177859566EE168| Application Note
Engineer To Engineer Note
EE-168
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Technical Notes on using Analog Devices' DSP components and development tools
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Using Third Overtone Crystals with the ADSP-218x DSP
Contributed by Larry Hurst August 8, 2002
Introduction
DSPs frequently require an input clock frequency (CLKIN) that is over 35MHz. Unfortunately fundamental mode crystals over 35MHz are not popular and tend to be expensive and fragile. Packaged clock oscillators cost considerably more than a crystal so, for some applications, using a 3rd overtone (3rd OT) crystal may be a sensible choice. While the current trend is to incorporate PLL frequency multiplication into the DSP, using a low frequency input clock to generate internal core clocks of several hundred MHz, there are still occasions when it might be useful to consider using a 3rd OT crystal. This note discusses using readily available 3rd overtone crystals, at frequencies over 35MHz, with the ADSP-218x family of DSPs. A design procedure is developed for calculating the optimum values for the support components. This procedure can be extended to CODECs and other applications requiring input clocks over 35MHz.
Second, a 3rd OT crystal has a lower activity, (i.e. requires a higher minimum drive level to start reliably). For these reasons, extra care should be taken when designing 3rd OT crystal oscillators and careful testing should be performed over temperature, voltage and with a representative batch of crystals to ensure that all parts operate reliably. Note that there is often no indication, marked on the crystal package, to show that a crystal is intended for 3rd OT operation verses fundamental mode operation. Care should be taken to determine this information. If a crystal is used in a traditional (two capacitor fundamental mode circuit) appears to be oscillating at approximately one third of the frequency marked on it's package, it is very likely that it is intended for 3rd OT operation.
Design Method
When a 3rd OT crystal is chosen, two additional circuit components must be added to the traditional parallel, or fundamental mode circuit, to force oscillation at the overtone frequency marked on the crystal. The added components consist of a series inductor and capacitor as shown in Figure 1. If L1 and C3 are not added to the circuit, the crystal will oscillate at its fundamental frequency, which is approximately one third of the desired overtone frequency.
Cautionary Note
There are a number of cautions that should be noted when deciding to use a 3rd OT crystal oscillator. First, a 3rd OT crystal normally has a higher ESR, typically more than twice that of a fundamental mode crystal at the same frequency.
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