Operation - Hameg HM 8122 User Manual

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The test runs for about 2 seconds. If an error is detected
it is indicated by an "E" at the leading digit and followed
by the number of the test that failed. The default function
after this test is frequency A.

Operation

This chapter describes the operation of the HM8122 from
the viewpoint of its application, and gives practical advice
to this end. The principle functions are described, as are
the use of auxiliary facilities. In addition, a few hints and
tips are included. Knowledge of the controls, indicators
and connectors is assumed.
The use of the IEEE-488 bus is described in the
manual of the HO88.
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 HM 8122 offers a variaty of signal shaping possibilities
to improve triggering, such as trigger slope, AC/DC coupling
and three trigger level ranges (–1....+1V, –10...+10V, –
100...+100V).
Also a switch selectable 50 kHz low pass filter prevents
false high frequency triggering at low frequency signals.
The trigger level can be set in one of two ways: either by
autotrigger or with the two trigger level potentiometers.
In Autotrigger mode the counter automatically measures
and sets the triggering to the 50% level of the input signal.
When this mode is selected AC coupling is necessary.
When the trigger level controls have been set in the
manual trigger mode, the functioning of the trigger circuits
can easily be checked on the trigger indicators, one for
each channel. The LEDs show the state of triggering.
On: the signal is above the trigger level
Off: the signal is below the trigger level
Blinking: the signal is crossing the hysteresis band,
correct triggering.
LED on LED flashes
Figure 1
For reliable triggering the trigger level should, in always
most cases, be at 50% of the signals peak-to-peak
voltage.
Selecting the correct attenuation is important to obtain
the best results from your instrument. If the attenuation
is to 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.
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Always try to set the controls 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 use a 50Ω
termination in 50Ω systems.
The C-input facilitates no input signal 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 frequency range from 100 MHz to 1600 MHz. For
frequencies lower than 100 MHz the measurement result
may be erroneous.
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 to 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).
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 frequencys. The low-pass filter is helpfull in those
cases were a low frequency signal is superimposed with
an unwanted high frequency.
Time interval measurement
In TIME A-B single mode, the time (e.g. number of 10 ns
clock pulses) is measured between a start event at
channel A and a stop event at channel B. The start and
stop triggering can be set individually with respect to:
coupling, trigger level, slope and attenuation.
In single source time measurements (e.g. Pulse width)
only input A is connected. The input B connector is
disconnected. However, when depressing COM, channel
B is internally connected to input A. The resolution of the
single measurement is 1 clock pulse (10ns).
By using the time interval average measuring technique,
0V
which means multiple measurements of a repetive signal,
the measuring accuracy and resolution are greatly
improved. Compared to single time interval measurements,
the basic 10 ns 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 start and stop
trigger point cause different delays between the trigger
level crossing and the trigger point, resulting in incorrect
measurements. By lowering the trigger level on positive
trigger slopes and raising it on negative slopes or vice
versa, one can compensate to the errors due to hysteresis.
The highest possible sensitivity which does not overload
the input stage, is the ideal.
HM8122 21