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E1437A User's Guide
VXI plug&play Programming Reference
sync when set to HPE1437_OFF allows an immediate frequency change. In
multiple-module systems, setting this parameter to HPE1437_ON prepares the
modules for a frequency change, but does not actually bring about the change until the
next ADC clock corresponding to the next assertion of the shared SYNC signal. The
SYNC transition is generated by calling the hpe1437_meas_control function. Note
that returning sync to OFF before the SYNC signal transition has occurred forces an
immediate asynchronous frequency change.
syncPtr returns the value of the sync parameter.
freq is a number between −0.5 and +0.5, which will be interpreted as a fraction of the
sample frequency. freq is the desired center frequency divided by the ADC sample
frequency. For example, selecting .25 with a sample clock frequency of 20 MHz will
yield a center frequency of 5.0 MHz. The ADC sample frequency is returned by the
hpe1437_clock_fs_get function. Negative frequencies select the negative image of the
signal, which is spectrally inverted from the input signal.
freqPtr contains the current actual value of the center frequency (as a fraction of the
sample clock frequency).
Comments
Although the freq parameter is a double floating point number, its effective resolution is
1/(1024*10^9) or 20 µHz when fs=20.48 MHz. The actual frequency will be set to the
nearest available value. This value is returned by the hpe1437_frequency_center_get
function. In multi-module systems this value represents the pending value rather than
the current value when a frequency change is incomplete due to a pending SYNC signal
transition,.
In multiple-module systems it is often desirable to force the frequency change to occur
synchronously in order to preserve the phase relationship of the LOs. This is
accomplished by setting the sync parameter to ON for all the modules which are to be
changed. See the first example below.
In configurations involving synchronous operation of multiple E1437 modules, the
hpe1437_frequency_setup function provides a mechanism to force all LOs to the
same phase. This can be done by first setting the frequency to zero. See the second
example below.
Example
The program multichan.exe described in Example Programs provides an example of
how to correctly perform synchronous frequency changes in a multi-module system.
Reset Values
cmplxDc OFF
sync
freq
Effect on Active
These commands do not abort any measurement in progress
Measurement
See Also
hpe1437_init, hpe1437_clock_fs_get, hpe1437_data_type , hpe1437_clock_multi_sync,
hpe1437_meas_control
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