April 2006, Issue 189

Low-Cost 2.4-GHz Spectrum Analyzer

WHY A SPECTRUM ANALYZER?

As UHF and microwave RF designs become more popular, you frequently need to “see” what’s being transmitted or received. Most of us are used to observing signals in the time domain with an oscilloscope; however, most oscilloscopes don’t have the bandwidth to see microwave signals. In addition, they aren’t much help in determining what a signal looks like in the frequency domain. This is where spectrum analyzers come in.

In a perfect world, all radio transmissions would be free from interference from other devices operating on the same frequency. In reality, though, many wireless devices must coexist in the industrial, scientific, and medical (ISM) bands. These portions of the frequency spectrum are allocated for unlicensed, low-power, short-range operation. Examples of devices operating on ISM bands are cordless phones, Wi-Fi networks, Bluetooth devices, and cordless mice for PCs. If you’re designing a wireless system that must coexist with these potential sources of interference, you must understand how these transmissions relate to your own communications. If you’ve ever had the experience of your Wi-Fi network going down while you were talking on your 2.4-GHz cordless phone, you know what I mean.

If you’re like me, you’ve had the experience of designing wireless systems without a means to debug transmissions. Is the transmitter sending its signal at all? Is it transmitting on the correct frequency? Using your 100-MHz oscilloscope on this 2.4-GHz signal will get you nowhere. My spectrum analyzer can go a long way in helping sort out these problems.

If you need to carry out calibrated, traceable measurements, or if you need to look for harmonics of 2.4 GHz, there’s no substitute for a high-end spectrum analyzer with its high-end price tag. However, many aspects of wireless design and the process of conducting site surveys don’t call for these features. Even if you have more modest requirements, you must dig pretty deep into your wallet to acquire the right equipment. In the past, if you attempted to build low-cost, hobbyist-grade spectrum analyzers, you probably ended up with a bulky systems based on off-the-shelf modules like cable TV tuners, which cost several hundred dollars.