Lambda SR1, Руководство по эксплуатации, Страница: 183 / 317

SR1 Operation 183
© 2014 Stanford Research Systems
2.4.11 Digitizer
SR1's optional Digitizer is a sophisticated tool for the analysis of digital audio carrier signals. The
Digitizer is designed to complement the real-time measurement capabilities of the
Jitter Analyzer with
off-line analysis capable of revealing additional signal details. The Digitizer is designed to work with the
Digitizer Display to produce full-color eye diagrams, histograms of carrier amplitude, jitter amplitude,
pulse width and rate, and spectra of both the carrier and the computed jitter signal.
The heart of SR1's digitizer is an 80 MHz 8-bit transient digitizer with an effective analog bandwidth of
approximately 20 MHz. The digitizer can store up to 2 Msamples of data in each record. A flexible
trigger generator allows synchronization of the digitizer record with a variety of points on the carrier
signal as well as external events. After acquiring the record the digitizer analyzes the zero-crossings of
the digitized signal, reconstructs the original clock, and then calculates the jitter of the signal relative to
the reconstructed clock.
Digitizer Input Selection
Selection of the input signal for the digitizer is done on the Input section of the Digital I/O panel. Select
either the BNC or XLR connectors, optical connector, or the digital audio output monitor. For BNC and
XLR connector C1 or C2 must also be specified. Checking "Term" applies the appropriate termination for
the selected connector which can substantially improve the quality of the digitized signal.
Digitizer Acquisition
The size of the digitizer record (specified in 80MHz samples) is set with the "Record Length" control.
Values from 8 kSamples to 2 MSamples can be selected. To set a scale for the record length recall that
the frame rate for a digital audio signal is equivalent to the embedded audio sample rate. So for a 48 kHz
sampled digital audio signal each frame takes about 20.8 μs or about 1.6 kSamples. Thus there are
about 4.9 48 kHz frames in an 8 kSample digitizer record and 1258 48 kHz frames in the longest 2M
sample record
Each frame contains two sub-frames of 32 bits for a total of 64 bits and each bit cell contains 2 "unit
intervals" (UIs) for a total of 128 UIs in each frame. Therefore, at a 48 kHz sample rate each UI contains
about 13 digitizer samples. This number grows proportionally larger (26 samples/UI at Fs = 24 kHz) at
lower sample rates and smaller (3.25 samples/UI at Fs = 192 kHz) at higher sample rates. Choosing
longer records increases the acquisition and analysis time for each record but provides more frequency
resolution in spectral measurements.
Digitizer Input Gain
The digitizer has a selectable front-end gain of x1, x2, and x4. The magnitude of the input relative to the
digitizer's full scale is shown by the colored bars at the bottom of the Acquisition box. Blue indicates the
signal is below 1/2 of full scale. Green indicates the signal level is optimally adjusted, while red indicates
the digitizer is overloaded. Usually setting the input gain to "Auto" will yield the best results.
Occasionally, with signals that have some transient component, it may be necessary to manually set