Hameg HM303-6 Manual

Also see for HM303-6: Operator's manual Service manual

Contents

16 Subject to change without noticetriggering should be noted. Again the correct slope setting iscritical, because the external trigger signal may not have thesame polarity or pulse edge as the test signal. This can bechecked, if the external trigger voltage itself is displayed first(with internal triggering).In most cases, the composite video signal has a high DCcontent. With constant video information (e.g. test pattern orcolor bar generator), the DC content can be suppressed easilyby AC input coupling of the oscilloscope amplifier. With achanging 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.Line triggering (~)A voltage originating from mains/line (50 to 60Hz) is used fortriggering purposes if the AT/NM and ALT pushbuttons aredepressed ( ~ symbol). When the AT/NM pushbutton isdepressed the instrument is automatically set to normaltriggering.This trigger mode is independent of amplitude and frequencyof the Y signal and is recommended for all mains/linesynchronous signals. This also applies within certain limits towhole number multiples or fractions of the line frequency.Line triggering can also be useful to display signals below thetrigger threshold (less than 0.5div). It is therefore particularlysuitable for measuring small ripple voltages of mains/linerectifiers or stray magnetic field in a circuit. In this triggermode the SLOPE pushbutton selects the positive or negativeportion of the line sinewave. The LEVEL control is operativefor trigger point adjustment.Magnetic leakage (e.g. from a power transformer) can beinvestigated for direction and amplitude using a search orpick-up coil. The coil should be wound on a small former witha maximum of turns of a thin lacquered wire and connected toa BNC connector (for scope input) via a shielded cable.Between cable and BNC center conductor a resistor of at least100Ω should be series-connected (RF decoupling). Often it isadvisable to shield statically the surface of the coil. However,no shorted turns are permissible. Maximum, minimum, anddirection to the magnetic source are detectable at themeasuring point by turning and shifting the coil.Alternate triggeringWith alternate triggering (ALT button depressed) it is possibleto trigger two signals which are different in frequency(asynchronous). In this case the oscilloscope must be operatedin alternate DUAL mode with signals of sufficient height ateach input. To avoid trigger problems due to different DCvoltage components, AC input coupling for both channels isrecommended.The peak (value) detection which is normally present inautomatic triggering mode (AT), is automatically switched off;the LEVEL control becomes inoperative and the triggerthreshold is set to 0 Volt.The internal trigger source is switched in the same way as thechannel switching after each time base sweep.Phase difference measurement is not possible in this triggermode.External triggeringThe internal triggering is disconnected by depressing the TRIG.EXT. button. The time base can be triggered externally via theTRIG. EXT. socket using a 0.3V pp to 3V pp voltage, which is insynchronism with the test signal. This trigger voltage may havecompletely different form from the test signal voltage. Triggeringis even possible in certain limits with whole number multiplesor fractions of the test frequency, but only with synchronoussignals.The input impedance of the TRIG. EXT. socket is approx.100kΩ II 10pF. The maximum input voltage of the input circuitis 100V (DC+peak AC).It must be noted that a different phase angle between themeasuring and the triggering signal may cause a display notcoinciding with the SLOPE pushbutton setting. The triggercoupling selection can also be used in external triggering mode.Unlike internal triggering, the lowest frequency for externaltriggering is 20Hz in all trigger coupling conditions.Trigger indicatorAn LED on condition (above the SLOPE switch) indicates thatthe trigger signal has a sufficient amplitude and the LEVELcontrol setting is correct. This is valid with automatic and withnormal triggering. The indication of trigger action facilitates asensitive LEVEL adjustment, particularly at very low signalfrequencies. The indication pulses are of only 100ms duration.Thus for fast signals the TR-LED appears to glow continuously,for low repetition rate signals, the LED flashes at the repetitionrate or at a display of several signal periods not only at the startof the sweep at the left screen edge, but also at each signalperiod.In automatic triggering mode the sweep generator startsrepeatedly without test signal or external trigger voltage. If thetrigger signal frequency is <20Hz the sweep generator startswithout awaiting the trigger pulse. This causes an untriggereddisplay and a flashing trigger LED (TR).Holdoff-time adjustmentIf it is found that a trigger point cannot be located on extremelycomplex signals even after repeated and careful adjustment ofthe LEVEL control, a stable display may be obtained using theHOLDOFF control. This facility varies the hold-off time betweentwo sweep periods approx. up to the ratio 10:1. Pulses or othersignal waveforms appearing during this off period cannottrigger the time base. Particularly with burst signals or aperiodicpulse trains of the same amplitude, the start of the sweep canbe delayed until the optimum or required moment.A very noisy signal or a signal with a higher interfering frequencyis at times displayed double. It is possible that LEVEL adjustmentonly controls the mutual phase shift, but not the double display.The stable single display of the signal, required for evaluation,is easily obtainable by expanding the holdoff time. To this endthe HOLD OFF knob is slowly turned to the right, until onesignal is displayed. A double display is possible with certainpulse signals, where the pulses alternately show a smalldifference of the peak amplitudes. Only a very exact LEVELadjustment makes a single display possible. The use of theHOLD OFF knob simplifies the right adjustment.After specific use the HOLD OFF control should be reset intoits calibration detent (fully ccw), otherwise the brightness ofthe display is reduced drastically. The function is shown in thefollowing figures.Triggering and time base