Inside The Dsp; Inputs; Demodulators; Phase Adjust - Stanford Research Systems SR844 User Manual

2-12 SR844 Basics

Inside the DSP

Much of the signal processing in the SR844 occurs inside the Digital Signal Processor
(DSP).
IF Demodulator
X-IF
IF
Chop
Y-IF
IF Demodulator
Aux In 1
A to D Converter
Aux In 2

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.
SR844 RF Lock-In Amplifier

Inside the DSP

÷
X offset
Y offset
Phase
÷
Shift
6, 12, 18, 24
dB/oct Filter
Ratio
and Gain
R offset
Compute
R and θ
Expands
X
R
θ
Y