Chapter 2
NI 5450 Overview
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National Instruments Corporation
2-29
NI PXIe-5450 User Manual
Using an External Clock with the OSP Block
Some applications may require lower jitter or phase noise than is provided
by the onboard High-Resolution clock. You can use an external clock
source to achieve spectral purity at any arbitrary I/Q rate. To determine the
frequency of the sample rate for the external clock source, complete the
following steps.
1.
Set the Sample Clock Source property or the
NIFGEN_ATTR_SAMPLE_CLOCK_SOURCE
attribute to the external
clock source you are using.
2.
Set the IQ Rate property or the
NIFGEN_ATTR_OSP_IQ_RATE
attribute.
3.
Read the value of the Sample Rate property or the
NIFGEN_ATTR_ARB_SAMPLE_RATE
attribute.
4.
Set the external clock source sample rate to the frequency of the
Sample Rate property or the
NIFGEN_ATTR_ARB_SAMPLE_RATE
attribute that you just read.
5.
Validate that the external Sample clock source is connected to the
NI 5450 connector specified in the Sample Clock Source property or
the
NIFGEN_ATTR_SAMPLE_CLOCK_SOURCE
attribute and is
generating a clock before you continue configuring the NI 5450.
For more information about using external clocks, refer to External Sample
Clock Sources.
Frequency Shift
The Frequency Shift property or the
NIFGEN_ATTR_OSP_FREQUENCY_SHIFT
attribute can be used to shift the
frequency of a baseband signal. This property can only be used when the
OSP mode has been configured for baseband by setting the OSP Mode
property or the
NIFGEN_ATTR_OSP_MODE
attribute.
When you are using the NI 5450 with an external I/Q modulator, the
Frequency Shift property or the
NIFGEN_ATTR_OSP_FREQUENCY_SHIFT
attribute modifies the baseband I/Q data such that the resulting RF signal is
shifted in frequency.
Note
When the NI 5450 is configured for single-channel operation in Real data processing
mode, you can use the active baseband channel as a direct, frequency-shifted output, in
addition to creating RF frequency shifts with an I/Q modulator.
