Stanford Research Systems SR844, User Manual

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1-14 Outputs, Offsets and Expands
SR844 RF Lock-In Amplifier
were generally used to remove DC output errors
from the lock-in itself. The SR844 demodulator is
digital and has no DC output errors, however, it
does have some coherent pickup at high
frequencies, which can be canceled using offsets.
Important!
Xoffset and Yoffset are applied to the X and Y
demodulator outputs directly. R and
θ
are computed
from the offset values of X and Y. Offsetting X or
Y changes the measurement of R and
θ
.
In addition, changing the Reference Phase will
modify the values of Xoffset and Yoffset. Think of
(Xoffset, Yoffset) as a signal vector relative to the
Reference (internal or external) which cancels an
actual signal at the input. This cancellation is
preserved even when the detection phase (Reference
Phase) is changed. This is done by circularly
rotating the
values of Xoffset and Yoffset by minus
the Reference Phase. This preserves the phase
relationship between (Xoffset, Yoffset) and the
signal input.
Since the vector (Xoffset, Yoffset) is used to cancel
a real signal at the input, Xoffset and Yoffset are
always turned on and off together. Turning either X
or Y offset on (or off) turns on (or off) both offsets.
Auto offsetting either X or Y performs auto offset
on both X and Y. These statements are true even if
only one of the quantities X or Y is currently being
displayed.
6 Press Phase to display the Reference Phase
in the Reference Display.
Since auto offset has set (Xoffset, Yoffset) to cancel
the signal input, changing the Reference Phase does
not affect the X and Y readings.
Press the
+90
°
key.
X and Y remain zero even as the phase is changed.
This allows phase coherent signals at the input to be
completely canceled. For example, to cancel
coherent pickup, turn the experimental signal off
while leaving all of the signal cabling in place,
perform auto offset X (or Y) and then turn on the
experimental signal and proceed normally. The
effects of the coherent pickup are removed
at the
input. The amplitude and phase of the experimental
signal are now measured normally.
Press the
Zero
key to return the phase to zero.