SRS Labs SR850 manual Trace SCANS, Sweeps & Aliasing

TRACE SCANS, SWEEPS & ALIASING

Trace and Scan parameters are selected in the TRACE/SCAN menu.

Trace Storage

Having defined up to four data traces for an exper- iment, the issue of data storage needs to be addressed. Only traces that are stored may be displayed in strip chart form. If a graph or record of a trace over time is desired, then that trace's data must be stored. The SR850 can record up to 64000 data points in memory. The data buffer can store 64000 points of a single trace, 32000 points of two traces, or 16000 points of all four traces. When defining the traces, the Store or Do Not Store option needs to be decided. The default is all four traces stored.

Data Points and Bins

Data points stored in a trace are sometimes referred to by their bin position within the trace buffer. The oldest data point is bin0, the next point is bin1, etc. A trace with N points numbers them from 0 to N-1.

Sample Rate

The Sample Rate can be varied from 512 Hz down to 62.5 mHz (1 point every 16 sec). The sample rate sets how often points are added to the stor- age buffers. All stored traces are sampled at the same rate (and at the same times).

In addition to the internal sample rates, samples can be triggered by an external TTL trigger. This mode is selected by increasing the sample rate past 512 Hz. In this mode, a sample is recorded within 2 ms of a rising edge trigger on the rear panel Trigger input. Triggers which occur faster than 512 Hz are ignored. When viewing an exter- nally triggered data trace on a chart graph, set the cursor readout to Bin (in the CURSOR SETUP menu). This displays the horizontal position of the cursor as bin or data point number rather than time (for scaling purposes, the time scale of the graph is based upon a 1 Hz sample rate - bins and seconds are equivalent).

Scan Length

The Scan Length is the time duration of a single scan expressed in seconds. The maximum scan length is determined by the number of stored traces (maximum storage buffer length) and the sample rate. When storing a single trace, the max-

Screen Display

imum scan length is 125 seconds at 512 Hz or 12 days at 62.5 mHz (64000 points). Changing the sample rate will only change the scan length if the maximum number of data points is already being used. Otherwise, the number of data points in the scan is changed to keep the scan length constant. The number of points in the buffer can vary from 1 to a maximum of 16000, 32000, or 64000 depend- ing upon the number of traces being stored.

There is only one Scan Length, i.e. the number of points stored will be the same for all traces being stored.

Sweep Time

The scan length is the sweep time for frequency sweeps and Aux Output sweeps. Swept parame- ters are synchronized with the data acquisition. For example, if the internal reference is pro- grammed to sweep from 1 kHz to 2 kHz, the sweep will take a scan length to finish. The fre- quency will change once per stored point. Thus, if the sample rate is 1 Hz and the scan length is 100 seconds, the frequency will change 100 times and move from 1 kHz to 2 kHz in 100 seconds. At each sample, the trace data is stored before the swept parameter is changed. The next data point is taken after one sample interval to allow the out- puts to settle as long as possible.

End of Scan

When the scan is complete, data storage can stop or continue.

The first case is called 1 Shot (data points are stored for a single Scan Length). At the end of the scan, data acquisition stops and swept parameters are held at their final stop values.

The second case is called Loop. In this case, a new scan is started at the end of each scan. Scans repeat indefinitely until halted by the user. The data buffer will store as many points as possi- ble (16000, 32000 or 64000 depending upon the number of stored traces). The buffer will start filling at the start and will hold as many scans as will fit. The buffer always holds at least one complete scan. If the scan is short, then the buffer will hold multiple scans of data. When the buffer end is reached, the buffer starts filling at the beginning again. The oldest data will be overwritten and lost. This looping continues indefinitely. In this mode,

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