Frequency Measurement; Period Measurement; Time Interval Measurement; Totalizing (Event Counting) - Hameg Instruments HM8021-4 Manuel

Signal inputs
The front panel of the HM8021-4 has two BNC
input sockets. one (Input A) with an impedance
of 1 MΩ II 40pF. As the frequency measuring
range of the HM8021-4 unit reaches up to 1 GHz,
this module offers also an input (C) for frequency
measurements from 100 MHz up to 1 GHz. It is also
provided as a BNC socket and has an impedance
of 50Ω.
Caution!
Particular care should be taken, when applying
signal voltages to the 1 GHz input of the HM8021-4
unit. A maximum voltage of 5 V (DC+AC
be applied to the input C (see "Specifications").
Any input voltage exceeding this value will de-
stroy the input stage of the frequency counter!

Frequency measurement

Counters are used for both, frequency and time
interval measurements. However, frequency and
time interval measurements have contradictory
requirements in respect of correct triggering. For
frequency measurements, too high a sensitivity
means that the counter is too sensitive to noise.
Therefore do not use higher sensitivity than needed
for correct triggering. Signals which are super-
imposed on a DC voltage, must be separated via
an input coupling capacitor (i.e. AC-coupling, DC
pushbutton released). The advantages of AC coup-
ling are: no DC-drift and good protection against
DC overload. AC-coupling however, gives a drop
in sensitivity for very low frequencies. The signal
frequency to be measured is applied to one of the
inputs, and the corresponding function is selected.
The trigger point is adjusted by use of the TRIGGER
knob , so that a stable value is displayed. This
stability is obtained, when the trigger signal dis-
play LED flashes (see "Input triggering"). Now the
test frequency can be read on the 8-digit display.
The obtained resolution depends on the gate time
and can be selected in 3 steps with the GATE TIME
pushbutton switch
range is exceeded, the oVERFLoW LED
up. A reliable indication is no longer ensured under
these circumstances. The maximum resolution of
0.1 Hz is obtained with a gate time of 10 sec.

Period measurement

For measurement of the period duration, the re-
ciprocal value of the frequency T=1/f is measured
) may
peak
. When the measurement
is light
A l i g n m e n t p r o c e d u r e
in seconds. The signal is applied as for frequency
measurement.

Time interval measurement

(Pulsewidth)
In TI mode, the time (e.g. number of 100ns clock
pulses) is measured between the positive slope
and the negative slope of an event at channel A.
(Corresponding for negative pulses in TI
In single source time measurements (e.g. Pulse
width) the resolution of the measurement is one
clock pulse (100 ns). By using the time interval
average technique, which means multiple mea-
surements of a repetitive signal, the measuring
accuracy and resolution are greatly improved.
Compared to single time interval measurements,
the basic 100ns resolution is improved by a factor
of N, where N is the number of time intervals
√
being averaged during the measuring time. Note
that the input signal must be repetitive and must
not have a phase relation with the reference fre-
quency. For time interval measurements, too low a
sensitivity means that different signal slopes at the
positive and negative edge cause different delays
between the trigger level crossing and the trigger
point, resulting in incorrect measurements. The
highest possible sensitivity which does not over-
load the input stage, is the ideal. DC-coupling,
attenuation and a continuously variable setting
of the trigger level is necessary for setting the
trigger level at any required point of the input
signal, independent of waveform and duty factor.
Autotriggering requiring AC-coupling is also
possible. The display resolution changes with the
number of measurements taken from the signal.
At single pulse measurements the resolution is
100ns, whereas the resolution may be as small
as 10ps, depending directly on the measurement
time set with the gate switch
rate of the input signal.

Totalizing (Event counting)

The kind of signal input is similar as for frequency
measurement. The counting starts at once. Reset
and restart counting is activating by pushing the
knob RESET.
External gate (back of mainframe)
The external gate function allows full control of
the start and stop of the measurement. When Ext.
(gate) is selected and the control input signal
Subject to change without notice
mode).
and the repetition
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