Stanford Research Systems SR830 Manual page 96

Aliasing Effects
In any sampled data stream, it is possible to sample a high frequency signal such that it will appear to be a
much lower frequency. This is called aliasing.
For example, suppose the lock-in is detecting a signal near 1 Hz with a relatively short time constant. The X
output will have a DC component and a 2 Hz component (2xf). If the sample rate is 2 Hz, then the samples
may be taken as illustrated below.
The samples represent a sine wave much slower than 2 Hz that isn't actually present in the output! In this
case, a much higher sampling rate will solve the problem.
Aliasing occurs whenever the output signal being sampled contains signals at frequencies greater
than 1/2 the sample rate. The effect is most noticeable when trying to sample an output frequency at an inte-
ger multiple of the sample rate (as above). The above aliasing problem will be the same for a 1 kHz output
(500 times the sample rate) as for the 2 Hz output.
Generally, the highest possible sample rate should be used given the desired storage time. The lock-in time
constant and filter slope should be chosen to attenuate signals at frequencies higher than 1/2 the sample rate
as much as possible.
SRAT (?) {i}
SEND (?) {i}
TRIG
TSTR (?) {i}
STRT
1 second
The SRAT command sets or queries the data sample rate. The parame-
ter i selects the sample rate listed below.
i
0
1
2
3
4
5
6
The SEND command sets or queries the end of buffer mode. The param-
eter i selects 1 Shot (i=0) or Loop (i=1). If Loop mode is used, make sure
to pause data storage before reading the data to avoid confusion about
which point is the most recent.
The TRIG command is the software trigger command. This command
has the same effect as a trigger at the rear panel trigger input.
The TSTR command sets or queries the trigger start mode. The parame-
ter i=1 selects trigger starts the scan and i=0 turns the trigger start fea-
ture off.
The STRT command starts or resumes data storage. STRT is ignored if
storage is already in progress.
5-13
Remote Programming
quantity
62.5 mHz
125 mHz
250 mHz
500 mHz
1 Hz
2 Hz
4 Hz
i quantity
7 8 Hz
8 16 Hz
9 32 Hz
10 64 Hz
11 128 Hz
12 256 Hz
13 512 Hz
14 Trigger