Philips PM 6670 Operating Manual page 14

16
rmjL
/OOns
clock cycles
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Measuring
Trigger let/ei 3
Trigger level A
Mam Gate
Fig. 3.9. Time interval average mode.
For a signal as illustrated in Fig. 3.9, approx 10000
time intervals are being averaged during a measuring
time of 10ms. Let's say that 6000 are measured
as 200ns (2 clock cycles) and 4000 as 100ns. The
statistical average is calculated in the microcomputer.
In this case the average is 1 60ns. The resolution
is 100ns/ VI0000 = Ins. Note that the input signal
must be repetitive and asynchronous with respect
to the time base and that the minimum dead time
from stop to start is 300ns.
Pulse width measurements
These measurementsare similarto singletime interval
measurements. Both start and stop triggering takes
place in channel A, with common trigger level setting
and automatic trailing edge trigger slope inversion
with respect to selected leading edge trigger slope.
Phase delay measurements
The timer/counters PM 6670...72 can measure the
phase delay between two signals connected to input A
and B. The measurement is performed as a simultan
eous measuring of time interval A-B and period.The
phase delay is calculated as:
Phase delay =
Time Interval A-B
Period
x360°
The measurement is made as an average measure
ment to improve accuracy and resolution; see
Fig. 3.10.
/7\\ /
\\ A A \\
\y/1 w/i
' 1 •'?/ 1 1
^ 1 1
n n
_j
—j
PHASC DELAY = " •
n^T
In order to get a high accuracy phase delay time
interval measurement, the setting of the trigger level
is very important. The trigger levels should be
identical for both channels and as close to zero
as possible. This is normally achieved with AC
coupling and max sensitivity setting. Unequal
settings of the trigger levels will result in inaccurate
time interval measurements; see Fig. 3.11.
Trigger level B
— L Trigger level A
Fig. 3.11. Unequal settings of the trigger levels will
result In inaccurate time interval measurements.
Very large differences in slew rate between the
signals, can result in a systematic phase error,
which can be up to 3...5.° This is caused by the
hysteresis band (typically ± 10mV). Although the
trigger level is set to OmV, the actual trigger point
will be + 10mV. With variations in slew rate, the time
before crossing the -FlOmV limit will vary; see
Fig. 3.12.
-WmV
Fig. 3.10. The phase delay measurement.
Fig. 3.12. Varitation in slew rate can result in phase
error.
It is therefore important to keep the signals at
about equal amplitude ( sine and triangular waves).
Since a normal time interval average measurement
is made, we also have the normal restriction
concerning minimum dead time between stop and
start of the time interval, i.e. 300ns. the dead
time also determines the maximum signal frequency,
which is 1.6MFIz.
Duty cycle measurements
A duty cycle measurement can be made by means of
a phase measurement. Connect the signal to Input A,
push COM via A and SLOPE for Input B. However,
the displayed read-out will be in degrees. Divide the
result by 3.6 to get 0...100% or by 360°to get 0...1
(duty factor). If the counter is equipped with a
PM 9696 IEC-625/IEEE-488 bus interface, this
calculation could easily be done by the controller.