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Keysight Technologies
Achieve High Speed, Multichannel
Data Acquisition with the
M9703A AXIe Digitizer
Application Note
Abstract
In many multichannel high speed data acquisition ap-
plications, such as radar, satellite (beam-forming), MIMO,
or high-energy experiments, it is critical to have phase-
coherent channels for accurate data acquisition whether
for demodulation, antenna calibration, channel sounding,
or event capture. Large-scale experiments may require
the use of tens or hundreds of synchronized channels,
and very precise time correlation between channels.
Therefore, synchronization across multiple instruments is
primordial. Figure 1. M9703A AXIe 12-bit digitizer and M9536A
embedded AXIe controller in M9505A 5-slot chassis.
This application note describes the measurement and
analysis of cross channel skew in multichannel high
speed digitizers over short and long durations to sub-pi- It is possible to achieve synchronous sampling across a
cosecond scales. Two measurement techniques using the great number of acquisition channels, and measure the
Keysight Technologies, Inc. M9703A High Speed Digitizer sub-nanosecond time delays between the samples of
are described and compared, and the results presented. different channels3. Synchronous sampling means any two
channels A and B acquiring data in synchronous sampling
mode, where the ith voltage sample uAi is the measured
Introduction signal voltage on channel A at time ti, there is a cor-
responding sample uBj measured on channel B at time tj = ti
High speed digitizer systems operating at well above + AB. Here AB is the delay between the channels, which
100 MS/s are being used in a diverse range of applica- is the combination of the sample clock delay plus the
tions from single-pulse linear induction accelerators to difference in analog path lengths of B with respect to A.
amplitude and phase measurements on multiple antenna Synchronous sampling is maintained when AB is constant
radars. A growing number of such applications require over all i values. By comparison of reconstructed single
simultaneous measurement of high frequency signals frequency waveforms from acquisitions on channels A and
over many channels. In some of these applications, high B, we can eliminate the sampling clock delay such that AB
speed data acquisition is used to capture events in the represents only the difference in analog path lengths, or
time domain, where the time parameter is often used as a the cross channel skew.
measure of other parameters such as energy or distance. Depending on the end-user application, two channel delay
In frequency domain measurements, phase parameters of parameters are commonly used: cross channel skew and
various signals can be of utmost importance. phase delay. Cross channel skew is measured in seconds
Most high speed digitizers or oscilloscopes feature a between two channels, where as the cross channel phase
maximum of only a handful of channels. For applications delay between channels is the delay in radians (rad) or
requiring tens or hundreds of channels and very precise degrees (deg) of the period of the input signal. One can
time correlation between channels or accurate phase of simply be derived from the other using the relation:
continuous signals, it is necessary to synchronize the rad = AB * fs * 2 (1)
sampling clocks of multiple instruments. or
Keysight developed a solution using front panel dongles deg = AB * fs * 360 (2)
for the interconnection of clock ASIC components for
transfer of high speed ADC clocks and trigger lines across where fs is the frequency of the signal at the input chan-
neighboring modules in a CompactPCI