Genesys Products - WhatIF Simulation Module

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Genesys WhatIF is a frequency planning tool that helps RF system architects choose a set of Intermediate Frequencies (IFs) that maximize system performance with a minimum of filtering and design margin. It reduces a task that often requires weeks of analysis down to an afternoon using an interactive, graphical approach that is unique in the industry. It accounts for tuned bandwidths, spurs and intermods, and multi-band front-end architectures so that System Architects begin with a workable frequency plan, and can concentrate on other areas of system performance.

Product Overview

The problem in picking intermediate frequencies is that the process of converting frequencies produces unwanted harmonics, or spurs. Computing the frequencies of the spurs is straightforward using the formula

F_IF = ¦m x F_RF ± n x F_LO ¦ or F_RF = ¦m x F_IF ± n x F_LO ¦

m = 0, 1, 2, … (up to 20 or even higher)
n = 0, 1, 2, … (up to 20 or even higher)
F_IF = IF frequency
F_RF = any frequency in the RF band
F_LO = any frequency in the LO band

Actual analysis is extremely difficult, however, because of the huge number of permutations that must be analyzed, and because the amplitudes must also be computed to see if spurs are large enough to actually interfere with system operation. Until now, this computation has been attempted using custom spreadsheets or arcane nomographs called "spur charts." Not all combinations could be analyzed because of the sheer volume of data. So, when an engineer found a solution that was "close enough" the analysis stopped.

WhatIF changes all that. Using a sophisticated synthesis technique, WhatIF analyzes all of the spurs within a given frequency range, based on the characteristics of the mixers to be used in the design, the RF frequencies and bandwidths of the radio system, and the desired IF bandwidth. The results are displayed in an easy-to-understand chart that shows "spur-free" regions that meet system requirements. For example, in the figure, the spur-free regions are clearly shown as the vertical green bars. Note that the extent and amplitude of each spurious region is displayed. The source of each spur can be identified by placing the mouse cursor over the bar.

Product Features

• Easy set-up. Just enter:
  • Mixer characteristics
  • RF frequencies and bandwidths
  • Desired IF bandwidth
• Analyzes all of the spurs in the frequency range
• Displays spur-free regions in an easy-to-read chart
• Cuts weeks to hours

Frequently-Asked Questions

1. Does WhatIF predict spurs in a non-linear mode?

The spurious amplitude prediction in WhatIF is based on the work that Bert Henderson did at Watkins-Johnson years ago. The attached PDF file contains these predictions for a double balanced mixer. As you know, exact prediction of spurious amplitudes is difficult at best since you need to have excellent models for the exact mixer you are trying to simulate. Our purpose of placing double balanced mixer amplitude prediction into WhatIF was to give the designer a rough estimate of the amplitude without going through the pains of mixer characterization. Furthermore, it is the bandwidths and frequencies of the conversion process that are really important since 'spur free' design is usually the goal. In this case the amplitude is somewhat irrelevant, but as you know 'spur free' applies across an amplitude dynamic range, i.e. spur free to 100 dBc or 150 dBc etc. WhatIF is not a tool that is used to predict spurious amplitude as much as it is a tool to help designers understand the performance of each and every potential IF for the given conversion process including the trade-offs between high and low side injection.

2. Can you enter a spur table into WhatIF?

Yes, WhatIF support mixers with Spur Tables beginning with version 2006.04. There's a browse button in WhatIF to search for a mixer part with a corresponding spur table from the part selector.

3. WhatIF predicts spurs for mixers in parallel, what about mixers in series?

WhatIF is not designed to deal with a cascade of series mixers. Generally, the other mixers in the cascade are fixed RF input, fixed LO, and fixed RF output.

Spectrasys is a great diagnostic tool to help understand the spurious mixer products in the cascaded case since it uses a new simulation technique called SPARCA (Spectral Propagation and Root Cause Analysis). As you think about the design process, the Frequency Planning step is usually the first design step. WhatIF is the tool to use to help the designer pick the best frequency plan. Once the frequency plan for the active mixer stage has been selected, Spectrasys is used to complete the entire frequency plan and RF architecture. Genesys can then be used along with the other synthesis products to complete the entire design.

Product Demos and Tutorials

• WhatIF Frequency Planning Tool - May 2007

Genesys New Features Videos

• What's New in Genesys 2005 - demonstrates Mixer Synthesis, WhatIF, and Cayenne
• What's New in ADS 2006A - demonstrates Spectrasys, WhatIF, and Synthesis being used in the bundles that add onto ADS

Publications and Comments

Articles

• Rulon VanDyke, Agilent Technologies/Eagleware-Elanix, Software Shaves Spurs In Frequency Planning, Microwaves & RF, June 2005. Current issue: Microwaves & RF.