Table of Contents
Advertisement
Chapter 2
The SR860 measures input signal noise AT the reference frequency. Many noise sources
have a frequency dependence which the lock-in can measure.
How Is Noise Measured?
Remember that the lock-in detects signals close to the reference frequency. How close?
Input signals within the detection bandwidth set by the low pass filter time constant and
roll-off appear at the output at a frequency f = f
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 the perfect rectangular filter which passes the same amount of noise as the
real filter.
The ENBW is determined by the time constant (T) and slope as shown below (for normal
RC type filters).
The noise is simply the standard deviation (root of the mean of the squared deviations) of
the measured X or Y values averaged over a period of time. This involves computing the
average of (X
output. The noise result is the square root of the average. The averaging time is roughly
200 time constants; for example, if the time constant is 100 mS, the noise measurement
would take about 20 seconds to settle.
If the Sync filter is enabled, that also affects the noise averaging time. The averaging time
due to the Sync filter is 100 Sync filter periods. The final noise averaging time is the
longer of these two averaging times, or 0.5 seconds.
Shorter averaging times yield a very poor estimate of the noise (the mean varies rapidly
and the deviations are not averaged well). Longer averaging times, while yielding better
results, take a long time to settle to a steady answer.
Noise and Sensitivity
Beginning with firmware version 1.20, noise is calculated using floating-point math, and
it is independent of the Sensitivity setting.
However, earlier firmware versions used data from the scaled output values of X and Y.
In order to accurately compute the noise on instruments running earlier firmware, the
Sensitivity must be set appropriately so that the displayed values of X or Y have enough
resolution. If the displayed values of X or Y are unchanging, then the computed noise
will be almost zero.
Noise Display
To display a noise measurement, choose X or Y noise (Xn or Yn) as one of the displayed
parameters in the [Config] screen. The SR860 is calculating the noise all of the time,
whether or not X or Y noise are being displayed. Thus, as soon as noise is displayed, the
Slope
ENBW
6 dB/oct
1/(4T)
12 dB/oct
1/(8T)
18 dB/oct
3/(32T)
24 dB/oct
5/(64T)
− X
2
)
where X
value
mean
− f
. Input noise near f
sig
ref
is the current X output and X
value
SR860 DSP Lock-in Amplifier
Basics
59
appears as
ref
is the mean X
mean
Advertisement
Table of Contents
