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Crystals vs. Oscillators - Which Are More Cost-Effective in Wireless Designs?
by Roger Burns, Fox Electronics -- ECN, 5/1/2002
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by Roger Burns, Fox Electronics
When designing a new wireless system, one of the first decisions to make is whether to use a crystal in conjunction with an oscillator circuit built on the IC, or a pre-packaged oscillator. While the designer may want to use the crystal to reduce costs, a closer look reveals that a pre-packaged oscillator may provide the most effective solution.
There are usually three primary factors to consider when determining which frequency source to use; they vary, depending on the end application. An extremely accurate, tight tolerance frequency source is essential when designing a wireless system. In addition, for portable devices and PCMCIA cards, size is also critical, as is cost.
This article provides a comparison of the costs and design times associated with the above options, in order to provide the designer with insight to select the best, most cost-effective solution for the system being developed.
Using Crystals in Conjunction with Oscillator Circuits — Shifting Specifications
While crystals are cost-effective, they may not be the best choice for meeting the high accuracy demands of wireless applications. For example, let's look at an application that requires a ±25 ppm frequency source. This would necessitate a crystal with a tolerance of ±10 ppm, a stability of ±10 ppm, and an aging rate of ±5 ppm. While this seems fairly straightforward, we also need to consider the crystal's frequency variation due to test equipment correlation variations and, most importantly, variances in load capacitance. The frequency of a crystal will shift with a change in load — this is commonly referred to as trim sensitivity and expressed in ppm/pF. (Note: All calculations in this article will assume the crystal's trim sensitivity is 13 ppm/pF.)
When crystals are tested on a crystal impedance meter, they are measured at the desired load capacitance. However, the meter is only accurate to 2 percent of the specified load. For example, if the desired load is 20 pF, the load the crystal actually sees from the meter can vary from 19.6 pF to 20.4 pF. If the crystal has a trim sensitivity of 13 ppm/pF, the actual reading will only have an accuracy of ±5.2 ppm. Therefore, the crystal that is rated at a tolerance of ±10 ppm could, in actuality, have a tolerance of ±15.2 ppm.
To account for this, we can change the crystal specification to ±10 ppm tolerance, ±6 ppm stability, and ± ppm aging. This allows ±6 ppm of cushion for testing variations. However, we still haven't accounted for variations in load capacitance.
Variations in the load capacitance the crystal "sees" in a circuit can be due to several contributing factors, including the load capacitors themselves. For example, the desired load for two 33 pF capacitors with a 1 percent tolerance, and stray capacitance at a constant 3.5 pF would be 20 pF. (See Figure 1) The actual load capacitance due to the tolerance of the load capacitors could vary from 19.835 pF to 20.165 pF or 20 pF ±0.165 pF, while the resulting crystal frequency could vary by ±2.15 ppm. This assumes that the stray capacitance is a constant 3.5 pf; however, the stray capacitance can vary from board to board and IC to IC.
Figure 1
If, as an example, the typical variance in stray capacitance from board to board is ±1.3 pF, from IC to IC the typical variance could be ±0.7 pF, for a total of ±2 pF. The combination of the stray variations and the load capacitor variations would result in a crystal with an actual load capacitance of 17.835 pF to 22.165 pF, or 20 pF ±2.165 pF. This means the crystal frequency could vary by ±28.15 ppm.
Variations in load capacitance
Since the target is an accuracy of ±25 ppm, the variances from the board alone would prevent the designer from being able to guarantee the desired ±25 ppm accuracy. Even if the designer uses one of the tightest crystals available — for example, one rated at ±18 ppm inclusive of tolerance, stability and first year aging, while allowing for ±2 ppm testing error — the designer would have to keep total board variation down to ±0.4 pF.
Pre-Packaged Oscillators — Tighter Tolerances, Less Design Work
When manufacturing a packaged crystal oscillator, the supplier trims the crystal frequency in the circuit. This allows the production of a tight tolerance oscillator that is devoid of the correlation issues that can arise when using a packaged crystal with the designer's own oscillator circuit. It also eliminates the need for the extensive design work required when using a discrete oscillator with a packaged crystal, while maintaining a reasonable cost.
When comparing the size of a crystal and a pre-packaged crystal oscillator, the length and width are the same, while the height generally increases by 0.3 mm to 0.4 mm, depending on which product is chosen. As an example, the F535L (see Figure 2) from Fox Electronics, which is the company's smallest oscillator, measures 5.0 mm × 3.2 mm × 1.3 mm maximum, while the equivalent FX532 crystal (see Figure 3) has a height of 1.0 mm maximum. Therefore, the only disadvantage for a pre-packaged crystal oscillator is a slight increase in height.
Fox's F535L oscillator for use with the FX532 crystal (click image to enlarge)
The FX532 crystal from Fox (click image to enlarge)
Overall Cost Differences
Several factors contribute to the overall price difference between a crystal and a pre-packaged crystal oscillator. Let's compare a crystal that was designed for 802.11A with a frequency accuracy of ±18 ppm inclusive of tolerance, stability and aging with an equivalent pre-packaged crystal oscillator with an accuracy of ±25 ppm inclusive of tolerance, stability, aging, load change, and voltage change. At first glance, the prices for this particular crystal and the oscillator in 100K quantities are virtually identical, with the crystal generally only $0.01 to $0.02 less in cost. However, the cost of the necessary capacitors, as well as manufacturing and sourcing costs required for additional components, must be added to the overall cost of the crystal. Moreover, once all variances are taken into consideration, a +25 ppm accuracy may still not be attainable with a crystal.
It may seem that conventional crystals may be the best choice for most applications because they are less expensive than oscillators. However, when all of the factors are taken into consideration, it becomes very apparent that, especially to meet the high accuracy requirements for wireless applications, oscillators are the easiest, safest, and most cost effective way to go. |
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