Hameg HM 1004-3 Series, Manual

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27
Subject to change without notice
Triggering and timebase
triggering purposes if the trigger coupling is set to ~ . This
trigger mode is independent of amplitude and frequency of
the Y signal and is recommended for all mains/line synchro-
nous signals. This also applies within certain limits, to whole
number multiples or fractions of the line frequency. Line
triggering can also be useful to display signals below the
trigger threshold (less than 0.5div). It is therefore particularly
suitable for measuring small ripple voltages of mains/line
rectifiers or stray magnetic field in a circuit. In this trigger
mode the slope direction pushbutton selects the positive or
negative portion of the line/mains sinewave.
The trigger level control can be used for trigger point adjust-
ment. Magnetic leakage (e.g. from a power transformer) can
be investigated for direction and amplitude using a search or
pick-up coil. The coil should be wound on a small former with
maximum turns of thin lacquered wire and connected to a
BNC connector (for scope input) via a shielded cable. Be-
tween cable and BNC center conductor a resistor of at least
100
Ω
should be series-connected (RF decoupling). Often it is
advisable to shield the surface of the coil. However, no
shorted turns are permissible. Maximum, minimum, and
direction to the magnetic source are detectable at the meas-
uring point by turning and shifting the coil.
Alternate triggering
This trigger mode can be selected in DUAL mode by pressing
and holding the
TRIG. pushbutton (19) if the preconditions are
met ( please note “Controls and Readout” ). In the case of
chopped DUAL mode, selecting alternate trigger mode auto-
matically sets the instrument to alternate DUAL mode. The
trigger point symbol and the peak value detection (in auto-
matic trigger mode) are internally deactivated. Only the
following trigger coupling modes are available in alternate
trigger mode: AC, DC, NR, HF and LF.
With alternate triggering it is possible to trigger two signals
which are different in frequency (asynchronous). Each input
signal must be of sufficient height to enable trigger. To avoid
trigger problems due to different DC voltage components, AC
input coupling for both channels is recommended.
The internal trigger source is switched in alternate trigger
mode in the same way as the channel switching system in
DUAL alternate mode, i.e. after each timebase sweep. Phase
difference measurement is not possible in this trigger mode
as the trigger level and slope setting are equal for both signals.
Even with 180° phase difference between both signals, they
appear with the same slope direction. If signals are applied
with a high frequency ratio (difference), the trace intensity
then becomes reduced if the timebase is set to smaller time
coefficients (faster sweep). This happens as the number of
sweeps does not increase because it depends on the lower
frequency signal, but with a faster sweep the phosphor
becomes less activated.
External triggering
The external trigger input is activated with the aid of the TRIG.
(19) pushbutton ( see “Controls and Readout” ), if the trigger
coupling is not set to line/mains trigger coupling. Then the
internal trigger source is deactivated.
As the external trigger signal applied at the TRIG. EXT socket
normally has no relation to the signal height of the displayed
signal, the trigger point symbol is switched off. The external
trigger voltage must have a minimum amplitude of 0.3Vpp
and should not increase above 3Vpp. The input impedance of
the TRIG. EXT. socket is approx. 1M
Ω
II 15pF.
The maximum input voltage of the input circuit is 100V
(DC+peak AC).
The external trigger voltage may have a completely different
form from the test signal voltage, but must be synchronous
with the test signal. Triggering is even possible in certain limits
with whole number multiples or fractions of the test frequency.
It must be noted that a different phase angle between the
measuring and the triggering signal may cause a display not
coinciding with the slope selection setting.
The trigger coupling selection can also be used in external
triggering mode.
Trigger indicator “TR”
The following description applies to the “TR” LED . Please
note item (12) under “Controls and Readout” .
An LED on condition indicates that the trigger signal has a
sufficient amplitude and the trigger level control setting is
correct. This is valid with automatic and with normal trigger-
ing. By observing the trigger LED, sensitive trigger level
adjustment is possible when normal triggering is used, par-
ticularly at very low signal frequencies. The indication pulses
are of only 100ms duration.
Thus for fast signals the LED appears to glow continuously,
for low repetition rate signals, the LED flashes at the repeti-
tion rate or at a display of several signal periods not only at the
start of the sweep at the left screen edge, but also at each
signal period. In automatic triggering mode the sweep gen-
erator starts repeatedly without test signal or external trigger
voltage. If the trigger signal frequency decreases the trigger
automatic repetition frequency the sweep generator starts
without awaiting the trigger pulse. This causes an untriggered
display and a flashing trigger LED.
HOLD OFF-time adjustment
For instrument specific information please note DEL.POS. -
HO (23) in section “Controls and Readout” .
If it is found that a trigger point cannot be found on extremely
complex signals, even after careful adjustment of the trigger
level control, a stable display may often be obtained using the
holdoff control. This facility varies the holdoff time between
two sweep periods approx. up to the ratio 10:1. Pulses or
other signal waveforms appearing during this off period
cannot trigger the timebase. Particularly with burst signals or
aperiodic pulse trains of the same amplitude, the start of the
sweep can be delayed until the optimum or required time.
A very noisy signal or a signal with a higher interfering
frequency is at times displayed double. It is possible that
trigger level adjustment only controls the mutual phase shift,
but not the double display. The stable single display of the
signal, required for evaluation, is easily obtainable by expand-
ing the hold off time until one signal is displayed.
A double display is possible with certain pulse signals, where
the pulses alternately show a small difference of the peak
amplitudes. Only a very exact trigger level adjustment makes
a single display possible. The use of the holdoff control
simplifies the right adjustment.
After specific use the holdoff control should be reset into its