Stanford Research Systems SR844, User Manual

Page: 128 / 246

4-26 Data Transfer
SR844 RF Lock-In Amplifier
The values of X and Y are recorded at a single instant, as are the values of R[V]
and
θ
. Thus reading (X,Y) or (R,
θ
) yields a snapshot of the input signal. A
SNAP?
measurement of X, Y, R and
θ
may give R,
θ
measured at a time up to 84
µ
s before
the X,Y measurement.
R[dBm] and the Aux Inputs are computed with a maximum interval of 84
µ
s. A
SNAP?
measurement of X, Y, and AUX IN 1 will give AUX IN 1 measured at a
time up to 84
µ
s before the X,Y measurement.
The reference frequency is computed 6-12 times a second.
The
SNAP?
command is a query only command. It is used to record various
parameters simultaneously, not to transfer data quickly.
Important!
The returned values of X, Y, R and RdBm have Offset and Ratio applied, but not
Expand, just like
OUTP?i
. The values of the CH1 and CH2 display have Offset,
Ratio and Expand applied, just like
OUTR?ch
.
SPTS?
The
SPTS?
command queries the number of points currently stored in the buffer.
The same number of points are stored from each display. If the buffer is reset, then
0 is returned. Remember,
SPTS?
returns N where N is the number of points – the
points are numbered from 0 (oldest) to N–1 (most recent). The
SPTS?
command
can be sent at any time, even while storage is in progress. It is a query only
command.
TRCA? ch,j,k
The
TRCA?
command queries the points stored in the Display
ch
buffer. The
values are returned as ASCII floating point numbers in the units of the display.
Multiple points are separated by commas and the final point is followed by a
terminator. For example, the response for two points might be
-1.2345E-6,+0.1234E-6,
The parameter
ch
selects the display buffer (
1
or
2
) and is required. Points are
read from the buffer starting at bin
j
(
j
≥
0
). A total of k bins are read (
k
≥
1
). To
read a single point, set
k=1
. Both
j
and
k
are required. If
j+k
exceeds the
number of stored points (as returned by the
SPTS?
query), then an error occurs.
Important!
If data storage is set to Loop mode, be sure to pause data storage before reading
any data. This is because the points are indexed relative to the oldest point, which
is continually changing.
TRCB? ch,j,k
The
TRCB?
command queries the points stored in the Display
ch
buffer. The
values are returned as IEEE format binary floating point numbers (in the units of
the display). There are four bytes per point. Multiple points are not separated by
any delimiter. The bytes can be read directly into a floating point array (in most
programming languages).
The parameter
ch
selects the display buffer (
1
or
2
) and is required. Points are
read from the buffer starting at bin
j
(
j
≥
0
). A total of
k
bins are read (
k
≥
1
). To
read a single point, set
k=1
. Both
j
and
k
are required. If
j+k
exceeds the
number of stored points (as returned by the
SPTS?
query), then an error occurs.