Philips PM 6670 Operating Manual page 11

13
Schmitt-trlgger function
The Schmitt-trigger function is illustrated in Fig. 3.3.
Tr,gg.
poin t
r Input Sign
at
Tnsger
I Hysteresis Dana
1 (trigger unndoui)
te vet
t f - \ / IS et^uQl to noise
i ' V \ 1 mmun i 1
Recoverg
point
Ou tpu t
from
tr,gg^r
C! r.cui t
Fig. 3.3. Visualization of the trigger function.
The hysteresis band (trigger window) is centered
around the trigger level and the width of the hyst
eresis band at the input, is the same as the effective
input sensitivity of Vpp.
The ideal hysteresis band is 50-60% of the signal's
peak-to-peak value. Since input signals can have
any amplitude within the specification, a continuously
variable input attenuator is prefered.
Signals, that are superimposed on a DC voltage, need
to be separated via a capacitor, i.e. AG-coupling. The
advantages of AG-coupling are:
• No DG drift.
• Good protection against DG overload.
However, AG-coupling gives a drop in sensitivity for
very low frequencies.
Time Interval measurements
For time interval measurements, too wide a hys
teresis band, i.e. too low a sensitivity, means that
different signal slopes at the start and stop trigger
point, cause different delays between the trigger level
crossing and the trigger point, see Fig. 3.5.
Frequency measurements
Timer/counters are used for both frequency and time
interval measurements. However, frequency and time
interval measurements have contradictory requi
rements in respect of correct triggering.
For frequency measurements, too narrow a hyste-
resisband, i.e. too high a sensitivity, means that the
counter is too sensitive to noise; see Fig.3.4. The
hysteresis band is equal to noise immunity.
Pulse duration
I Measured ft
r
\pulse duration I I
, Measured time (
r n
Fig. 3.5. Too wide a hysteresis might cause incorrect
time interval measurements.
WRONG
Three counts per cycle
CORRECT
One count per cycle
Fig. 3.4. Do not use a higher sensitivity than needed
for correct triggering.
The highest possible sensitivity with low noise is
ideal. However, a calibrated input attenuator is needed
to enlarge the trigger level setting range. A separate
xlO step attenuator, which expands the trigger level
range to -25V... + 25V is available on PM 6670...72.
A continuously variable setting of the trigger level
is necessary for setting the trigger level at any
required point of the input signal.
If the duty factor of the input signal changes, the
average DG-component will also change. In case of
AG-coupling, the trigger level follows the average
DG-component. This is not acceptable if the time
interval measurements with accurate trigger level
settings are to be made. Hence, DG-coupling is
necessary. Two identical inputs and slope selection
are also necessary.