Hameg HM2005-2 User Manual

Contents

Hameg HM2005-2 User Manual Manual pdf 17 page image

17Subject to change without noticeFrame sync pulse triggeringRemark:Using frame sync triggering in dual trace chopped mode mayresult in interference, then the dual trace alternate modeshould be chosen. It may also be necessary to turn the read-out off.In order to achieve frame sync pulse triggering call MODE,select video signal triggering and then FILTER to select frametriggering. It may be selected further whether ”all“, ”only even“or ”only odd“ half frames shall trigger. Of course, the correct TVstandard must be selected first of all (625/50 or 525/60).The time base setting should be adapted, with 2 ms/cm a com-plete half frame will be displayed. Frame sync pulses consistof several pulses with a half line rep rate.Line sync pulse triggeringIn order to choose line snyc triggering call MODE and selectVIDEO, enter FILTER, make sure that the correct video standardis selected (625/50 or 525/60) and select Line.If ”ALL“ was selected each line sync pulse will trigger. It is alsopossible to select a line number ”LINE No.“.In order to display single lines a time base setting of TIME/DIV.= 10 μs/cm is recommended, this will show 1½ lines. In generalthe composite video signal contains a high dc component whichcan be removed by ac coupling, provided the picture is steady.Use the POSITION control to keep the display within the screen.If the video content changes like with a regular tv program onlyDC coupling is useful, otherwise the vertical position wouldcontinuously move.The sync separator is also operative with external triggering.Consult the specifi cations for the permissible range of triggervoltage. The correct slope must be chosen as the externaltrigger may have a different polarity from the composite video.In case of doubt display the external trigger signal.LINE triggerConsult SOURCE 18 in ”Controls and Readout“ for specificinformation.If the readout shows Tr:Line the trigger signal will be internallytaken from the line (50 or 60 Hz).This trigger signal is independent of the scope input signals andis recommended for all signals synchronous with the line. Withinlimits this will also be true for multiples or fractions of the linefrequency. As the trigger signal is taken off internally there isno minimum signal height on the screen for a stable display.Hence even very small voltages like ripple or line frequencyinterference can be measured.Please note that with line triggering the polarity switching willselect either the positive or negative half period of the line, notthe slope. The trigger level control will move the trigger pointover most of a half wave.Line frequency interference may be checked using a searchcoil which preferably should have a high number of turns anda shielded cable. Insert a 100 Ω resistor between the centerconductor and the BNC connector. If possible the coil shouldbe shielded without creating a shorted winding.Alternate triggerThis mode is selected with SOURCE 18 > Alt. 1/2. The readoutwill display Tr:alt, but no trigger point symbol indicating leveland time position.This trigger mode is to be used with greatest care and should bean exception rather than the rule, because the time relationshipsvisible on the screen are completely meaningless, they dependonly on the shape of the signals and the trigger level!In this mode the trigger source will be switched together withthe channel switching, so that when CH1 is displayed in thedual channel alternate mode the trigger is taken from CH1and when CH2 is displayed the trigger is taken from CH2. Thisway two uncorrelated signals can be displayed together. If thismode is inadvertently chosen the time relationships betweenthe signals will also be lost when both signals are correlated!(Except for the special case that both happen to be square waveswith extremely fast rise times). Of course, this trigger mode isonly possible in the dual channel alternate mode and also notwith external or line trigger. AC coupling is recommended formost cases.External triggeringThis trigger mode may be selected with SOURCE 18 > Extern.The readout will display Tr:ext. AUXILIARY INPUT 33 will be theinput for the external trigger signal, all internal trigger sourceswill be disconnected. In this mode the trigger point symbol (leveland time position) will not be displayed, only the trigger timeposition will be indicated. External triggering requires a signalof 0.3 to 3 VPP , synchronous with the vertical input signal(s).Triggering will also be possible within limits with multiples orfractions of the vertical input signal frequency. As the triggersignal may have any polarity, it may happen that the verticalinput signal will start with a negative slope in spite of havingselected positive slope; slope selection refers now to the ex-ternal trigger.Indication of triggered operation (TRIG’D LED)Refer item 19 in ”Controls and Readout“. The LED labelledTRIG’D indicates triggered operation provided:– Sufficient amplitude of the internal or external trigger sig-nal.– The trigger point symbol is not above or below the signal.If these conditions are met the trigger comparator will outputtriggers to start the time base and to turn on the trigger indi-cation. The trigger indicator is helpful for setting the triggerup, especially with low frequency signals (use normal trigger)and very short pulses.The trigger indication will store and display triggers for 100 ms.With signals of very low rep rate the indicator will flash accor-dingly. If more than one signal period is shown on the screenthe indicator will flash each period.Hold off time adjustmentConsult ”Controls and Readout“ HOR VAR 26 > Hold off timefor specifi c information.After the time base has deflected the trace from left to right,the trace will be blanked so the retrace is invisible. The nextT r i g g e r i n g a n d t i m e b a s e