Stanford Research Systems SR844 User Manual page 76

3-18 CH1 Display and Output
Important!
•
Except for R[dBm], 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 Ratio key selects the ratio mode. The ratio mode may be Off, divide by AUX IN 1
or AUX IN 2. The AUX IN 1 and AUX IN 2 indicators above the Ratio key show the
ratio mode if it is on. Both indicators are off when ratio mode is Off. The instrument has a
single ratio mode, which applies to both X and Y. The CH2 AUX IN 1 and AUX IN 2
indicators follow the CH1 indicators.
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
ratio shows up as a dBm offset in R[dBm]. Xnoise is computed from the ratioed X. For
example, if the ratio mode is AUX IN 2 and the AUX IN 2 input is a steady 2 volts, X and
R[V] will be ½ their non-ratioed values, R[dBm] will be down by 6 dB and Xnoise will
also be down a factor of 2. Ratio is not applied to AUX IN 1.
Note that the effects of ratio mode on Xnoise may be several. A steady Aux Input will
linearly scale the Xnoise 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 Xnoise 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 Xnoise in ratio mode may be greater than the
non-ratioed value.
OVERFLOW The OVERFLOW indicator shows that the ratio Aux Input exceeds the input range
±
( 10.5 V). The ratioed outputs are no longer correct in this case.
UNDER-
FLOW
SR844 RF Lock-In Amplifier
θ
are computed from ratioed X and Y. Thus, R is ratioed the same as X and Y. The