Stanford Research Systems SR844, User Manual

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2-26 SR844 Basics
SR844 RF Lock-In Amplifier
External Noise Sources
In addition to the intrinsic noise sources discussed in the previous section, there are a
variety of external noise sources within the laboratory.
Many noise sources are asynchronous, i.e. they are not related to the reference and do not
occur at the reference frequency or its harmonics. Examples include lighting fixtures,
motors, cooling units, radios and computer screens. These noise sources affect the
measurement by increasing the required dynamic reserve or time constant.
Some noise sources, however, are related to the reference and, if picked up in the signal
path, will add or subtract from the actual signal and cause errors in the measurement.
Typical sources of synchronous noise are ground loops between the experiment, detector
and lock-in, and electronic pick-up from the reference oscillator or experimental apparatus
and cables.
Many of these noise sources can be reduced with good laboratory practice and experiment
design. There are several ways in which noise sources are coupled into the signal path.
Capacitive coupling
An RF or AC voltage from a nearby piece of apparatus can couple to a detector via a
stray capacitance. Although C
STRAY
may be very small, the coupled noise may still exceed
a weak experimental signal. This is especially damaging if the coupled noise is
synchronous (i.e. at the reference frequency).
We can estimate the noise current caused by a stray capacitance by
I =
C
STRAY
×
(dV/dt) =
ω⋅
C
STRAY
⋅
V
NOISE
(2-22)
where
ω
/2
π
is the noise frequency, V
NOISE
is the noise amplitude, and C
STRAY
is the stray
capacitance. This type of coupling is especially damaging since it is proportional to
frequency and the SR844 operates at very high frequencies.
For example, if the noise source is a computer clock line,
ω
/2
π
might be 33 MHz and
V
NOISE
might be 5 V/2. C
STRAY
can be crudely estimated using a parallel plate equivalent
capacitor, perhaps 0.1 cm
2
at a distance of 10 cm, which yields C
STRAY
≅
10
–15
F. The
resulting noise current is 0.5
µ
A, or 25
µ
V across 50
Ω
.
If the noise source is at the reference frequency, then the problem is much worse. The
lock-in rejects noise at other frequencies, but pick-up at the reference signal appears as
signal !
Signal
Source
Noise
Source
Cstray
Detector