Stanford Research Systems SR844 User Manual page 31

Chapter 2
SR844 Basics
In This Chapter
What is a Lock-In Amplifier ?
Why Use a Lock-in Amplifier ?
What is Phase-Sensitive Detection ?
Units
RMS or Peak ?
Degrees or Radians ?
Volts or dBm ?
What About Signals at Other Frequencies ? 2-5
What About DC Offset and Drift ?
Where Does the Reference Come From ?
The Functional SR844
RF Signal Input Path
Input Impedance
200 MHz Low Pass Filter
20 dB Attenuator
20 kHz High Pass Filter
20 dB Gain
Reference Channel
Auto-Threshold Comparator
Phase Locked Loop and Divider Chain
20 MHz Reference/ Synthesizer
X and Y Reference Generator
IF Section
180 kHz Low Pass Filter
Gain Stages
Anti-Aliasing Filter
16-Bit ADC
Inside the DSP
Inputs
Demodulators
Phase Adjust
Offsets
Ratio
2-3
Time Constant Filters
2-3
R, θ, dBm Computation
2-3
Output Select
2-4
The Host Processor
2-4
2-4
Analog Outputs and Scaling
2-5
CH1 and CH2 Outputs
Output Scales
2-6
Output Offset and Expand
2-6
Display Scales
2-8
What is Dynamic Reserve ?
Wide and Close Reserves
2-9
2-9
Sources of Error
2-9
Spurious Responses
2-9
Square Wave Response
2-9
IF Sidebands
2-9
Coherent Pickup
2-10
Using the SR844 as a Double Lock-In
2-10
Noise Measurements
2-10
How Does a Lock-in Measure Noise ?
2-10
Noise Estimation
2-10
Intrinsic (Random) Noise Sources
2-11
Johnson Noise
2-11
Shot Noise
2-11
1/f Noise
2-11
Total Noise
2-11
External Noise Sources
2-12
Capacitive coupling
2-12
Inductive coupling
2-12
Resistive coupling or Ground Loops
2-12
Microphonics
2-13
SR844 RF Lock-In Amplifier
2-1
2-13
2-13
2-14
2-14
2-14
2-15
2-15
2-15
2-15
2-16
2-17
2-17
2-19
2-19
2-19
2-19
2-20
2-22
2-23
2-23
2-23
2-25
2-25
2-25
2-25
2-25
2-26
2-26
2-27
2-27
2-28