V.J. Wilson anxiously awaits the return of his prototype of a factory automation wireless link. He has thoroughly — he believes — analyzed his entire design. With a carefully crafted spreadsheet, he defined a system architecture and passed specifications on to the circuit designers. The design team completed block designs and simulated their performance. As time is of the essence, and prototyping costs are high, the team is putting together the transceiver in a single pass prototype.
The prototype comes back, and disaster strikes. Absolutely nothing works. Apparently, the duplexer designer misread the lower corner of the transmit channel from the spreadsheet. Then, the LNA designer created an oscillator instead of an amplifier because he incorrectly entered the fT of his transistor. Third, and worst of all, V.J. himself mis-defined a splitter isolation term causing the receiver LO to leak out onto the antenna.
Such design issues are all too common. Even when using powerful modern design systems, engineers too often work alone and the same errors are made over and over. With GENESYS 2004, engineers move to the next stage of design automation — workgroup automation. At this stage, groups of engineers work together as a team, reuse each others’ work, and eliminate errors of communication.
Electronic design automation (EDA) progresses orderly through five stages, starting with manual design and ending with enterprise integration (see sidebar on page 118). While EDA in areas such as digital PCB design has progressed to the highest level, RF and microwave design has been lodged just below workgroup automation for a number of years, primarily because design software and operating systems have been limited.
GENESYS 2004 changes that. The release offers new capabilities that make sharing designs easy. A key component is the XML-based library manager. Included in all GENESYS professional products, the library manager makes it easy to create and share custom libraries. Leading companies are already creating such libraries with GENESYS, investing time to qualify parts with careful measurements, and including company and vendor reference information. These qualified libraries allow engineers to readily access complete part information with confidence, saving time and eliminating errors. Figure 1 shows an example part library entry.
Also included in GENESYS 2004 are updated sets of device models from 16 vendors. Engineers can now search for specific bias conditions or package sizes from over 30,000 parts from within the library manager. No longer do users have to look through file lists to find a part. This data serves as an excellent starting point for developing one’s own qualified part libraries. With a simple button click, a part can be moved into a company or personal library.
In V.J.’s case, using a qualified library would have prevented the data entry error that ruined the LNA design. The use of the library also would have saved the designer wasted time — time spent finding the part, its datasheet and the company part number, and re-entering parameters that other engineers at his facility had done a dozen times before.
In addition to libraries, a wide range of customization is possible with GENESYS 2004. Using Visual Basic the possibilities are almost unlimited. For example, custom menu picks can be added, and advanced optimization algorithms can be developed and used.
Integrated RF Architecture Design
The most important part of RF design is getting the architecture right so that the workgroup can develop a system that works. The best LNA, filter, mixer and LO designs are useless if they do not work together. At first glance, putting together a receiver block diagram seems simple. However, as anyone who has been through such a design will testify, it is all too easy to overlook important details. The SPECTRASYS simulator in GENESYS 2004 completes detailed analysis of systems at all frequencies, rapidly and accurately. As a result, SPECTRASYS is rapidly becoming the industry standard for sophisticated RF architecture design. Now in its fourth major release, this technology is helping engineers solve problems they often did not even know existed. Figure 2 shows a typical spectral domain simulation.
In V.J.’s case, the oversight was small and easy to fix — by simply increasing the splitter isolation term, he would have eliminated the LO leakage. But he was unaware of the problem. Had he entered his design into GENESYS and simulated the performance prior to prototype, the unwanted radiating signal would have been seen immediately.
In many cases, interference can be subtle and almost impossible to find. For example, engineers at Hittite Microwave Corp. recently showed1 how the second harmonic of an interfering signal mixes with the second harmonic of the LO to produce an in-channel tone. The tone is below the signal level, but is large enough to have a measurable effect on the bit error rate (BER) of the receiver. The spectral domain simulation method used by SPECTRASYS is unique in its ability to identify signals hidden under noise and under larger signals.
In GENESYS 2004, SPECTRASYS now includes an active frequency divider, new filter models that include the effects of Q, faster calculation speeds, a new warning message that identifies potential stability problems in the RF architecture, and the ability to graph any sweep of a SPECTRASYS path measurement at any node in the schematic. Improvements have also been made to the log detector, LO harmonics in the table mixer, phase accuracy in multipliers and dividers, and more.
Converting Architecture to Circuits
Obtaining first-pass designs can be tedious and error prone. More importantly, without expert knowledge in the design of each given component of a design, an engineer may develop far from optimal circuits. A central strength of GENESYS are the eight synthesis products that create first-pass splitters, filters, oscillators and matching networks for the engineer, directly from specifications. With GENESYS’ built-in component expertise and direct connection to the RF architecture design, the engineer will get better circuits with less tedium and fewer errors, as shown in Figure 3.
Looking again at V.J.’s case, it can be seen that with GENESYS, the filter engineer could have synthesized the filter directly from the architecture schematic, eliminating the error in communication on the corner frequency. He would simply have right-clicked on the block-level filter and directed the system to synthesize a microwave, LC, or active filter using the specifications on the block filter. Not only would there have been no confusion about the corner frequency, the filter designer would have seen the impact of the realized filter on the overall system performance prior to prototype.
In GENESYS 2004, five new topologies have been added for microwave filters, LC filters and oscillators. The Step Penetrating Lowpass, shown in Figure 4, is one of two new microwave filter topologies that have been added to the M/FILTER synthesis program. It features a space-saving topology that organizes stepped impedance lines to minimize space, providing designers with improved size reduction. Quarter Wave Coupled Bandpass, based on quarter wavelength resonators, gives the designer complete freedom and control over resonator parameters, including impedance, to produce filters with bandwidths up to 100 percent.
GENESYS 2004 also includes a new LC filter topology. The Top-C TEM Resonator Bandpass topology makes use of traditional capacitively coupled LC resonator filter design with the resonators replaced by transmission line resonators. Ceramic transmission lines are excellent candidates for this filter because they provide a physically small structure with excellent Q values.
The oscillator synthesis module has been updated with two new oscillator configurations. A completely new class of oscillator is supported — the LC Hybrid, developed by Eagleware’s founder Randy Rhea, features rapid startup characteristics and excellent harmonic performance. In addition, a Negative Conductance VCO is now available for frequencies above 200 MHz requiring octave tuning.
A New Way to Design
When a workgroup adopts GENESYS 2004, productivity increases through improved teamwork. Common errors, such as those seen by V.J. and his team, are eliminated. Repetitive model entry and communication errors are reduced and better designs are created faster.
GENESYS 2004 is now shipping and is immediately available. Additional information is available at www.eagleware.com, by e-mail to email@example.com, or by telephone.
- Hittite Microwave Corp., Application Note, “High IP3 Mixers for Cellular Applications,” http://www.hittite.com.