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(Previously published in Planet Analog 6/12/07, and EE Times 7/23/07)
Most people are familiar with applications connecting a crystal to a crystal oscillator (XO) IC to generate a frequency. However, many applications also use XO ICs with a reference input, rather than a crystal, connected to the oscillator input pin, as shown in Figure 1.
For this reason, this pin is often labeled Xin/REF to indicate that it may be used with either a crystal (i.e. Xin) or reference (i.e. REF) inputs.
There are many reasons to use reference inputs with XO ICs. The wide variety of XO ICs available provides designers many options in filling out a clock tree. In such cases the XO is treated just like any other clock IC. For example, many XO ICs integrate additional functionality, making them attractive for saving board space, reducing component count, and saving cost.
Additionally, connecting a reference input to an XO IC is the only option when the input waveform does not meet industry-standard logic levels (such as LVCMOS, LVDS or LVPECL). For example, TXCO outputs are ∼1 Vpp clipped sine waves, making them incompatible with ICs designed for standard logic-level inputs. Therefore, TCXO outputs are typically ac-coupled into an XO IC, since an XO's input is an amplifier stage accepting signals down to a few 100 mV.
In all of these applications, the potential for introducing high output jitter exists if the following two conditions are met...
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