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Chapter 3
Hardware Overview
©
National Instruments Corporation
3-7
The remainder of this chapter describes how these signals are used,
acquired, and generated by the NI PXIe-6672 hardware, and explains
how you can route the signals between various locations to synchronize
multiple measurement devices and PXI chassis.
Clock Generation
The NI PXIe-6672 can generate two types of clock signals. The first clock
is generated using the onboard DDS chip, and the second is generated with
a precise 10 MHz oscillator. The following sections describe the two types
of clock generation and explain the considerations for choosing either type.
Direct Digital Synthesis (DDS)
DDS is a method of generating a clock with programmable frequency.
DDS consists of a
frequency tuning word
, an accumulator, a sine-lookup
table, a D/A converter (DAC), and a comparator.
The frequency tuning word is a number that specifies the desired
frequency. Each master clock cycle, the frequency tuning word is added to
the accumulator, which rolls over when it gets to its maximum value. The
accumulator value is used to get a point in the sine-lookup table, which is
converted to an analog voltage by the DAC. For example, if the sine table
is 128 points long, and the frequency tuning word is one, the accumulator
takes 128 clock cycles to output one sine wave. If you change the frequency
tuning word to 3, the accumulator steps through the sine table three times
as fast, and outputs a sine wave in 128/3, or 42.6, clock cycles.
The output of the DAC is run through an analog filter to smooth the sine
wave. The filtered output is then run through a comparator, which changes
the output to a square wave with the specified frequency.
You can specify the programmable DDS frequency on the NI PXIe-6672
with a precision of approximately .07 Hz within the range 1 Hz to
105 MHz. The accuracy of the frequency depends on the PXI_CLK10
reference clock, so a precise 10 MHz source improves the accuracy of the
DDS output. You can replace the 10 MHz clock with the TCXO for more
accurate DDS timing.
When the DDS is programmed an update signal must be sent to it before it
will begin operating as programmed. The source for this update signal is
either immediate (DDS starts outputting the programmed frequency as
soon as software programs it) or one of the eight PXI triggers. When one