Power-On Test; Input Triggering; Measuring Time And Resolution - Hameg Instruments HM 8021-3 Manuel

Power-on test

A practical test of the correct operation of the
HM 8021-3 is run automatically at power on. As
soon as power is applied, the display shows type
and version of the actual instrument and the GATE
indicator appears, LEDs are lit and the Eprom and
all functions of the counter are tested. The test
runs for about 2 seconds. If an error is detected it
is indicated by an "I" at the leading digit and
followed by the number of the test that failed.
I 1 microprocessor RAM error
I 2 program ROM error
I 3 counting chain error
When the tests are completed satisfactorily,
the counter sets the display to zero and selects
the default measuring function A.
Measuring functions
FA/FC: Sets the instrument to measure the
frequencyofthesignalconnectedtoinputA/C.
TOT.: The counter will totalize events (pulses or
cycles) on input A. Measurement stops and
display freezes as soon as the input signal
is removed or DISPLAY HOLD is depressed.
Depressing RESETcluring totalizing
clearsthe display and starts a new
measurement when releasing the button.
Reset is active as long as the switch (7) is
depressed. Totalizing A during External
Gate is performed applying a TTL signal to
the Ext. Gate input (16).
PA: Sets the counter to measure the period
duration of the signal connected to input A.
TI : Sets the counter to measure the average
(AVG)pulsewidth between positive slope and the
negative slope of an input signal at input A.

Input Triggering

As the input signal can have very different
waveforms, it is necessary to shape the signals
so that the counting circuits can handle the
signals. The HM8021-3 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.
Sous réserve de modifications / Reservado el derecho de modificación
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 possible for frequency
measurements and DC-coupling with no
attenuation for time measurements. In many
cases it is vital to have a good impedance
matching to avoid reflections which might make
the trigger level setting very difficult. Always usea
50Ω termination in 50Ω systems. The C-input
facilitates no input conditioning controls and needs
no trigger level setting. The input signal is triggered
f rom 50 mV up to the maximum input voltage of
5V. The input f requency for the C-input must
always be in the range f rom 100MHz to 1000MHz.
For frequencies lower than 100MHz the
measurement result may be erroneous.

Measuring time and resolution

The measuring time can be varied in 3 steps
between 100ms and 10sec. The gate time may
be modified during a measurement. In the
reciprocal mode (atall frequencies with HM8021-
3), the counter totalizes the input cycles until the
set measuring time has elapsed and the
synchronization conditions are met. Hence, the
effective measuring time (also called gate time)
is longer than the set measurement time.The
measurement in the HM8021-3 is always
synchronized to the input signal. This is called the
input synchronized 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
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