What Is A Lock-In Amplifier; Why Use A Lock-In Amplifier; What Is Phase-Sensitive Detection - Stanford Research Systems SR844 User Manual

What is a Lock-In Amplifier ?

Lock-In amplifiers are used to detect and measure very small AC signals — all the way
down to a few nanovolts. Accurate measurements may be made even when the small
signal is obscured by noise sources many thousands of times larger.
Lock-in amplifiers use a technique known as phase sensitive detection to single out the
component of the signal at a specific reference frequency and phase. Noise signals at
frequencies other than the reference frequency are rejected and do not affect the
measurement.

Why Use a Lock-in Amplifier ?

Let's consider an example. Suppose the signal is a 1
some amplification is required. A good low noise amplifier will have about 3 nV/
input noise. If the amplifier bandwidth is 200 MHz and the gain is 1000, then we can
expect our output to be 1 mV of signal and 43 mV of broadband noise ( 3 nV/
√
200 MHz
single out the frequency of interest.
Now try following the amplifier with a phase sensitive detector (PSD). The PSD can
detect the signal at 10 MHz with a bandwidth as narrow as 0.01 Hz (or even narrower if
you have the patience to wait for several time constants). Using a 1 Hz detection
bandwidth, the output noise will be only 3
considerable less than the amplified signal of 1 mV. The signal to noise ratio is now
300 and accurate measurement is possible.

What is Phase-Sensitive Detection ?

Lock-in measurements require a frequency reference. Typically an experiment is excited
at a fixed frequency (from an oscillator or function generator) and the lock-in amplifier
detects the response from the experiment at the reference frequency. Suppose the
reference signal is a square wave at frequency
function generator. If the sine output from the function generator is used to excite the
experiment, the response might be V
The lock-in amplifier multiplies the signal by the reference V
(Note: The SR844 uses a more complicated reference signal for reasons discussed below,
but the principle is the same.) The mixer generates the product of its two inputs as its
output V
M1
V
M1
Since the two inputs to the mixer are at exactly the same frequency, the first term in the
mixer output is at DC. The second term is at a frequency 2
frequency and can be readily removed using a low pass filter. After filtering
V
M1+FILT
×
1000 ). We won't have much luck measuring the output signal unless we
.
=
V V sin(ω t+θ )sin(ω
I R R I
=
½ V V cos(θ –θ ) + ½ V
I R R I
=
½ V V cos(θ –θ )
I R R I
µ
V sinewave at 10 MHz. Clearly
µ √ × √
V ( 3 nV/ Hz
ω
. This might be the sync output from a
R
ω θ
sin( t+ ) where V is the signal amplitude.
I R I I
t+θ )
R R
V sin(2ω t+θ +θ
I R R R I
ω
SR844 RF Lock-In Amplifier
SR844 Basics
√
Hz of
√ ×
Hz
×
1 Hz 1000 ) which is
θ
sin(ω t+ ) using a mixer.
R R R
(2–1)
(2–2)
)
, which is at a high
R
(2-3)
2-3