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Chapter 3, TECHNICAL DESCRIPTION
3.3.23 Main Output Processor - Noise Measurements
3-14
X-Y mode, with the phase-shifter adjusted to maximize the X output and to bring the
mean Y output to zero. (Refer to section 3.3.29 for further information on the correct
use of the Auto-Phase function for this purpose.)
The noise measurement facility uses the output processor to perform a noise
computation on the X output of the demodulator. A noise buffer continuously
calculates the mean level of X, representing the measured output signal, by summing
the last n samples of the X output and dividing by n. The processor then calculates
the modulus of the difference between each X-output value and the mean value and
uses this figure to derive the noise. The displayed noise value is correct for input
noise where the amplitude distribution of the waveform is Gaussian, which is
normally the case. The indicated value (in V/Hz or A/Hz) is the square root of the
mean spectral density over the equivalent noise bandwidth defined by the setting of
the output filter time constant and slope.
When used for noise measurements, the available range of output time constants is
restricted to 500 µs to 10 ms inclusive, , and the slope to 6 or 12 dB/octave. The
corresponding actual bandwidth for the present time constant and slope settings can
be found from the table 3-2 below, or by using the ENBW. command. In addition,
the synchronous time constant control is turned off, and the fast analog outputs mode
is turned on.
Time
Constant
500 µs
1 ms
2 ms
5 ms
10 ms
Table 3-2, ENBW vs. Time Constant and Slope
The noise buffer length n can be set to 1, 1000, 2000, 3000 or 4000. Since new input
values to the buffer are supplied at a 1 kHz rate, these correspond to averaging times
of zero, 1 second, 2 seconds, 3 seconds and 4 seconds respectively. Hence the
control on the Configuration Menu that adjusts this buffer length is labeled Noise
Buffer Length and can be set to Off, 1 s, 2 s, 3 s or 4 s. Setting a shorter time means
that the system responds more quickly to changes in the mean X-output level, but the
noise reading itself exhibits more fluctuation. Conversely, the fluctuation can be
reduced by setting a longer time, but at the expense of increased settling time
following changes in the mean X-output level.
If a noise output (N calibrated in volts or amps per root hertz, or as a percentage of
full scale) is selected as one of the outputs on the Main Display or for conversion to
an analog signal for output to the DAC 1 to DAC 4 outputs, and the time constant is
not within the permitted range then a warning message is displayed on the screen.
Similarly, if a noise output value is read via the computer interfaces while the time
constant or slope are outside the permitted range, or if the synchronous time constant
control is enabled, then the response will be -1. Since noise readings can only be zero
or positive, this negative number clearly indicates that the reading is invalid and
should be ignored.
In order to make noise measurements easier, the instrument includes a Noise
Measurement Mode, activated by a control on the Configuration menu or by a
Equivalent Noise Bandwidth at Output Filter Slope (Hz)
6 dB/octave
500.00
250.00
125.00
50.00
25.00
12 dB/octave
333.33
166.67
83.33
33.33
16.67
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