Chapter 3, TECHNICAL DESCRIPTION
3-15
3.3.24 Main Microprocessor - General
All functions of the instrument are under the control of a microprocessor, which in
addition supports the computer interfaces. It also controls the instrument's 8-bit
digital (TTL) programmable input/output port, which may be used for controlling
auxiliary apparatus or reading the status of external logic lines.
A particularly useful feature of the design is that only part of the controlling
firmware program code, which the microprocessor runs, is permanently resident in
the instrument. The remainder is held in a flash EEPROM and can be updated via the
USB or RS232 computer interface, using an Update Pack that can be downloaded
from the www.signalrecovery.com website.
3.3.25 Main Microprocessor - Auto Functions
The microprocessor also controls the instrument's auto functions. These allow easier,
faster operation in most applications, although direct manual operation or special
purpose control programs may give better results in certain circumstances. During
operation of several of the auto functions, decisions are made on the basis of output
readings taken at a particular moment. Where this is the case, it is important for the
output time constant set by the user to be long enough to reduce the output noise to a
sufficiently low level so that valid decisions can be made and that sufficient time is
allowed for the output to settle.
The following sections contain brief descriptions of the auto functions.
Auto-Sensitivity
This function only operates when the reference frequency is above 1 Hz. A single
Auto-Sensitivity operation consists of decreasing the full-scale sensitivity range if
the magnitude output is greater than 90% of full-scale, or increasing the full-scale
sensitivity range if the magnitude output is less than 30% of full-scale. After the
Auto-Sensitivity function is called, Auto-Sensitivity operations continue to be made
until the required criterion is met.
In the presence of noise, or a time-varying input signal, it may be a long time before
the Auto-Sensitivity sequence comes to an end, and the resulting setting may not be
what is really required.
Auto-Phase
In an Auto-Phase operation the value of the signal phase is computed and an
appropriate phase-shift is then introduced into the reference channel so as to bring
the value of the signal phase to zero. The intended result is to null the output of the Y
channel while maximizing the output of the X channel.
Any small residual phase can normally be removed by calling Auto-Phase for a
second time, after a suitable delay to allow the outputs to settle.
The Auto-Phase facility is normally used with a clean signal that is known to be of
stable phase. It usually gives very good results provided that the X channel and Y
channel outputs are steady when the procedure is called.
If a zero error is present on the outputs, such as may be caused by unwanted coupling
between the reference and signal channel inputs, then the following procedure should
be adopted:-
1) Remove the source of input signal, without disturbing any of the connections to
the signal input which might be picking up interfering signals from the reference
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