Stanford Research Systems SR844 User Manual page 74

3-16 CH1 Display and Output
Key features and parameters for the various displayed quantities are shown below.
Quantity
[Unit]
X [Volts]
R [Volts]
R [dBm]
Xnoise
[Volts]
AUX IN 1
[Volts]
Note 1
Note 2
Note 3
Offset
User entered offsets can be added to X and Y. These offsets are added before taking
ratios, output time-constant filtering, and computing R and
Offsets are useful for making relative measurements or to cancel the contribution from an
unwanted phase coherent signal. In analog lock-ins, offsets were generally used to remove
DC output errors from the mixer outputs. The SR844 demodulator is digital and has no
DC output errors, however, the SR844 does have coherent pickup at high frequencies,
which can be canceled using offsets.
Important points about offsets:
•
Xoffset and Yoffset are applied to X and Y before ratios, filtering and expands. R and
θ
are computed from the offset values of X and Y. Adding offsets to X or Y changes
the value of R and
SR844 RF Lock-In Amplifier
Display Bar
Range Graph
Range
± ±
110% f.s. f.s.
± ±
110% f.s. f.s.
± ±
220 dBm 200 dBm
± ±
110% f.s. f.s.
± ±
10 V 10 V
If ratio mode has been selected, the reciprocal of the appropriate input
(1.0V/AUX IN 1 or 1.0V/AUX IN 2) is computed, and both X and Y are
multiplied by this quantity. Since the value of R is computed after the ratio,
R is also scaled by the ratio. R[dBm] will show an offset, and Xnoise will be
scaled in the same proportion as X.
This shows the worst-case output update rate for the CH1 analog output.
The update rate is fastest at the high end of an octave band. See the IF
frequency display in the Reference Section for more details.
The X output updates 4 times slower when 18 and 24 dB/oct filtering is
used.
The digital display is always updated at a 2 Hz rate. The bar-graphs are
updated at 64 Hz. Display quantities may be read out via remote interface or
stored in the internal data buffers (see Chapter 4, Data Storage) at a
maximum rate of 512 Hz.
The noise is computed at 512 Hz for all time constants ≤ 30 ms. For longer
time constants the noise is updated 25.6 times per time constant.
θ
.
Ratio Offset Expand
±
Yes 110% f.s. Yes
±
Yes 110% f.s. Yes
±
Note 1 110% of Yes
200 dBm
Note 1 No Yes
No No No
θ
.
Max Output
Update
Period
µ
22 s, Note 2
µ
88 s, Note 2
µ
88 s, Note 2
1.953 ms,
Note 3
µ
88 s, Note 2