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4-13
Screen Display
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-
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,