Output Processor - Noise Measurements - Ametek 7280 Instruction Manual

as a result of the inherent non-linearity of the magnitude formula: this error is always
positive and its value, expressed as a fraction of the signal level, is half the ratio of
the mean-square value of the noise to the square of the signal.
These considerations lead to the conclusion that when the magnitude output is being
used, the time constants of the demodulator should be set to give the required signal-
to-noise ratio at the X channel and Y channel demodulator outputs; improving the
signal-to-noise ratio by averaging the magnitude output itself is not to be
recommended.
For analogous reasons, the magnitude function also shows signal-dependent errors
when zero offsets are present in the demodulator. For this reason, it is essential to
reduce zero offsets to an insignificant level (usually by the use of the Auto-Offset
function) when the magnitude output is to be used.
Note that the majority of signal recovery applications are scalar measurements, where
the phase between the required signal and the reference voltage is constant apart from
possible phase reversals corresponding to changes in the sign of the quantity being
measured. In this situation the lock-in amplifier is used in the normal 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 3.3.21 for further information on the correct use of the
Auto-Phase function for this purpose.)

3.3.16 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.
Time
Constant
500 µs
1 ms
2 ms
4 ms
5 ms
10 ms
Chapter 3, TECHNICAL DESCRIPTION
Equivalent Noise Bandwidth at Output Filter Slope (Hz)
6 dB/octave
335
209
115
60
48
24
Table 3-2, ENBW vs. Time Constant and Slope
12 dB/octave
276
158
82
42
33
17
3-13