Input Triggering; Measuring Time And Resolution - Hameg HM8021-4 Manual Del Usuario

O p e r a t i o n a l c h e c k

Input Triggering

As the input signal can have very different wave-
forms, it is necessary to shape the signals so
that the counting circuits can handle the signals.
The HM8021-4 offers a variaty of signal shaping
possibilities to improve triggering, such as AC/
DC coupling and two trigger level ranges (–2...
2V, –40...+40V). The trigger level can be set in
one of two ways: either by Autotrigger or with the
trigger level potentiometer. In Autotrigger mode
the counter automatically sets the triggering to
the 50% level of the input signal. When this mode
is selected AC coupling is necessary. When the
trigger level control has been set in the manual
trigger mode, the functioning of the trigger circuit
can easily be checked on the trigger indicator. The
LED shows the state of triggering.
LED on: the signal is above the trigger level
LED off: the signal is below the trigger level
LED
blinking: the signal is crossing the hysteresis
band, correct triggering.
For reliable triggering the trigger level should,
in always most cases, be at 50% of the signal's
peak-to-peak voltage.
Selecting the correct attenuation is important to
obtain the best results from your instrument. If
the attenuation is too high, the measurement will
be affected by the noise of the input comparator.
This results in an unstable display. With an input
signal too great, the input stage may saturate
and thus producing overshoots which result in a
display which is twice too high e.g. at frequency
measurements. Always try to set the control to
AC-coupling and use as much attenuation as pos-
sible for frequency measurements and DC-coup-
ling with no attenuation for time measurements.
In many cases it is vital to have a good impedance
matching to avoid refl ections which might make
the trigger level setting very diffi cult. Always use
a 50 Ω termination in 50 Ω systems. The C-input
22
Subject to change without notice
facilitates no input conditioning controls and
needs no trigger level setting. The input signal is
triggered from 50 mV up to the maximum input
voltage of 5V. The input frequency for the C-input
must always be in the range from 100 MHz to
1000 MHz. For frequencies lower than 100 MHz the
measurement result may be incorrect.

Measuring time and resolution

The measuring time can be varied in 3 steps
between 100ms and 10sec. The gate time may be
modifi ed during a measurement. In the recipro-
cal mode (at all frequencies with HM8021-4), the
counter totalizes the input cycles until the set
measuring time has elapsed and the synchro-
nization conditions are met. Hence, the effective
measuring time (also called gate time) is longer
than the set measurement time. The measure-
ment in the HM8021-4 is always synchronized to
the input signal. This is called the input synchro-
nized or reciprocal method.
In this mode, both the opening and closing of the
main gate are synchronized with the input signal,
so that only completed input cycles are counted.
This means that a ±1 input cycle error is avoided.
During the gate time, the counter totalizes the
number of clock cycles. When the preselected gate
time is over, the counter waits for the next active
transition of the input signal to stop counting. If
the recurrence of this signal is low, e.g. with long
period times, the stop synchronization time may
be long compared to the preset gate time. In that
case the effective gate time may be very different
from the preset value (if the signal was removed
during measurement, this time becomes infi nite
and the measurement fi nishes never). The reso-
lution in the input synchronized mode is caused
by trunctation of the clock pulses, which results
in +1clock pulse error (100ns). The resolution
of the measurement thus only depends on the
measurement time. For example, the resolution
for 1s measuring time is 10
frequency. In conventional counters the gate time
is synchronized with the clock signal. The fi rst
and last input cycle can therefore be truncated,
causing a ±1 cycle error. This resuits in a good
resolution for high frequency measurements, but
a poor resolution for low frequency measurements
(±1: frequency, for 1sec. measuring time).
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, independent of input