Hameg HM304 Manual

Hameg HM304 Manual Manual pdf 14 page image

Subject to change without notice14whereU T= unmodulated carrier amplitudeΩΩΩΩΩ = 2πF = angular carrier frequencyωωωωω = 2πf = modulation angular frequencym = modulation factor (i.a. œ 1 100%).The lower side frequency F-f and the upper side frequencyF+f arise because of the modulation apart from the carrierfrequency F.Amplitude and frequency spectrum for AM display (m = 50%)The display of the amplitude-modulated HF oscillation can beevaluated with the oscilloscope provided the frequencyspectrum is inside the oscilloscope bandwidth. The time baseis set so that several cycles of the modulation frequency arevisible. Strictly speaking, triggering should be external withmodulation frequency (from the AF generator or ademodulator). However, internal triggering is frequentlypossible with normal triggering (NM LED lit) buttondepressed) using a suitable TRIG. LEVEL setting and possiblyalso using the time variable adjustment.Oscilloscope setting for a signal according to figure 2:• Y: CH. I; 20mV/div.; AC.• TIME/DIV.: 0.2ms/div.• Triggering: Normal (NM LED lit); with LEVEL-setting;internal (or external) triggering.Figure 2Amplitude modulated oscillaton: F = 1 MHz; f = 1 kHz;m = 50 %; UT = 28.3 mVrms.If the two values a and b are read from the screen, themodulation factor is calculated fromwhere a = UT (1+m) and b = UT (1-m).The variable controls for amplitude and time can be setarbitrarily in the modulation factor measurement. Theirposition does not influence the result.Triggering and time baseTime related amplitude changes on a measuring signal (ACvoltage) are displayable in Yt-mode. In this mode the signalvoltage deflects the beam in vertical direction while thetimebase generator moves the beam from the left to theright of the screen (time deflection).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.,through which the time base generator starts, the display isstable, and the trigger LED (located in the X field above thetrigger coupling scale) lights.The internal trigger threshold of the HM304 is given as ≤5div. When the trigger voltage is externally supplied, it canbe measured in Vpp at the TRIG. INP. socket. Normally, thetrigger threshold may be exceeded up to a maximum factorof 20.The HM304 has two trigger modes, which are characterizedin the following.Automatic Peak-TriggeringThe triggerring mode is indicated by the NM LED beside theNORM pushbuttons on the X field of the front panel.Automatic triggering is selected if the NM LED is unlit,otherwise simultaneously depress both NORM pushbuttonsto select automatic triggering. Then the sweep generatorwill be running without test signal or external trigger voltage.A base line will always be displayed even with no signal.With an applied AC signal the peak value triggering enablesthe user to select the voltage point on the trigger signal, bythe adjustment of the TRIG. LEVEL control. The TRIG. LEVELcontrol range depends on the peak to peak value of the signal.This trigger mode is therefore called Automatic Peak (Value)-Triggering. Operation of the scope needs only correctamplitude and timebase settings, for a constantly visible trace.Automatic mode is recommended for all uncomplicatedmeasuring tasks. However, automatic triggering is also theappropriate operation mode for the „entry“ into difficultmeasuring problems, e.g. when the test signal is unknownrelating to amplitude, frequency or shape. Presetting of allparameters is now possible with automatic triggering; thechange to normal triggering can follow thereafter. AUTO SETtherefore sets the instrument to automatic peak-triggeringmode in combination with AC trigger coupling.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 (aboveTRIG.) this is still blinking. Break down of triggering is bestrecognizable at the left screen edge (the start of the trace indiffering display height).The automatic peak triggering operates over all variations orfluctuations of the test signal above 20Hz. However, if thepulse duty factor of a square-wave signal exceeds a ratio of100:1, switching over to normal triggering will be necessary.As the peak value detection makes no sense in combinationwith DC signals, it is switched off automatically in DC triggerΩ Ω ω Ω ω