Hameg HM1008-2 Manual

Hameg HM1008-2 Manual Manual pdf 18 page image

18 Subject to change without noticeof 10:1. Manipulation of the hold off time and thus of the time fora complete sweep period from start to start can be useful e.g.when data packets are to be displayed. It may seem that suchsignals can not be triggered. The reason is that the possiblestart of a new sweep does not coincide with the start of a datapacket, it may start anywhere, even before a data packet. Byvarying the hold off time, a stable display will be achieved bysetting it so that the hold off ends just before the start of a datapacket. This is also handy with burst signals or non periodicpulse trains.A signal may be corrupted by noise or hf interference so a doubledisplay will appear. Sometimes varying the trigger level cannotprevent the double display but will only affect the apparent timerelationship between two signals. Here the variable hold off timewill help to arrive at a single display.Sometimes a double display will appear when a pulse signalcontains pulses of slightly differing height requiring delicatetrigger level adjustment. Also here increasing the hold off timewill help.Whenever the hold off time has been increased it should resetto its minimum for other measurements, otherwise the bright-ness will suffer as the sweep rep rate will not be maximum. Thefollowing pictures demonstrate the function of the hold off:Fig. 1: Display with minimum hold off time (basic setting).Double image, no stable display.Fig. 2: By increasing the hold off a stable display is achieved.Time base B (2nd time base). Delaying, DelayedSweep. Analog modeConsult ”Controls and Readout“ HOR VAR 30 and TIME/DIV. 28for specific information.As was described in ”Triggering and time base“ a trigger willstart the time base. While waiting for a trigger, after completionof the hold off time, the trace will remain blanked. A trigger willcause trace unblanking and the sweep ramp which deflectsthe trace from left to right with the speed set with TIME/DIV.At the end of the sweep the trace will be blanked again andreset to the start position. During a sweep the trace will also bedeflected vertically by the input signal. In fact the input signaldoes continuously deflect the trace vertically, but this will beonly visible during the unblanking time. This is, by the way, onemarked difference to digital operation where the input signal isonly measured during the acquisition time, for most of the timethe digital oscilloscope will not see the signal. Also, in analogmode the signal itself will be seen on the screen in real time,whereas a digital oscilloscope can only show some time latera reconstruction of the signal acquired.In analog mode the display will always start on the left. Let usassume one period of a signal is displayed at a convenient timebase setting. Increasing the sweep speed with TIME/DIV. willexpand the display from the start, so that parts of the signalwill disappear from the screen. It is thus possible to expandthe beginning of the signal period and show fine detail, but itis impossible to show such fine detail for ”later“ parts of thesignal.The x10 Magnifier (MAG x10) may be used to expand the displayand the horizontal positioning control can shift any part of thedisplay into the centre, but the factor of 10 is fixed.The solution requires a second time base, called time base B.In this mode time base A is called the delaying sweep andtime base B the delayed sweep. The signal is first displayedby TB A alone. Then TB B is also turned on giving the mode”A intensified by B“. TB B should always be set to a higher sweeprate than A, thus its sweep duration will be also shorter thanthat of A. The TB A sweep sawtooth is compared to a voltagewhich can be varied such that TB A functions as a precisiontime delay generator. Depending on the amplitude of the com-parison voltage a signal is generated anywhere between sweepstart and end.In one of two operating modes this signal will start TB B imme-diately. The TB A display will be intensified for the duration ofTB B, so that one sees which portion of the signal is covered byTB B. By varying the comparison voltage the start of TB B canbe moved over the whole signal as it is displayed by TB A. Thenthe mode is switched to TB B. The signal portion thus selected isnow displayed by TB B. This is called „B delayed by A“. Portionsof the signal can thus be expanded enormously, however, thehigher the speed of TB B the darker the display will become asthe rep rate will remain that of the accepted signal triggers whilethe duration of TB B is reduced with increasing speed.In cases where there is jitter the TB B can be switched to waitfor a trigger rather than starting immediately. When a triggerarrives TB B will be started by it. The jitter is removed, however,the effect is also, that the TB B start now can be only from signalperiod to signal period, no continuous adjustment is possiblein this mode.Alternate sweepIn this mode the signal is displayed twice, with both time bases.An artifi cial Y offset can be added in order to separate the twodisplays on the screen. The operation is analogous to Y dualtrace alternate mode, i.e., the signal is alternately displayed byboth time bases, not simultaneously which is not possible witha single gun crt. TB B operation is the same here.T r i g g e r i n g a n d t i m e b a s eperiodheavy parts are displayedsignaladjustingHOLD OFF timesweepFig. 1Fig. 2