Output Processor - Output Offset And Expand; Output Processor - Vector Magnitude And Phase - Ametek 7280 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION

3.3.14 Output Processor - Output Offset and Expand

3.3.15 Output Processor - Vector Magnitude and Phase

3-12
Where random noise is relatively small, synchronous filter operation gives a major
advantage in low-frequency measurements by enabling the system to give a constant
output even when the output time constant is equal to only 1 reference cycle.
Following the output filter, an output offset facility enables ±300% full-scale offset
to be applied to the X, Y or both displays and to the analog outputs.
The output expand facility allows a ×10 expansion, performed by simple internal
digital multiplication, to be applied to the X, Y, both or neither outputs, and hence to
the bar-graph displays and the CH 1 and CH 2 analog outputs, if these are set to
output X or Y values.
The processor also implements the magnitude and signal phase calculation which is
useful in many situations. If the input signal V
constant amplitude, and the output filters are set to a sufficiently long time constant,
the demodulator outputs are constant levels V
dependent only on the amplitude of the required signal V
on the phase of V (t) with respect to the reference input) and is computed by the
s
output processor in the lock-in amplifier and made available as the magnitude output.
The phase angle between V
phase: this is equal to the angle of the complex quantity (V
square root of -1) and is also computed by the processor by means of a fast arc tan
algorithm.
The magnitude and signal phase outputs are used in cases where phase is to be
measured, or alternatively where the magnitude is to be measured under conditions of
uncertain or varying phase.
One case of varying phase is that in which the reference input is not derived from the
same source as that which generates the signal, and is therefore not at exactly the
same frequency. In this case, if the input signal is a sinusoid of constant amplitude,
the X channel and Y channel demodulator outputs show slow sinusoidal variations at
the difference frequency, and the magnitude output remains steady.
However, the magnitude output has disadvantages where significant noise is present
at the outputs of the demodulator. When the required signal outputs (i.e. the mean
values of the demodulator outputs) are less than the noise, the outputs take both
positive and negative values but the magnitude algorithm gives only positive values:
this effect, sometimes called noise rectification, gives rise to a zero error which in the
case of a Gaussian process has a mean value equal to 0.798 times the combined root-
mean-square (rms) value of the X and Y demodulator noise. Note that unlike other
forms of zero error this is not a constant quantity which can be subtracted from all
readings, because when the square root of the sum of the squares of the required
outputs becomes greater than the total rms noise the error due to this mechanism
disappears.
A second type of signal-dependent error in the mean of the magnitude output occurs
(t) is a reference frequency sinusoid of
s
and V
x
(t) and the X demodulation function is called the signal
s
2
. The function √(V
+ V
y x
(t) (i.e. it is not dependent
s
+ jV ) (where j is the
x y
2
) is
y