met Christel, whom he would marry and with whom he would move permanently to Andechs.
Christel Wainwright said that to her knowledge, Telonic was the first company to offer filters as separate products and to customer specifications. Previously, filters were part of larger circuits or products, such as the sweep generators from Telonic. “It was practically impossible to purchase a filter in those days or even specify one, ” she said, adding that after Claire Wainwright realized he could not find anyone who would sell him a band-pass filter, he recognized an opportunity, leading him to found Telonic Engineering. In one key innovation, he provided a clear, concise method of allowing customers to specify filters—a method that can be found in most filter catalogs to this day.
Attracting skilled employees
The technical challenges that Wainwright Instruments faces include attracting skilled employees. “We need people who are neither fish nor meat,” who have the skill sets of mechanics and electrical engineering, Christel Wainwright said. “There are no integrated circuits or transistors here,” she added, and prospective employees trained in electrical engineering might feel out of place.
The present filter-building and customer-service teams, she said, consist of experienced employees who have been with Wainwright for 10 or even 25 years, and she added that several talented new technicians are being trained to enable the company to expand.
To compete with larger prospective employers, Wainwright Instruments has been flexible and innovative in providing a good place to work—establishing a 36-hour work week spread over four days, for example. Christel Wainwright emphasized, however, that customers can contact the office every business day.
Of course, the employees, too, need to be adaptable. “Sometimes a customer will place an order for a lot of filters, keeping the staff working Fridays and Saturdays, only to suddenly not need so many filters,” she said. That customer, she said, has always been replaced by another, and the company tries to maintain sufficient flexibility to meet shifting demand. In general, business is good, she said, with the company having posted a record year in 2010. The company is looking to expand and is in the process of building a new facility a few kilometers away.
Wainwright Instruments doesn’t employ a sales force or make use of distributors. The Website is the company’s primary selling tool, where each datasheet lists full price information, in euros. The company used to provide US dollar prices as well, including import/export fees, but found it became impractical to update the datasheets as currency-exchange rates fluctuate. US customers may pay in euros and assume import and freight charges themselves, or they can ask for a quote in US dollars.
“The Internet changed everything,” Christel Wainwright said, adding that previously, the company spent a considerable amount of money printing catalogs that would likely be out of date by the time they reached their worldwide destinations. “Now, we don’t print anything.” She added, “By selling to the user directly, our service department keeps close contact to the customer’s engineers and helps them identify and solve their filter problems, even if they need only one filter.”
The challenge of RF engineering
Good RF engineers who can design and build filters are hard to find. Explained Heine, “You can never learn all you need to know at school. You need a mentor—someone who works with filters all the time and can show you all the tricks. That’s the only way to learn it.”
Filter-design software has its place, he said, but the software is limited, and an accomplished filter designer must “understand what is actually happening in the metalwork and what effect his physical manipulations have on filter performance. What will happen if you bend a wire like this or like that,” he said, trying out a few configurations on a sample filter. Beginners must not only learn how to use complex instruments like VNAs, they must also learn the basics through experience: “Don’t touch the silver plating, don’t over-torque the screws, handle the connectors with care, etc., etc.,” Heine said, concluding that when building filters, “It’s important to be very, very careful, because with RF devices, any small thing you do can affect performance.” T&MW
1. Browne, Jack, “The Odyssey of Claire Wainwright,” Microwaves & RF, June 2007. www.mwrf.com/Article/ArticleID/15801/15801.html.
|An abundance of filters |
Wainwright Instruments offers standard filters whose guaranteed specs you can find in the many datasheets posted on the company Website.
The company offers low-pass Chebyshev filters with cutoff frequencies to 16 GHz, low-pass Cauer filters with cutoff frequencies to 1 GHz, high-pass Chebyshev filters with cutoff frequencies to 10 GHz and with an upper pass band to 26.5 GHz, high-pass Cauer filters with cutoff frequencies to 1 GHz, narrow and wide L/C band-pass filters, and very wide high-pass/low-pass combinations. The company also offers a variety of cavity-design fixed frequency and tunable band reject and notch filters as well as wide and narrow band-pass filters—fixed and tunable. Also available are diplex/duplex-filters and multiplexers in L/C and cavity designs, including band-pass/reject diplexers.
Wainwright Instruments will also develop filters to meet a customer’s requirements. Using an online form, you can specify parameters such as pass-band upper and lower frequencies, pass-band loss, return loss, VSWR, reject-band frequencies, reject-band attenuation, and storage and operating temperatures. You can also specify power-handling requirements, mounting provisions, and the types of connectors you want, and the company will try to accommodate any dimensional restrictions you may have. —Rick Nelson
|Vector network analyzers|
Unlike their scalar counterparts, VNAs (vector network analyzers) measure phase as well as amplitude, allowing users to fully characterize RF and microwave devices such as filters. A typical VNA includes an RF generator, receivers, and a test set (Ref. A). The test set routes incident waves to a DUT (device under test) and to a receiver reference channel, and it routes transmitted and reflected waves to receiver-measurement channels, thereby providing sufficient information for the instrument to derive S-parameters (Ref. B) for the DUT. An onboard computer handles tasks such as system error correction as well as embedding and de-embedding, and it can display measurement results in a Smith chart (Ref. C) or in other graphical formats.
VNAs are complex instruments, and using them correctly requires detailed understandings of topics such as measurement accuracy and calibration, random and systematic measurement errors, calibration standards and traceability, error models, and measurement uncertainty. Nevertheless, the instruments are powerful and flexible RF/microwave measurement tools, and they can make linear and nonlinear frequency-domain measurements as well as time-domain measurements.
A VNA example is the R&S ZVB, which Wainwright Instruments uses to tune its filters and generate the datasheet curves it sends to its customers. The ZVB offers frequency ranges from 300 kHz to 4 GHz, 8 GHz, 14 GHz, and 20 GHz; it comes with two or four test ports to support multiport and balanced measurements.
A key feature of the ZVB is high speed—measurement time is less than 4.5 ms for a 201-test-point frequency sweep, switching time between channels is less than 1 ms, switching time between instrument setups is less than 10 ms, and data transfer time is less than 0.7 ms for 201 test points. The instrument permits simultaneous measurement of more than one DUT.—Rick Nelson
A. Hiebel, Michael, Fundamentals of Vector Network Analysis, 2nd ed., Rohde & Schwarz, 2007. www.books.rohde-schwarz.com.
B. Nelson, Rick, “What are S-parameters, anyway?” Test & Measurement World, February 2001.
C. Nelson, Rick, “How does a Smith chart work?” Test & Measurement World, July 2001.