Stanford Research Systems SR844, User Manual

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CH2 Display and Output 3-23
SR844 RF Lock-In Amplifier
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, it 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
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 offset on turns on
both offsets. Auto offsetting either X or Y performs auto offset on both quantities.
These statements are true even if only one of the quantities X or Y is currently being
displayed.
•
Use the Reference Phase controls to adjust the measured and displayed phase of the
input signal. This is discussed in the section on Reference Phase later in this chapter.
Note that reference phase adjustment is
not equivalent to having an offset adjustment
on the displayed phase. This will change the measured values of X and Y.
On/Off This key turns the Offset On or Off for the current CH2 display quantity. As indicated in
the table above, Y is the only CH2 display quantity which may be offset. This key has
no effect if the currently displayed quantity is not Y. When Yoffset is turned On, the
Yoffset value last used is applied.
Turning Yoffset on and off also turns on and off Xoffset. The
XYOffs
indicator in the
display indicates that the displayed quantity is affected by X and Y offsets.
Xoffset and Yoffset are applied before R and
θ
are calculated. Thus, the
XYOffs
indicator will be on if the display is showing
θ
and XYoffsets are on.
The Offsets can be set by pressing the Modify key and then using the knob, or using the
Auto key. The Offsets can also be set from the remote interface.
Auto This key sets the Yoffset equal to the negative of its current displayed value, so that the
display, with Yoffset applied, is equal to zero. The Yoffset is turned On if it is not
already On. This key has no effect for CH2 displays other than Y.
Important!
If the display is Y,
Auto
performs Auto Offset for both X and Y and turns on both X and
Y offsets. This is true even if the CH1 display is not displaying X at the time.