Stanford Research Systems SR844, User Manual

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SR844 Basics 2-15
SR844 RF Lock-In Amplifier
Analog Outputs and Scaling
CH1 and CH2 Outputs
The SR844 has two analog outputs, CH1 and CH2, on the front panel. These outputs can
be configured to output voltages proportional to the CH1 and CH2 displays or X and Y.
X and Y are the traditional outputs of an analog lock-in. The output voltage is
proportional to the X and Y components of the signal with low-pass output filtering,
offset, ratio and expand. In this case, a different quantity (R or
θ
for example) may be
shown on the displays.
If the outputs are set to
DISPLAY
, the output voltage is proportional to the quantity
shown on the corresponding display. The CH1 display can show X, R, R[dBm], Xnoise or
AUX IN 1. The CH2 display can show Y,
θ
, Ynoise, Ynoise[dBm] or AUX IN 2. Offset,
ratio and expand may be applied to many of these quantities.
Output Scales
The sensitivity of the lock-in is the rms amplitude of an input sine (at the reference
frequency) which results in a full scale DC output. Full scale means 10 Vdc at the CH1
or CH2 analog output. The overall gain (input to output) of the amplifier is then
10 V/sensitivity. This gain is distributed between RF gain (before the mixer), IF gain (after
the mixer) and DC gain (in the DSP). Changing the dynamic reserve at a given sensitivity
changes the gain distribution while keeping the overall gain constant.
The SR844 considers 10 Vdc to be full scale for any output proportional to simply X, Y
or R. Values of X, Y and R are always rms values. Noise is also measured in rms Volts
and Xnoise and Ynoise are scaled the same as X and Y.
Phase is a quantity which ranges from -180
°
to +180
°
regardless of the sensitivity. When
CH2 outputs a voltage proportional to
θ
, the output scale is 18
°
/V or 180
°
= 10 V.
Outputs proportional to quantities measured in dBm (R[dBm] and Ynoise[dBm]) have an
output scale which is independent of sensitivity. The output is
±
10 V =
±
200 dBm or
20 dBm/V.
Output Offset and Expand
The SR844 has the ability to offset the X, Y and R outputs. This is useful when
measuring deviations in the signal around some nominal value. The offset can be set so
that the output is offset to zero. Further changes in the output can then be read directly
from the display or output voltages. The offset is specified as percentage of full scale and
the percentage does
not change when the sensitivity is changed. Offsets may be set up to
±
110% of full scale. For dBm displays, the offset range is
±
110% of 200 dBm or
±
220 dB.
The measured phase may be offset by adjusting the Reference phase.
The X, Y, R and
θ
outputs may also be expanded. This simply takes the output (minus its
offset) and multiplies by an expansion factor. Thus, a signal which is only 10% of full
scale can be expanded to provide 10 V of output rather than only 1 V. The normal use for