Block Diagram - Stanford Research Systems SR865 Operation Manual

Chapter 2
Thus, in the previous example with a 2 V pk–pk square wave input, the SR865 would
detect the first sine component, 1.273 sin(ωt). The measured and displayed magnitude
would be 0.90 V (rms), e.g. 1.273/√2.
Degrees or Radians?
In this discussion, frequencies have been referred to as f (Hz) and ω (2πf radians/sec).
This is because people measure frequencies in cycles per second and math works best in
radians. For purposes of measurement, frequencies as measured in a lock-in amplifier are
in Hz. The equations used to explain the actual calculations are sometimes written using
ω to simplify the expressions.
Phase is always reported in degrees. Once again, this is by custom. Equations written as
sin(ωt + θ) are written as if θ is in radians mostly for simplicity. Lock-in amplifiers
always manipulate and measure phase in degrees.

Block diagram

A simplified block diagram of the SR865's lock-in circuit is shown below and explained
in the following sections.
Reference input
Sine/TTL
Ref in
1MΩ
50Ω/1MΩ
50Ω
Signal input
AC/DC couple
Differential
amplifier
A input
B input
A/A–B
Gnd/
Float
10Ω 10kΩ
Range
100Ω
I input
1kΩ
Virtual
ground
Phase
locked Internal
loop oscillator
ADC
θ
ref
phase
shift
X PSD
90°
Input
phase
Range
shift
ADC
Y PSD
x0.1,
x0.3,
...x100
Signal
monitor
Timebase input
Timebase
in
PLL
10 MHz
crystal Clock
generator
Digital signal processor
CPU
Low pass filters
RC
BlazeX
Filter
RC FIR
Filter Filter
IIR Sync
X
Filter Filter
R, θ
RC FIR
Calc
Filter Filter
IIR Sync
Y
Filter Filter
SR865 DSP Lock-in Amplifier
Basics
Timebase
10 MHz
out
System
clock
Annenuator
+
Sine
DAC
Sync –
BlazeX
DAC
out
Gain/
Offset/
Expand
DAC
X out
DAC Ch 1 out
R
θ
DAC Ch 2 out
DAC Y out
Sensitivity
41
Sine
out