Main Output Processor - Noise Measurements - Ametek 7230 Instruction Manual

3.3.20 Main Output Processor - Noise Measurements

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
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 Main Controls web panel 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 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 Main Controls web panel or by a
computer command. When this is turned on, the outputs are set to the four types
most commonly required, and the filter time constant, slope and synchronous time
constant setting are forced to values within the permitted ranges. When turned off,
these restrictions are removed.
When making noise measurements the user is strongly advised to use an oscilloscope
to monitor the signal at the SIG MON output on the rear panel as this is the best way
of ensuring that a random process is being measured rather than line pick-up or other
Chapter 3, TECHNICAL DESCRIPTION
Equivalent Noise Bandwidth at Output Filter Slope (Hz)
6 dB/octave
500.00
250.00
125.00
50.00
25.00
Table 3-2, ENBW vs. Time Constant and Slope
12 dB/octave
333.33
166.67
83.33
33.33
16.67
3-13