Stanford Research Systems SR844, User Manual

Page: 82 / 246

3-24 CH2 Display and Output
SR844 RF Lock-In Amplifier
Modify
When
Modify
is pressed, the Yoffset appears on the Reference Display (above the
knob), even if the Offset is Off. This key has no effect if the display is not Y. Press
Phase
,
Freq
or
AuxOut
to return the Reference Display to its previous state.
The Offset is displayed as a percentage of Full Scale. The knob may be used to modify
the offset. Turn the Offset on to apply the displayed offset.
Important!
•
The offset is specified as a percentage of full scale sensitivity. Changing the
sensitivity requires the offsets be changed to offset the same input signal.
•
Changing the Reference Phase will modify the values of Xoffset and Yoffset.
(Xoffset, Yoffset) is 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.
Ratio Ratio mode divides both X and Y by the input voltage AUX IN 1 or AUX IN 2. The
ratio input is normalized to 1.000 Volt, so that ratioing by an Aux Input that is a steady
1.000 V is exactly the same as having the ratio mode Off. The useful range of the Aux
Inputs, when in ratio mode, is from about 0.1 Volt to 10 Volts. Both positive and
negative voltages are permitted.
The SR844 has a single ratio mode that is common to both channels. The control for the
ratio mode is the
Ratio key in the Channel 1 Display section. The instrument’s ratio
mode will be applied to the currently displayed Channel 2 quantity as shown by the
AUX IN 1 and AUX IN 2 indicators. As shown in the table above, ratioing may be
applied to Y, Ynoise [Volts] and Ynoise [dBm], but may not be applied to
θ
and
AUX IN 2.
If the instrument is in Ratio Mode, the non-ratioed quantities are not available.
When the ratio mode is on, the ratio is performed after X and Y offsets are applied and
before the output time-constant filters. This allows the offsets to cancel a signal at the
input before applying the ratio.
R and
θ
are computed from ratioed X and Y. If the ratioing input is negative, the signs of
X and Y will both be changed, and the phase
θ
will differ by 180
°
from the non-ratioed
value. Ynoise is computed from the ratioed Y. For example, if the ratio mode is
AUX IN 2
and the AUX IN 2 input is a steady 2 volts, Y will be ½ its non-ratioed value,
θ
will be unchanged and Ynoise will also be down a factor of 2.
Note that the effects of ratio mode on Ynoise may be several. A steady Aux Input will
linearly scale the Ynoise as just mentioned. If the variations of the Aux Input are
positively correlated with signal variations, as might be expected in situations where the
input signal is dependent on the Aux Input, then Ynoise in ratio mode may be much lower
than the non-ratioed value. If the variations of the Aux Input are uncorrelated with signal
variations, or negatively correlated, then the Ynoise in ratio mode may be greater than the
non-ratioed value.