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38
Basics
The SR865A generates its own sine wave at frequency f
below. The lock-in reference is sin(ω
The SR865A amplifies the signal and then multiplies it by the lock-in reference using a
phase-sensitive detector (PSD) or multiplier. The output of the PSD is simply the product
of two sine waves.
The PSD output is two ac signals, one at the difference frequency (ω
at the sum frequency (ω
If the PSD output is passed through a low pass filter, the ac signals are removed. What
will be left? In the general case, nothing. However, if ω
frequency component will be a dc signal. In this case, the filtered PSD output will be
This is a very nice signal — it is a dc signal proportional to the signal amplitude.
Narrow band detection
Now suppose the input is made up of signal plus noise. The PSD and low pass filter only
detect signals whose frequencies are very close to the lock-in reference frequency. Noise
signals at frequencies far from the reference are attenuated at the PSD output by the low
pass filter (neither ω
close to the reference frequency will result in very low frequency ac outputs from the
PSD (|ω
and roll-off. A narrower bandwidth will remove noise sources very close to the reference
frequency, a wider bandwidth allows these signals to pass. The low pass filter bandwidth
determines the bandwidth of detection. Only the signal at the reference frequency will
SR865A DSP Lock-in Amplifier
sin(ω
V
=
V
psd
sig
r
=
1/2 V
cos([ω
sig
1/2 V
cos([ω
sig
+ ω
).
r
L
cos(θ
V
=
1/2 V
psd
sig
− ω
noise
ref
− ω
| is small). Their attenuation depends upon the low pass filter bandwidth
noise
ref
t + θ
) where ω
L
ref
t + θ
) sin(ω
t + θ
)
sig
L
ref
− ω
]t + θ
− θ
r
L
sig
ref
+ ω
]t + θ
+ θ
r
L
sig
ref
− θ
)
sig
ref
nor ω
+ ω
are close to dc). Noise at frequencies very
noise
ref
, shown as the lock-in reference
L
= 2πf
.
L
L
) −
)
− ω
) and the other
r
L
equals ω
, the difference
r
L
Chapter 2
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