SR844 Basics
2-23
SR844 RF Lock-In Amplifier
Noise Measurements
Lock-in amplifiers can be used to measure noise. Noise measurements are usually used to
characterize components and detectors.
The SR844 measures input signal noise
at
the reference frequency. Many noise sources have
a frequency dependence which the lock-in can measure.
How Does a Lock-in Measure Noise ?
Remember that the lock-in detects signals close to the reference frequency. How close? Input
signals within the detection bandwidth set by the time constant and filter rolloff appear at the
output at a frequency f=f
SIG
–f
REF
. Input noise near the reference frequency appears as noise at
the output with a bandwidth of DC to the detection bandwidth.
For Gaussian noise, the equivalent noise bandwidth (ENBW) of a low-pass filter is the
bandwidth of a perfect rectangular filter which passes the same amount of noise as the real
filter. The ENBW is determined by the time constant and slope as shown below.
Slope [dB/octave]
ENBW for Time Constant T
6
1/(4T)
12
1/(8T)
18
3/(32T)
24
5/(64T)
Noise Estimation
The noise is simply the standard deviation (root of the mean of the squared deviations) of the
measured X or Y. This formula, while mathematically exact, is not suited to providing a real-
time output proportional to the measured noise. Therefore the SR844 uses a simplified
algorithm to estimate the X or Y noise.
The moving average of X is computed over some past history, and subtracted from the
present value X to get the deviation. The Mean Average Deviation (MAD) is computed as a
moving average of the absolute value of the deviations. For Gaussian noise, the MAD is
related to the RMS deviation by a constant factor. The MAD is scaled by this factor and by
the ENBW to obtain noise in units of Volts/
√
Hz. X and Y noise are displayed in units of
Volts/
√
Hz. The average reading is independent of the time constant and slope but the
variations or noisiness in the reading is not. For more stable readings, use longer time
constants.
In the SR844 the X and Y noise are computed in the host processor; the MAD algorithm is
used because it requires less computation and is a moving average. The X and Y data values
are sampled (from the DSP) at a 512 Hz rate; the moving average and MAD are then
updated. The moving averages have an exponential time constant that varies between 10 to
80 times the filter time constant. Shorter averaging times settle quickly but fluctuate a lot and
yield a poor estimate of the noise, while longer averaging times yield better noise estimates
but take a long time to settle to a steady answer.
The SR844 performs the noise calculations all the time, whether or not X or Y noise is being
displayed. Thus, as soon as X noise is displayed, the value shown is up to date and no extra
Summary of Contents for SR844
Page 10: ...viii SR844 RF Lock In Amplifier...
Page 12: ...1 2 Getting Started SR844 RF Lock In Amplifier...
Page 32: ...2 2 SR844 Basics SR844 RF Lock In Amplifier...
Page 60: ...3 2 Operation SR844 RF Lock In Amplifier...
Page 102: ...3 44 Shift Functions SR844 RF Lock In Amplifier...
Page 108: ...4 6 Index of Commands SR844 RF Lock In Amplifier...
Page 144: ...4 42 Example Program SR844 RF Lock In Amplifier...
Page 146: ...5 2 Performance Tests SR844 RF Lock In Amplifier...
Page 150: ...5 6 Performance Tests SR844 RF Lock In Amplifier...
Page 156: ...5 12 Performance Tests SR844 RF Lock In Amplifier...
Page 158: ...5 14 Performance Tests SR844 RF Lock In Amplifier...
Page 162: ...5 18 Performance Tests SR844 RF Lock In Amplifier...
Page 166: ...5 22 SR844 Test Record SR844 RF Lock In Amplifier...
Page 168: ...6 2 Circuitry Parts Lists and Schematics SR844 RF Lock In Amplifier...
Page 246: ...Parts Lists SR844 RF Lock In Amplifier 6 80 Schematic Diagrams...