Stanford Research Systems SR780, Руководство по эксплуатации и программированию

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2-30 Capture Buffer
SR780 Network Signal Analyzer
Capture Buffer
Input Capture samples the analog inputs at a selected sample rate and stores the samples
in memory. Storage is continuous and real-time without interruption in the data stream.
Triggering only starts the capture, it does not synchronize individual time records.
Captured data can be used as the measurement input for the FFT and Octave
Measurement Groups (capture playback). The capture buffer can neither be filled nor
measured from in the Swept Sine Measurement Group.
Use the [Capture] <Memory Allocation> menu to allocate memory between the capture
buffer, waterfall storage and the arbitrary source waveform. Capture memory must be
allocated before the capture buffer may be used.
Input Sampling
The input data is always digitized at the maximum sample rate. The maximum sampling
rate is 262.1 kHz when the FFT Base Frequency is 102.4 kHz and 256 kHz when the FFT
Base Frequency is 100.0 kHz OR the Measurement Group is Octave. When capturing
data in the FFT Measurement Group, it is important to choose the 100.0 kHz Base
Frequency if the captured data will be played back in the Octave Group.
After the inputs are digitized, they may be filtered and downsampled before being stored
in the capture buffer. This allows longer capture times at lower sampling rates. The
capture data represents a frequency span from 0 Hz to the sampling rate/2.56. All
captured data is baseband. Capturing at less than the maximum sampling rate restricts the
playback measurement span to sampling rate/2.56.
The Capture Length is selected in 2 kPoint (2048) increments limited by the user memory
allocation. A single input or both inputs may be captured. The maximum capture length
for a single input is twice the length for both inputs at a given memory allocation.
Capture Fill
To start capture, press the [Start Capture] key. If the Trigger Mode is Free Run, capture
starts immediately. Otherwise, the selected Trigger will be in effect and the next trigger
event will start capture. The Capture Progress indicator shows how much of the desired
capture length has been filled. If the Capture Mode is 1-Shot, capture stops when the
buffer is full. Press [Stop Capture] to halt capture before the buffer is full.
If the Capture Mode is Continuous, once capture is started, it continues indefinitely and
fills the capture buffer in a circular fashion. In this case, press [Stop Capture] to halt
capture with the most recently acquired data stored in the buffer.
While capture is in progress, the displays do not update unless the measurement is a
Capture buffer. Use Zoom and Pan to inspect a portion of the buffer. The capture buffer
display can automatically pan as the capture progresses through the buffer. During
capture, if the capture buffer accumulates points faster than can be displayed, some points
are not displayed. This speeds up the display update so that it keeps up with the real time
capture data but allows visual aliasing to occur. Once capture is complete, the display is
redrawn showing the envelope of all points, eliminating any visual aliasing effects.