Outputs And Scales - Stanford Research Systems SR865A Operation Manual

46 Basics
In the SR865A, synchronous filters are available at detection frequencies up to 4.8 kHz.
At higher frequencies, synchronous filters are not required because 2f is easily removed
with the other lowpass filters. The synchronous filter is applied after the other lowpass
filters have removed the noise components from the PSD output leaving, just the
synchronous signals for the synchronous filter to remove. This combination of filters
removes all multiples of the reference frequency and provides overall noise attenuation as
well.
Synchronous Filters and Sensitivity
It is important to note that the synchronous filter requires the Sensitivity to be set
appropriately. This is because the synchronous filter is an integer filter and requires the
floating point output to be converted to integer values. This conversion is based upon the
Sensitivity. The output of the synchronous filter is then converted back to floating point.
Choose a Sensitivity as if you will be using the analog X and Y outputs. This will
typically result in the optimum scale. If the Sync filters are overloaded, the Sync error
indicators on the display will light.

Outputs and Scales

The SR865A has X and Y outputs on the rear panel and Channel 1 and 2 (CH1 and CH2)
outputs on the front panel.
X and Y Rear Panel Outputs
The X and Y rear panel outputs are the outputs from the two phase sensitive detectors
with low pass filtering, offset and expand. These outputs are the traditional outputs of an
analog lock-in. The X and Y outputs update at 2.222 MHz.
CH1 and CH2 Front Panel Outputs
The two front panel outputs can be configured to output voltages proportional to X or R
and Y or θ. These outputs update at 2.222 MHz.
If the outputs are set to X or Y, these outputs duplicate the rear panel outputs.
X, Y, R and θ Output Scales
The Sensitivity setting of the SR865A determines what input signal corresponds to 10 V
full scale output for X, Y and R. For example, a Sensitivity of 100 mV means that a
signal at f
maximum of 10 V, and a minimum of –10 V, depending upon the phase of the signal.
The Sensitivity also sets the scale for the displayed bar graphs and numeric readouts.
If the signal input exceeds the sensitivity, the outputs will overload. The actual
measurement is typically unaffected since it is done in floating point and has no overload.
The data displayed in the chart will still be accurate, provided the Sync filter is not
overloaded.
Lock-in amplifiers are designed to measure the RMS value of the ac input signal. All
values of X, Y and R outputs and displays are RMS values.
SR865A DSP Lock-in Amplifier
of 100 mVrms will result in a 10 V output of R. X and Y will reach a
ref
Chapter 2