Signal Inputs; Frequency Measurement; Period Measurement; External Gate - Hameg Instruments HM 8021-3 Manuel

measurement, this time becomes infinite and the
measurement finishes never). The resolution 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 first 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).

Signal inputs

The front panel of the HM8021-3 has two BNC
input sockets. One (Input A) with an impedance
of 1MΩ II 40pF. As the frequency measuring range
of the HM8021-3 unit reaches up to 1GHz, this
module offers also an input (C) for frequency
measurements from 100MHz up to 1GHz. 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 HM 8021-3 unit.
A maximum voltage of 5V (DC+AC
applied to the input C (see "Specifications").
Any input voltage exceeding this value will
destroy 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
superimposed on a DC voltage, must be separated
via an input coupling capacitor (i.e. AC-coupling,
DC pushbutton released). The advantages of AC
coupling 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 (9, so that a stable value is
displayed. This stability is obtained, when the
trigger signal display 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
(3). When the measurement range is exceeded,
the red OVERFLOW LED (1) is light up. A reliable
indication is no longer ensured under these
18
-7 , independent of input
) may be
peak
circumstances. The maximum resolution of 0.1
Hz is obtained with a gate time of 10sec.

Period measurement

For measurement of the period duration, the
reciprocal value of the frequency T=1/f is
measured 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
mode). In single source time measurements
(e.g.Pulse width ) the resolution of the measurement
is 1clock pulse (100ns).By using the time interval
average technique, which means multiple
measurements of a repetitive signal, the
measuring accuracy and resolution are greatly im
proved.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 frequency. For time interval
measurements, too low a sensitivity means that
different signal slopes at the positive and negati-
ve edge cause different delays between the trigger
level crossing and the trigger point, resulting in
incorrect measurements. The highest possible
sensitivity which does not overload 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 (3) and the repetition rate of
the input signal.
Totallizing (Event counting)
The signal is applied as for frequency
measurement.

External gate

The external gate function allows full control
of the start and stop of the measurement. When
Ext. (gate) is selected (3) and the control input
signal (16) is low, the counter makes all necessary
preparations for a measurement. With the high
level of the gate signal, measurement starts when
the input signal triggers after a synchronization
delay. Measurement stops on the first trigger after
the gate signal changes from high to low. The
external gate overrides the set measurement
time. The external gate signal must be in the range
Änderungen vorbehalten / Subject to change without notice