When Would I Use An External Sample Clock?

Return to Fundamentals of High-Speed Digitizers

Typically, a high-speed digitizer uses an internal sample clock to determine the rate to acquire data. However, there are several occasions when using an external clock source is advantageous. These situations are discussed below.

When considering whether to use an external clock, first note that the internal time clock of NI high-speed digitizers depends upon the model (either 100MHz or 200MHz). Additionally, any external clock used should still not exceed the maximum sampling rate specified for your high-speed digitizer.

Note: Not all high-speed digitizers support external sample clocks. Refer to the specifications document for your device.

Capturing Data at the Rate of Incoming Signals

You may want to use an external sample clock if you are sampling a signal where an irregular sampling period is necessary. Typically, you will configure your system so that the incoming signal that you want to sample is accompanied by a TTL pulse train, which acts as your external sample clock. Each pulse of this signal is used to sample data for all channels simultaneously.

Synchronizing With Other Devices

External clocking provides a method to synchronize your high-speed digitizer to other devices in a measurement system by distributing a common clock to multiple devices. Additionally, an external clock can provide a consistent timebase between multiple PXI chassis.

Using a More Accurate Clock

Applications that require an extremely accurate timebase will benefit from the use of rubidium or oven-controlled crystal oscillator (OCXO)-based frequency sources. The accuracy of these devices can be better than ±100 parts per billion (ppb). For example, an OCXO source with ±100 ppb accuracy yields a 10 MHz clock with ±1 Hz uncertainty. The NI PXI-6653 Slot 2 timing and synchronization controller is ideal for such applications with an OCXO accurate to ±50 ppb. It can drive its OCXO clock onto the PXI 10 MHz reference clock lines instead of the PXI backplane clock. Thus, all instruments with VCXOs locked to the 10 MHz OCXO inherit the ±50 ppb accuracy.
Related Links:
Fundamentals of High-Speed Digitizers
National Instruments Synchronization and Memory Core -- a Modern Architecture for Mixed-Signal Test
How Do National Instruments PXI Boards Map to the PXI Backplane?

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