April 2006, Issue 189

Low-Cost 2.4-GHz Spectrum Analyzer

PUT IT IN PLAY

The Low-Cost Spectrum Analyzer (LCSA) PC software is a 32-bit Windows application that should run on any reasonably modern PC (see Photo 3). It has modest memory requirements. There is no special installation process. Just double-click the LCSA icon.

Prior to using the software for the first time, you must install a driver for the CP2102 USB chip. This is easy in Windows XP. Let Windows’s Add Hardware wizard search the Internet for a driver after plugging the spectrum analyzer into the USB port. Alternately, you can tell Windows to use the Silicon Laboratories driver, which you may download from the Circuit Cellar FTP site.

After plugging in the spectrum analyzer, installing the CP2102 driver, and running LCSA, the software should immediately begin acquiring data and showing you the 2.4-GHz ISM band spectrum in real time. You can open the Options dialog box to adjust the amplitude and frequency scales. Toolbar buttons allow you to turn on and off Peak Hold, color amplitude bars, and the white and black backgrounds. The Save menu item will export a comma separated value (CSV) file of the currently displayed spectrum.

In the simplest situation, you can perform a single sweep of the band and see the spectrum almost instantly on the screen. However, this works well only for continuous wave (CW) signals in which the spectrum is always the same from one instant to the next. 

In most ISM communication protocols, the carriers are turned on and off and/or hop to different frequencies during a transmission. Table 1 shows some of the differences between popular 2.4-GHz protocols. For protocols labeled as direct-sequence spread spectrum (DSSS) or frequency-hopping spread spectrum (FHSS), you must use special techniques to get a true picture of the frequency spectrum.

If a transmitter is hopping between different frequencies as you are scanning across the frequency spectrum, you might miss a transmission at a particular frequency if you are monitoring a different frequency at that moment. For this reason, the microcontroller can be instructed to over-sample several times at each frequency and save the highest value it finds at each frequency. This is controlled by the Sweep Time parameter in the PC software application. Slower sweep speeds mean more samples are acquired at each frequency in the hopes of monitoring that frequency when the transmitter is transmitting on that frequency.

In practice, you’ll try to strike a balance between sweeping so fast that transmissions are missed and sweeping so slowly that the transmission ends before the sweep is complete or that the screen updates become too sluggish. Typically, you’ll need to capture the spectrum for somewhere between a few seconds and a minute to completely acquire a pulsed spectrum. Alternately, the spectrum analyzer can be put into Zero Span mode, during which it rapidly refreshes the amplitude at one particular frequency.

In addition to adjusting the sweep speed, you can remember the highest value you have found at each frequency. This is done by using the Peak Hold feature, which allows the true shape of the spectrum to be accumulated after many sweeps of the frequency band. The maximum amplitudes at each frequency in the spectrum are saved until Peak Hold mode is turned off.