Hameg HM404-2.02 Manual

Hameg HM404-2.02 Manual Manual pdf 24 page image

Subject to change without notice24Triggering and timebaseFigure 1:Amplitude and frequency spectrum for AM display (m = 50%)Oscilloscope setting for a signal according to figure 2:• Y: CH. I; 20mV/div; AC.• TIME/DIV.: 0.2ms/div.• Triggering: Normal; with LEVEL-setting; internal(or external) triggering.Figure 2: Amplitude modulated oscillation:F = 1 MHz; f = 1 kHz;m = 50 %; UT = 28.3 mVrmsIf the two values a and b are read from the screen, themodulation factor is calculated fromwherea = UT (1+m) and b = U T (1-m).The variable controls for amplitude and time can be setarbitrarily in the modulation factor measurement. Their positiondoes not influence the result.Triggering and time baseAll controls regarding trigger and time base are located on theright of the VOLTS/DIV. knobs. They are described in thesection ”Controls and Readout”.Time related amplitude changes on a measuring signal (ACvoltage) are displayable in Yt-mode. In this mode the signalvoltage deflects the beam in vertical direction (Y) while thetime base generator moves the beam from the left to the rightof the screen (time deflection = t).Normally there are periodically repeating waveforms to bedisplayed. Therefore the time base must repeat the timedeflection periodically too. To produce a stationary display,the time base must only be triggered if the signal height andslope condition coincide with the former time base startconditions. A DC voltage signal can not be triggered as it is aconstant signal with no slope.Triggering can be performed by the measuring signal itself(internal triggering) or by an external supplied but synchronousvoltage (external triggering).The trigger voltage should have a certain minimum amplitude.This value is called the trigger threshold. It is measured witha sine signal. Except when external trigger is used the triggerthreshold can be stated as vertical display height in div, atwhich the time base generator starts, the display is stable,and the trigger indicator LED lights or flashes.The internal trigger threshold of the oscilloscope is given as ≤0.5div. When the trigger voltage is externally supplied, it canbe measured in Vpp at that input. Normally, the triggerthreshold may be exceeded up to a maximum factor of 20.The instrument has two trigger modes, which are characterizedas Automatic Peak and Normal triggering.Automatic Peak (Value) -TriggeringInstrument specific information can be drawn from the itemsNM - AT - (10) and LEVEL (12) in the section ”Controlsand Readout”.This trigger mode is automatically selected after the AUTOSET pushbutton was pressed. As the peak value detectionmakes no sense in combination with DC and TV (television)signals, it is switched off automatically in DC, TVL and TVFtrigger coupling conditions as well as in alternate triggermode. In this case the automatic is still present, but a wrongtrigger level setting causes an untriggered display.In automatic trigger mode the sweep generator can runwithout test signal or external trigger voltage. A base line willalways be displayed even with no signal. With an applied ACsignal the peak value triggering enables the user to select thevoltage point on the trigger signal (trigger point), by theadjustment of the trigger level control. The control rangedepends on the peak to peak value of the signal. This triggermode is therefore called Automatic Peak (Value)- Triggering.Operation of the scope needs only correct amplitude and timebase settings, for a constantly visible trace. Automatic modeis recommended for all uncomplicated measuring tasks.However, automatic triggering is also the appropriate operationmode for the ”entry” into difficult measuring problems, e.g.when the test signal is unknown relating to amplitude,frequency or shape. Presetting of all parameters is nowpossible with automatic triggering; the change to normaltriggering can follow thereafter.The automatic triggering works above 20Hz. The failure ofautomatic triggering at frequencies below 20Hz is abrupt.However, it is not signified by the trigger indicator LED this isstill blinking. Break down of triggering is best recognizable atthe left screen edge (the start of the trace in differing displayheight).The automatic peak (value) triggering operates over allvariations or fluctuations of the test signal above 20Hz.However, if the pulse duty factor of a square-wave signalexceeds a ratio of 100:1, switching over to normal triggeringwill be necessary. Automatic triggering is practicable withinternal and external trigger voltage.Normal TriggeringInformation specific to the instrument are given in the articlesNM - AT - (10), LEVEL (12) and TRIG. MODE (21) in thesection ”Controls and Readout”. The time fine adjustment(VAR.) and the hold off time setting assist in triggering underspecially difficult signal conditions. The following description