Stanford Research Systems SR844, User Manual

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2-12 SR844 Basics
SR844 RF Lock-In Amplifier
Inside the DSP
Much of the signal processing in the SR844 occurs inside the Digital Signal Processor
(DSP).
Inputs
The DSP receives the digitized X-IF and Y-IF signals from the IF section. In addition
there is an IF chop signal that allows the DSP to demodulate the X-IF and Y-IF signals at
the correct IF frequency.
Demodulators
The two data streams are multiplied by a digital IF chop waveform which converts the
X-IF and Y-IF signals back to DC. The advantage of demodulating inside the DSP is to
eliminate the DC output errors of analog mixers.
Phase Adjust
The two demodulated signals are subject to a matrix rotation that selects the detection
phase (reference phase set by the user) and compensates for phase delays internal to the
instrument. These phase-rotated signals are hereafter referred to as X and Y.
Y-IF
IF
Chop
Compute
R and
θ
÷
÷
X offset
Y offset
Ratio
6, 12, 18, 24
dB/oct Filter
and Gain
R offset
Phase
Shift
IF Demodulator
IF Demodulator
X-IF
X
Y
R
θ
Aux In 1
Aux In 2
A to D Converter
Inside the DSP
Expands