Agilent Technologies VXI E1439 - Manual (Page 50)

40
Using the Agilent E1439
Managing multiple modules
Trigger and phase in multi-module systems
When you use triggering in multiple modules, you do not need to measure phase differences
between two or more channels if the channels are set up identically in terms of digital filtering and
LO frequency, and the digital filters and LOs are correctly synchronized. Since the filters and LOs
are synced together, their actual trigger delays and LO phases are identical and will cancel out of
relative phase measurements. Any remaining delay should be less than 10ns between two
modules in the same mainframe.
Only the module that generates the trigger has knowledge of the delay between the trigger event
and the start of data collection. Therefore, if you need the actual delay from the trigger, you
should use the trigger delay correction from the module that generated the trigger. Likewise, you
should obtain the LO phase at the time of the trigger from the module that generated the trigger.
See “Delay and phase in triggered measurements” on page 25 .
External sample synchronization in multi-module systems
There are two general instances where you might want to use an external sample clock in a system
with multiple E1439s:
• You wish to have the ADC's sample at a rate other than the 95 MHz clock supplied with the
E1439.
• You wish more precise simultaneous sampling than can be provided by the normal scheme
that uses the internal VCXOs within the modules locked together by a 2.5 MHz reference that
is distributed from module to module. By exercising care in matching the skew of the sample
clocks fed into each module, channel-to-channel group delays at low frequencies can be well
below a nanosecond.
Note
External sample is specified only for use with baseband path.
To use external sample clocks with multiple modules and still perform synced measurements, you
need to use either the AGE1439_FRNT_SYNC_EXT_SAMP or AGE1439_REAR_SYNC_
EXT_SAMP clock setups (see
“age1439_clock_setup” on page 78 ). These setups use the signal
that you feed into the Ext Clock/Ref BNC input of the E1439 as a sample clock for the ADC. A
counter within the E1439 generates two lower frequency clocks, one for the DSP circuitry and
one to clock the measurement SYNC signal between multiple modules. Since these clocks are
generated independently within each module, the counters in each module must be synced
together with a common externally generated signal in order to make properly synced and
triggered measurements involving multiple channels. You feed this "external sample sync" signal
into the External Trigger BNC and the module uses the signal to reset the counters to a known
phase.
The external sample sync signal should be generated on the falling edge of the external sample
clock, and fed into each module in the system by an identical length coax cable. Likewise, the
sample clock should be fed into each Ext Clock/Ref BNC by an identical length coax cable from a
common driver.