Philips PM 6670 - Theory of Measurements; Introduction and Input Triggering

12
Theory of
Measurements
Introduction
The microcomputer-based PM 6670...72 provide a
wide range of frequency and time measuring func-
tinons, including period average, time interval average
as well as three count modes. Resolution down to
lOps is possible with time interval average. In ad
dition, these timer/counters offer pulse width
measurements, phase delay directly in degrees and
tachometer readings i RPM.
PM 6670...72 feature automatic truncation of digits
and automatic calculation of displayed LSD ( Least
Significant Digit). Only significant digits are displayed
and no overflow can occur, except for RPM measure
ments.
A block diagram is shown in Fig. 3.1. To identical
channels are used for accurate time interval measure
ments.
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COUNT RCC.
{INPUT CYCLES)
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BCD OUTPUT
ANALOG OUTPUT
Fig. 3.1. Block diagram
Input triggering functions
As the input signal can have very different wave
forms, it is necessary to pulse shape the signal.
Ohterwise, the counting circuits could not handle the
signal. The input circuits consist of:
• AC/DC-coupling selector;
• an input step attenuator (xlO), to attenuate exces
sive input signals to fit the± 2.5V trigger level
off-set range;
• a switchable low-pass filter for noise rejection;
• a differential amplifier that allows trigger level
setting;
• trigger circuits with variable hysteresis band;
The functional difference between AC- and DC-
coupled input is illustrated in Fig. 3.2.
Input A, AC - coupled
Input A, DC'COupled
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A ^NSITIVITV ( HYS TERLS / 5)
(XlO)
Fig. 3.2. Block diagram of the input circuits.
In AC-coupled mode, which is the normal case for
frequency measurements, the trigger level/sensi
tivity potentiometer controls the Schmitt-trigger
sensitivity. In combination with the xlO attenuator,
this enables a variable trigger hysteresis band, thus
a variable input sensitivity (variable noise immunity).
This is essential for correct triggering for frequency
measurements of noisy signals.
In DC-coupled mode, which is the normal mode for
time measurements, the trigger level/sensitivity
potentiometer controls the differential amplifier. This
results in a continuous variable trigger level setting.
The Schmitt-trigger sensitivity is not influenced, but
is set to minimum. It is of utmost importance to
have as narrow hysteresis band as possible for time
measurements.