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Inside Frequency Control

Warning! EFC Over-Specification May Be Costing You (Do this instead!)

Posted by Rob Rutkowski on Oct 2, 2019 11:14:09 AM

EFC Over-Specification May Be Costing You!

What if I told you that specifying more Electronic Frequency Control (EFC) than you need could actually be hurting you pocketbook? Well, it very well may be! Paying attention to whether your supplier is using AT cut vs SC cut crystals will help you save money in the long run with OCXOs.

AT Cut vs. SC Cut Quartz Crystals

To understand why EFC Pull might be creating more costs than benefits, we need to take a closer look at these 2 types of crystals inside your quartz crystal oscillators. For the most part, AT Cut crystals are typically cheaper, however you'll likely end up sacrificing performance in multiple areas such as

  • Frequency vs. Temperature
  • Crystal Aging
  • G-sensitivity

While SC cut crystals can be more expensive, they typically provide significantly better performance in the same areas mentioned above (and below).

crystal-cuts-comparison-chart

We talk a lot more about the types of crystals inside your oscillators in this post. Consider checking that out for more on performance qualities of AT and SC cut crystals.

2 Common Impacts of EFC Pull on AT Cut & SC Cut Quartz Crystals in OCXOs

1. Costs

It's common for design engineers to try and save costs by using less-expensive AT cut crystals in their OCXO designs and using more EFC pull to try and compensate for the lower performance quality. However, this is a myth in many cases and doesn't work out in the long-run. Here's why...

If you spec more EFC than you need to cover frequency deviation, costs will significantly increase for the supplier because their overall yield will go down. So in reality, there's really no cost savings to be found when going with an AT cut crystal and compensating with more EFC pull.

This is one reason why we recommend just sticking with an SC cut crystal in your designs instead of trying to save costs with an AT cut crystal and high EFC pull.

2. Crystal Aging

The second benefit to sticking with an SC cut crystal in an OCXO is aging. Using EFC to maintain crystal stability over many months and years is actually very inefficient when compared to using the "natural" stability of a SC cut crystal.

With a 100MHz SC cut crystal, it'd be realistic to see aging of around 0.1 ppm over a year, where as an AT cut would see significantly higher aging of around 1 ppm/yr. Expecting EFC to cover for aging in AT cut crystals is, many times, not the best option.

Quick Summary

We recommend sticking with SC cut crystals opposed to AT cut crystals combined with EFC pull when it comes to OCXOs. AT Cut crystals combined with EFC don't actually save on costs and can lead to poor performance in the long-run.

Common Oscillators

Topics: crystal oscillators, Clocks & Crystals, RF Technology