Page 115 - Photodetection and Measurement - Maximizing Performance in Optical Systems

System Noise and Synchronous Detection

108 Chapter Five

5.7 The Lock-in Amplifier
Commercial, general-purpose lock-in amplifiers provide the multiplication and
integration functions of the basic synchronous detector and usually also provide
the Walsh demodulator. Figure 5.17 shows a generic device. They are addition-
ally endowed with many powerful and sometimes useful functions. In particu-
lar they often include:

Current input front ends (similar to a transimpedance amplifier)
Voltage input front ends (very high impedance)
Differential inputs, for use in reducing ground loop problems
Wide-range gain controls, allowing measurements from nanovolts to volts
Input filters, low-pass, high-pass, bandpass, and 50/60Hz notch
Reference channels that accept a wide variety of waveforms (sinusoidal,
pulsed, etc. and lock in to the correct frequency)
f/2f reference generators
Wide-range and stable phase shifting (continuous and ±90°)
Wide-range postdetection filter time-constants
Different order postdemodulation filters (6, 12, 18dB/octave)
Two-phase (sine/cosine) detection.

The 2f reference generators are useful in situations where excitation of a system
at frequency f produces primarily effects at 2f. An example is a heating system,
where an AC current drive I at a frequency of f hertz provides power propor-
2
tional to I and a temperature variation with much of its energy at 2f hertz. You
would also use it for filament and gas discharge lamps, for example if you needed

Signal filters Display 1
Input stage Postdemodulation filters
Low-pass
Bandpass
Single-ended 500 Low-pass X
200 Flat
Differential 100 50/60Hz Notch
50 500
Float 20 +90° 200 X R
10 V 100
GND 5 mV Reference Generation Lowpass 50 Display 2
2 μV 20
A-Input 1 nV and Phase Shift X 10
f 5
2f Int. 35.8° 2
B-Input Inc. Dec. 1
Ext. 6dB/octave s
12dB/octave ms
Y q
Channel 1,2
Ref. input outputs
Figure 5.17 Generic lock-in amplifier, showing wide-range gain stage, reference generation, demodulation, and
filtration.
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