VCC6-Q/R Series, 2.5 and 3.3v PECL Crystal Oscillator
Vectron International 267 Lowell Rd, Hudson NH 03051
Tel: 1-88-VECTRON-1
e-mail: vectron@vectron.com
Terminating PECL Outputs
The VCC6 incorporates a standard PECL output scheme, which are un-terminated emitters as shown in
Figure 5. There are numerous application notes on terminating and interfacing PECL logic and the two most
common methods are a single resistor to ground, Figure 6, and a pull-up/pull-down scheme as shown in
Figure 7. An AC coupling capacitor is optional, depending on the application and the input logic
requirements of the next stage.
One of the most important considerations is terminating the Output and Complementary Outputs equally.
An unused output should not be left un-terminated, and if it one of the two outputs is left open it will result in
excessive jitter on both. PC board layout must take this and 50 ohm impedance matching into account.
Load matching and power supply noise are the main contributors to jitter related problems.
Figure 5. Standard PECL Output Configuration
Figure 6. Single Resistor Termination
Figure 7. Pull-up Pull-down Termination
Resistor value are typically:
Resistor values are typically:
120 to 240ohms for 3.3V
130 and 82 ohms for 3.3V
82 to 120 ohms for 2.5V
240 and 62 ohms for 2.5V
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