Stanford Research Systems SR844, User Manual

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Outputs, Offsets and Expands 1-13
SR844 RF Lock-In Amplifier
The CH1 display may not read exactly 0.54 V for a
number of reasons:
•
The Ref Out amplitude is only a nominal
specification.
•
The basic accuracy of the unit is
±
0.25 dB
(3%).
•
Since the phase may not be exactly 0
°
, X=Rcos
θ
is slightly less than R (amplitude).
4 Connect the CH1 OUTPUT to the DVM. Set
the DVM to read DC Volts, on the 20 Vdc
scale.
The CH1 output is preset to X as indicated by the
X
LED above the CH1 OUTPUT. The output voltage
is given by the formula:
(X/Sensitivity – Xoffset)
×
Expand
×
10V
In this case X
≅
0.54 Vrms, Sensitivity = 1.0 Vrms,
Xoffset = 0, Expand = 1 (no output expand), so we
expect the DC output voltage to be about 5.4 V.
The DVM should read about this value (depending
upon the exact X reading).
5 Press CH1 Offset Auto (this key is two keys
left of the CH1 OUTPUT connector).
X, Y and R may all be offset and expanded
independently.
Since Channel 1 is displaying X, the Offset ( On/Off ,
Auto
and
Modify
) and
Expand
keys below the
Channel 1 Display set the offset and expand for X.
The
display selection determines which quantity the
Offset and Expand keys operate on.
Offset Auto
automatically adjusts the offset of the
displayed quantity to make the result zero. In this
case, X is offset to zero. (Y is also offset to zero.
See below for an explanation of X and Y offsets.)
The offset affects both the displayed value of X and
the CH1 analog output (X). Thus, after the auto
offset function is performed, both the displayed
value of X and the DVM should show readings very
close to zero.
The XYOffs indicator in the Channel 1 Display has
turned on to indicate that the display quantity is
affected by XY offsets.
Offsets are useful for making relative measurements
or to cancel the contribution from an unwanted
phase coherent signal. In analog lock-ins, offsets