Hameg HM 407A Manual

29Subject to change without noticecoupling is not set to line/mains trigger coupling. Then theinternal trigger source is deactivated. As the external triggersignal applied at the TRIG. EXT socket normally has no relationto the signal height of the displayed signal, the trigger pointsymbol is switched off. The external trigger voltage must havea minimum amplitude of 0.3Vpp and should not increase above3Vpp. The input impedance of the TRIG. EXT. socket isapprox. 1MΩ II 15pF.The maximum input voltage of the input circuit is 100V(DC+peak AC).The external trigger voltage may have a completely differentform from the test signal voltage, but must be synchronouswith the test signal. Triggering is even possible in certain limitswith whole number multiples or fractions of the test frequency.It must be noted that a different phase angle between themeasuring and the triggering signal may cause a display notcoinciding with the slope selection setting.The trigger coupling selection can also be used in externaltriggering mode.Trigger indicator “TR”The following description applies to the “TR” LED.Please noteitem (16) under “Controls and readout”.An LED on condition indicates that the trigger signal has asufficient amplitude and the trigger level control setting iscorrect. This is valid with automatic and with normal triggering.By observing the trigger LED, sensitive trigger level adjustmentis possible when normal triggering is used, particularly at verylow signal frequencies. The indication pulses are of only100ms duration.Thus for fast signals the LED appears to glow continuously, forlow repetition rate signals, the LED flashes at the repetition rateor at a display of several signal periods not only at the start ofthe sweep at the left screen edge, but also at each signal period.In automatic triggering mode the sweep generator startsrepeatedly without test signal or external trigger voltage. If thetrigger signal frequency decreases the trigger automaticrepetition frequency the sweep generator starts withoutawaiting the trigger pulse. This causes an untriggered displayand a flashing trigger LED.HOLD OFF-time adjustment(Only in analog mode)For instrument specific information please note DEL.POS. -HO (27)in section “Controls and readout”.If it is found that a trigger point cannot be found on extremelycomplex signals, even after careful adjustment of the triggerlevel control, a stable display may often be obtained using theholdoff control. This facility varies the holdoff time betweentwo sweep periods approx. up to the ratio 10:1. Pulses or othersignal waveforms appearing during this off period cannottrigger the time base.Particularly with burst signals or aperiodic pulse trains of thesame amplitude, the start of the sweep can be delayed untilthe optimum or required time.A very noisy signal or a signal with a higher interferingfrequency is at times displayed double. It is possible thattrigger level adjustment only controls the mutual phase shift,but not the double display. The stable single display of thesignal, required for evaluation, is easily obtainable by expandingthe hold off time until one signal is displayed.A double display is possible with certain pulse signals, wherethe pulses alternately show a small difference of the peakamplitudes. Only a very exact trigger level adjustment makesa single display possible. The use of the holdoff controlsimplifies the right adjustment.After specific use the holdoff control should be reset into itscalibration detent (fully ccw), otherwise the brightness of thedisplay is reduced drastically. The function is shown in thefollowing figures.Fig. 1 shows a case where the holdoff control is in the minimumposition and various different waveforms are overlapped on thescreen, making the signal observation unsuccessful.Fig. 2 shows a case where only the desired parts of the signal are stablydisplayed.Delay / After Delay Triggering(Only in analog mode)The instrument specific information regarding this mode ispart of the section “Controls and readout” paragraph DEL.POS.- HO (27), SEA./DEL. - ON/OFF (29)and DEL.TRIG. - VAR.(30).As mentioned before, triggering starts the time base sweepand unblanks the beam. After the maximum X deflection to theright, the beam is blanked and flies back to the (left) startposition. After the hold off period the sweep is startedautomatically by the automatic trigger or the next triggersignal. In normal triggering mode the automatic trigger isswitched off and will only start on receipt of a trigger signal.As the trigger point is always at the trace start position, traceexpansion in X direction with the aid of the time base is limitedto the display on the left of the trace. Parts of the signal to beexpanded which are displayed near the trace end (right side ofthe screen) are lost when the time base speed is increased(time coefficient reduced).The delay function delays the trace start by a variable time fromthe trigger point. This allows the sweep to begin on any portionof a signal. The time base speed can then be increased toexpand the display in X direction. With higher expansion rates,the intensity reduces and within certain limits this can becompensated by the INTENS knob setting.Triggering and time base