Hameg HM404-2.02 Manual

Hameg HM404-2.02 Manual Manual pdf 26 page image

Subject to change without notice26Triggering and timebaseexpanded with the knob TIME/DIV. But note that this canresult in an apparently unsynchronized display as each frame(half picture) triggers. This is due to the off set of half a linebetween frames.The influence of the integrating network which forms a triggerpulse from the vertical sync pulses may become visible undercertain conditions. Due to the integrating network time constantnot all vertical sync pulses starting the trace are visible.On the 10μs/div setting and line TV triggering selected, approx.1½ lines are visible. Those lines originate from the odd andeven fields at random.The sync-separator-circuit also operates with external triggering.It is important that the voltage range (0.3Vpp to 3Vpp) forexternal triggering should be noted. Again the correct slopesetting is critical, because the external trigger signal may nothave the same polarity or pulse edge as the test signaldisplayed on the CRT. This can be checked, if the externaltrigger 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 ( ~)The instrument specific information regarding this mode ispart of the section ”Controls and Readout” paragraph TRIG.MODE (21).This trigger mode is present if the ~ LED is lit (21). The triggerpoint symbol is inactive in line/mains trigger mode as there isno direct amplitude relationship between the trigger voltageand the signal voltage.A voltage originating from mains/line (50 to 60Hz) is used fortriggering purposes if the trigger coupling is set to ~. Thistrigger mode is independent of amplitude and frequency ofthe Y signal and is recommended for all mains/line synchronoussignals. This also applies within certain limits, to wholenumber multiples or fractions of the line frequency. Linetriggering 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 direction pushbutton selects the positive ornegative portion of the line/mains sinewave. The trigger levelcontrol can be used for 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 connectedto a 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 triggeringThis trigger mode can be selected in DUAL mode by pressingand holding the TRIG. pushbutton (18) if the preconditions aremet (please note ”Controls and Readout”). In the case ofchopped DUAL mode, selecting alternate trigger modeautomatically sets the instrument to alternate DUAL mode.Under TV-L, TV-F and line/mains triggering conditions alter-nate triggering can not be chosen. Thus only the followingtrigger coupling modes are available in alternate trigger mode:AC, DC, HF and LF. The trigger point symbol is not displayedin alternate trigger mode.With alternate triggering it is possible to trigger two signalswhich are different in frequency (asynchronous). In this casethe oscilloscope must be operated in DUAL alternate modeand internal triggering each input signal must be of sufficientheight to enable trigger. To avoid trigger problems due todifferent DC voltage components, AC input coupling for bothchannels is recommended.The internal trigger source is switched in alternate triggermode in the same way as the channel switching system inDUAL alternate mode, i.e. after each time base sweep. Phasedifference measurement is not possible in this trigger modeas the trigger level and slope setting are equal for both signals.Even with 180° phase difference between both signals, theyappear with the same slope direction.If signals are applied with a high frequency ratio (difference),the trace intensity then becomes reduced if the time base isset to smaller time coefficients (faster sweep). This happensas the number of sweeps does not increase because itdepends on the lower frequency signal, but with a fastersweep the phosphor becomes less activated.External triggeringThe external trigger input is activated with the aid of the TRIG.(18) pushbutton (see ”Controls and Readout”), if the triggercoupling is not set to line/mains trigger coupling. Then theinternal trigger source is deactivated. As the external triggersignal applied at the TRIG. EXT socket normally has norelation to the signal height of the displayed signal, the triggerpoint symbol is switched off. The external trigger voltagemust have a minimum amplitude of 0.3Vpp and should notincrease above 3Vpp. The input impedance of the TRIG. EXT.socket is approx. 1MΩ II 20pF.The maximum input voltage of the input circuit is 100V(DC+peak AC). The external trigger voltage may have acompletely different form from the test signal voltage, butmust be synchronous with the test signal. Triggering is evenpossible in certain limits with whole number multiples orfractions of the test frequency.It must be noted that a different phase angle between themeasuring and the triggering signal may cause a display notcoinciding with the slope selection setting.The trigger coupling selection can also be used in externaltriggering mode.