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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 PXI-6653 and NI PXI-6652 with
a precision of approximately 1
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 OCXO for more
accurate DDS timing.
PXI_CLK10 and OCXO or TCXO
The NI PXI-6653 features a precision 10 MHz OCXO. The NI PXI-6652 features a precision
10 MHz TCXO. The frequency accuracy of this clock is several orders of magnitude greater than
the frequency accuracy of the native 10 MHz PXI backplane clock (PXI_CLK10).
The main source of error in most frequency reference oscillators is temperature variation. The
OCXO houses the oscillator circuit inside a sealed oven. A resistive heater and automatic
feedback circuit maintain a precisely controlled operating temperature for the oscillator. This
temperature-control scheme minimizes frequency error to low enough levels that other sources
of error become considerable. The user can do two things to help mitigate these other error
sources and achieve the best possible performance from the OCXO:
1.
Use a chassis that has as little variance in the 5 V supply as possible. The OCXO has an
error of 1 ppb for every 1% of inaccuracy in the voltage level of the 5 V supply. The
CompactPCI Specification
requires all chassis to regulate the 5 V supply to -3% to +5%
which results in a -3 ppb to +5 ppb possible error referenced to 5 V. Improved performance
can be obtained by using a chassis that specifies a tighter accuracy on the 5 V supply.
NI recommends using the NI PXI-1044 or NI PXI-1045 chassis as they have tighter power
supply regulation performance.
2.
Avoid power-cycling the OCXO. The longer the OCXO is consistently powered, the more
stable the aging rate will be.
For the tightest performance requirements, the OCXO can be calibrated by the user in the same
system in which the NI PXI-6653 is normally used. Doing so will calibrate out error from the
power supply variance and, if done at the normal operating temperature, reduce the error from
temperature variance. Refer to the
NI PXI-665x Calibration Procedure
for more information on
calibrating the OCXO.
The TCXO contains circuitry to measure the temperature of the oscillator. It uses the temperature
to adjust its frequency output according to the crystal’s known frequency variation across its
operating temperature range.
