Activity dips, also referred to as frequency perturbations, can be difficult to spot in oscillating crystals. However, a single activity dip can cause major problems in an application’s signal, including within GPS or missile systems. Imagine launching a missile and then losing control of its navigation. Or getting a very inaccurate GPS velocity reading of an aircraft. These (and many other problems) can be caused by a failure to spot frequency dips in crystals. It’s important to understand what activity dips are and how to avoid them from occurring in crystals...and that's exactly what you're going to learn right now!
There are many different types of modes that can be excited during resonance of a crystal. One of which is “coupled modes” that are commonly referred to as “activity dips”. Activity dips occur when 2 or more excited modes/harmonics pulse at the same rate. Many modes with their corresponding harmonics can couple into the primary mode at a specific temperature and ruin the performance of a crystal. This dip in performance is where the term “activity dip” originates.
An activity dip also causes a frequency changes over certain temperature ranges. In some cases, this frequency deviation can lead to the oscillation to stop all together. These activity dips are more common on smaller sized crystals.
These activity dips can have negative impacts during signal processing. For example, frequency spikes caused by activity dips can cause 2 major problems in GPS receivers:
- The frequency spikes can be falsely interpreted as changes in velocity by the GPS
- A strong frequency spike can lead to complete GPS disconnection from satellites
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Amplitude and Gradient
Amplitude and gradient must be taken into account for activity dips.
An activity dip’s amplitude = peak-to-peak ppm
An activity dip’s gradient = ppm/oC
Activity dips can occur anywhere from a half a degree deviation to multiple degrees. Different applications can tolerate different levels of frequency perturbations over different temperature ranges. Therefore, it’s important to note how much of an amplitude or gradient spike an application can tolerate. The type of chipset, software, or system application can all impact an application’s tolerance. Going back to our GPS example, GPS receivers can track frequency spikes at some level, but are restricted by their carrier-tracking loop. Their tolerance can be impacted by loop type, order, and bandwidth.
Preventing Activity Dips in Crystal Oscillators
It’s extremely difficult to completely eliminate frequency perturbations in all crystals. In fact, it's next to 'impossible'...but we don't believe in that word here.
Therefore, every crystal must be carefully tested in production to spot activity dips over even half a degree temperature deviation. As previously mentioned, even a small activity dip can cause significant operating problems for a short period of time, or even completely disrupt the signal for good. Testing the crystals in an environment controlled chamber over the operating temperature range is one of the most effective methods for spotting those pesky frequency perturbations.
One of the most important ways to be activity dip free is by choosing a trusted crystal manufacturer with state-of-the-art testing equipment to catch even the smallest of frequency perturbations. Bliley has over 85 years of experience creating some of the highest quality and trusted crystals in the entire universe!
Another way of preventing frequency perturbations and drift is by locating and eliminating the top sources of frequency drift...duh! The challing part is that there are so many possible sources out there waiting to throw off your application's frequency. We made it easy for you and compiled a checklist of some of the most common sources of frequency drift. Download your free copy!