Ericsson Shortens Design Cycle with VisSim/Comm

By Charles Gore, Research and Development Engineer, Ericsson, Inc., RF Advanced Development Group in Research Triangle Park, NC, USA

Ericsson CH388 Personal Communication Services (PCS) Phone

Ericsson, Inc., a leading manufacturer of cellular and PCS consumer electronics has turned to VisSim/Comm, a Windows® -based modelling and simulation package, for the rapid prototyping of new transceiver architectures for its next generation products. VisSim/Comm uses a block diagram-based approach for the modeling and simulation of end-to-end communication systems at the signal or physical level. With a full complement of communication blocks and powerful, time-domain simulation engine, VisSim/Comm provides fast and accurate solutions for analogue, digital and mixed-mode communication system designs. VisSim/Comm's intuitive block diagram-based interface makes it easy to build, modify and maintain even the most complex system models.

The goal of Ericsson's RF Advanced Development Group in Research Triangle Park, North Carolina, USA, is to evaluate and implement new technologies that will reduce cost, size and power consumption in future products. A system-level simulation tool such as VisSim/Comm allows engineers to investigate new concepts and provide first order results to upper management before committing to a more costly hardware prototype phase.

The Challenge: Investigating New Digital Modulation Synthesizer Architectures
Critical to the design of a new digital modulation synthesizer was a novel approach employing a modified phase locked loop (PLL) circuit. The new concept involved generating the desired digital modulation at the output of the synthesizer’s voltage controlled oscillator (VCO) by appropriately manipulating the phase within the PLL's feedback loop. This approach would eliminate the need for a subsequent I/Q modulator stage, thus resulting in cost and board real estate savings.

Our challenge was to prove the feasibility of this new synthesizer design without an extensive hardware prototype development cycle. What we needed was a system-level tool that would allow us to create a block diagram model of the modulation synthesizer and predict its theoretical performance mathematically through appropriate simulations.

The Solution: VisSim/Comm
Among the applications we considered to achieve our goal was the VisSim/Comm rapid prototyping tool. VisSim/Comm proved to be the perfect choice for the task at hand. Our initial evaluation proved that it was not only easy to use, but also powerful enough to model and simulate the behavior of a system involving complex feedback loops. Its intuitive graphical interface provided an easy way to develop communication system models, enabling us to quickly manipulate transceiver elements and achieve the desired results without having to build a hardware prototype.

We started by modeling the synthesizer using a standard second order PLL. We then modified the PLL by adding a phase rotation element in the feedback path after the VCO. Coupled with appropriate control logic, this block would introduce either a +90° or a -90° phase rotation in the feedback signal to produce the desired BPSK modulation at the output of the synthesizer.

During the simulation we were able to adjust all the key PLL parameters to achieve our desired transient behavior and produce the desired modulated output (a simplified portion of the VisSim/Comm simulation is shown to the right.) It shows the synthesizer control loop, and includes the data-driven phase shift logic. The output plot shows the input digital data signal and the corresponding synthesizer output. For reference, the local oscillator output is also shown to more easily observe the BPSK modulation effect at the output of the VCO.

Based on the simulation results, we immediately moved on to a successful hardware implementation with a minimum of tweaking required.

Future plans now include implementation of the approach in an ASIC, which could easily be used in millions of digital cellular phones. We have also applied for a patent for this new design.

Synthesizer Control Loop modeled in VisSim

The Benefits
Much of the success for this project we owe to VisSim/Comm. Using VisSim/Comm, we designed the new modulation synthesizer much faster than if we had followed the conventional hardware prototype cycle for proof-of-concept. We were able to validate our approach in a matter of days as opposed to the several weeks required to design, assemble and test a breadboard.

In addition, the task of convincing upper management to pursue this innovative approach would have been much more difficult without the results obtained from the VisSim/Comm simulations.

Overall, using VisSim/Comm shortened our design cycle and simplified the development of new and innovative architectures for the next generation of Ericsson transceiver products.

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