April 2006, Issue 189

Low-Cost 2.4-GHz Spectrum Analyzer

PC CHIPs TO THE RESCUE

You can thank the Pentium processor line (and other similar chips) for some interesting new capabilities in wireless circuits. How does computer technology affect RF circuits? In the constant push for smaller feature sizes and faster clock speeds, chips for high-volume products like PCs set the standard for the semiconductor fabrication processes used throughout the industry. This benefits other chip product lines because smaller features mean more capabilities at lower costs. In addition, the faster circuits needed for today’s PCs allow microwave frequency circuits to be integrated onto a single chip that in the past required many external components.

Examples of state-of-the-art low-power ISM band circuits are the recent offerings from companies like Xemics, Nordic Semiconductor, and Chipcon. If you’re familiar with conventional radio circuits, you’ll look at Figure 1 and ask, “Where are all the tuned circuits?” Unlike conventional tuners like the venerable superhet with its big IF coils, these new frequency-synthesized chips require virtually no external parts other than bypass capacitors, a crystal, and (sometimes) an antenna matching network.

In spite of these semiconductor advances, there are no dedicated, low-cost spectrum analyzer chips on the market right now. But if a chip has the right combination of features, you can use a radio chip intended for other purposes in your spectrum analyzer design. The basic approach for a swept-tuned spectrum analyzer is to tune a single-chip radio receiver to a given frequency, measure the signal strength, tune to the next frequency, and so on until you’ve acquired the signal strength at all frequencies in the spectrum. Plotting the spectrum is relatively straightforward after you have the data.