A GPS-Disciplined Frequency Counter using the PSoC 5LP

In this month's column, Brian tasks one of his mixed-signal favorite MCUs, the Infineon PSoC 5LP, to build a frequency/ period counter using a GPS satellite module to discipline the project's time base.

- By Brian Millier

Frequency counters and signal generators generally derive their accuracy from whatever accurate time base they have onboard.

This is often a crystal oscillator, which may take the form of a Temperature Compensated Crystal Oscillator (TCXO) if better accuracy is needed. Oven Controlled Crystal Oscillators (OCXOs), however, are incorporated in the “Cadillac” of time-bases (ignoring very high-tech solutions like a rubidium-based frequency standard), and are much more expensive than TCXOs.

While most people are quite familiar with the location information that various GPS satellite constellations provide, everything about GPS boils down to being able to measure time extremely accurately. GPS location services work by comparing timing signals received from several different GPS satellites (the more the better) and calculating the GPS receiver's location from these time relationships. The GPS satellites themselves have extremely accurate time-bases onboard and know their own positions accurately, allowing this scheme to work. This GPS timing signal can be used to implement an accurate time-base.

The cost of GPS receiver modules has dropped substantially over the last decade or so. Most of this can be attributed to the fact that tiny GPS receivers are built into virtually every cell phone currently sold. Although this drop in price made GPS modules readily available for electronics enthusiasts and makers, they still weren't particularly useful in their earlier iterations because the receiver chip in them was not sensitive/noise-free enough to allow them to work indoors. I bought a modestly priced GPS module a few years ago, hoping it would work indoors—at least near a window, anyway. It didn’t—and is buried in my junk box somewhere—since I didn’t have any outdoor use for it in mind.