Hameg HM304 Manual

Hameg HM304 Manual Manual pdf 15 page image

15Subject to change without noticecoupling mode. In this case the automatic is still present, buta wrong TRIG. LEVEL setting causes an untriggered display.Automatic triggering is practicable with internal and externaltrigger voltage.In alternate triggering mode (TR I and TR II lit) the peak valuedetection is switched off.Normal TriggeringWith normal triggering (both NORM pushbuttons depresseduntil the NM LED is lit) and TRIG. LEVEL adjustment, thesweep can be started by AC signals within the frequencyrange defined by the TRIG. coupling setting. In the absenceof an adequate trigger signal or when the trigger controls(particularly the TRIG. LEVEL control) are misadjusted, notrace is visible, i.e. the screen blanked completely.When using the internal normal triggering mode, it is possibleto trigger at any amplitude point of a signal edge, even withvery complex signal shapes, by adjusting the TRIG. LEVELcontrol. Its adjusting range is directly dependent on the displayheight, which should be at least 0.5div. If it is smaller than1div., the TRIG. LEVEL adjustment needs to be operated witha sensitive touch. In the external normal triggering mode,the same applies to approx. 0.3Vpp external trigger voltageamplitude.Other measures for triggering of very complex signals arethe use of the time base variable control and HOLDOFF timecontrol, hereinafter mentioned.SlopeThe time base generator can be triggered by a rising or fallingedge of the test signal. The ± pushbutton marking selectsthe slope polarity. If the LED above the pushbutton is lit, the( - ) falling edge is used for triggering. This is valid withautomatic and normal triggering. The positive (+) slopedirection (LED dark) means an edge going from a negativepotential and rising to a positive potential. This has nothingto do with zero or ground potential and absolute voltagevalues. The positive slope may also lie in a negative part of asignal. A falling ( - ) edge will trigger the timebase when theminus symbol is lit.However the trigger point may be varied within certain limitson the chosen edge using the LEVEL control. The slopedirection is always related to the input signal and the noninverted display.Trigger couplingThe coupling mode and accordingly the frequency range ofthe trigger signal can be changed using the upper or lowerNORM pushbutton. The selected coupling mode is indicatedon the LED scale above.AC: Trigger range <20Hz to 100MHz.This is the most frequently used trigger mode. The triggerthreshold is increasing below 20Hz and above 100MHz.The AUTO SET function always selects AC triggercoupling.DC: Trigger range DC to 100MHz.DC triggering is recommended, if the signal is to betriggered with quite slow processes or if pulse signalswith constantly changing pulse duty factors have to bedisplayed.With DC- or LF-trigger coupling, always work with nor-mal triggering (NM) and TRIG.LEVEL adjustment. Ifautomatic (peak-value) triggering was in use, the peakvalue detection is then switched off automatically.LF: Trigger range DC to 1.5kHz (low-pass filter).The LF coupling is often more suited for low-frequencysignals than the DC coupling, because the (white) noisein the trigger voltage is strongly suppressed. So jitter ordouble-triggering of complex signals is avoidable or atleast reduced, in particular with very low input voltages.The trigger threshold increases above 1.5kHz.TV-L / TV-F: The built-in active TV-Sync-Separator providesthe separation of sync pulses from the video signal.Evendistorted video signals are triggered and displayed in astable manner.Video signals are triggered in the automatic mode (NM LEDdark). The internal triggering is virtually independent of thedisplay height, but the sync pulse must exceed 0.5div. height.TV-L is for line sync pulse separation and triggering, whileTV-F is for field sync pulse separation and triggering.The slope of the leading edge of the synchronization pulse iscritical for the SLOPE selection. If the displayed sync pulsesare above the picture (field) contents (leading edge positivegoing), then the positive going SLOPE (+) must be chosen.In the case of sync pulses below the field/line, the leadingedge is negative and - (minus) symbol above the ± pushbuttonmust lit. Since the INV (invert) function may cause amisleading display, it must not be activated (INV LED dark).On the 2ms/div setting and field TV triggering (TV-F) selected1 field is visible if a 50 fields/s signal is applied. If the hold offcontrol is in fully ccw position, it triggers without lineinterlacing affects caused by the consecutive field. Moredetails in the video signal become visible if in delayed triggermode the timebase speed is increased (see DELAY / AFTERDELAY TRIGGERING). The X-MAG. (x10) expansion may alsobe used (x10 LED lit). The influence of the integrating networkwhich forms a trigger pulse from the vertical sync pulsesmay become visible under certain conditions. Due to theintegrating network time constant not all vertical sync pulsesstarting the trace are visible.Disconnecting the trigger circuit (e.g. by double depressingand releasing the EXT. button next to the TRIG. INP.BNC socket in the Y field) can usually result in triggering theconsecutive (odd or even) field.On the 10μs/div setting and line TV triggering (TV-L) selected,approx. 1½ lines are visible. Those lines originate from theodd and even fields at random.The sync-separator-circuit also operates with externaltriggering. It is important that the voltage range (0.3Vpp to3Vpp) for external triggering should be noted. Again thecorrect slope setting is critical, because the external triggersignal may not have the same polarity or pulse edge as thetest signal displayed on the CRT. This can be checked, if theexternal trigger voltage itself is displayed first (with internaltriggering).In most cases, the composite video signal has a high DCcontent. With constant video information (e.g. test patternor color bar generator), the DC content can be suppressedeasily by AC input coupling of the oscilloscope amplifier.Witha changing picture content (e.g. normal program), DC inputcoupling is recommended, because the display varies itsvertical position on screen with AC input coupling at eachchange of the picture content. The DC content can becompensated using the Y-POS. control so that the signaldisplay lies in the graticule area. Then the composite videosignal should not exceed a vertical height of 6div.