Hameg HM 407A Manual

28 Subject to change without noticeTriggering of video signalsIn TV-L and TV-F trigger coupling mode the instrument isautomatically set to automatic triggering and the trigger pointindicator is switched off. As only the separated synchronizationpulses are used for triggering the relationship between thedisplayed signal and the trigger signal is lost. In TV-F modeinterference may occur if chopped DUAL mode is chosen orthe readout is active.Video signals are triggered in the automatic mode. The internaltriggering is virtually independent of the display height, but thesync pulse must exceed 0.5div height.The polarity of the synchronization pulse is critical for the slopeselection. If the displayed sync pulses are above the picture(field) contents (leading edge positive going), then the slopesetting for positive going edges must be chosen. In the caseof sync pulses below the field/line, the leading edge is negativeand consequently the slope selection must be set for fallingedges. Since the invert function may cause a misleadingdisplay, it must not be activated.On the 2ms/div setting and field TV triggering selected, 1 fieldis visible if a 50 fields/s signal is applied. If the hold off controlis in fully ccw position, it triggers without line interlacingaffects caused by the consecutive field.The display can be expanded by switching on the X-MAG. x10function so that individual lines are recognizable. Commencingwith a frame synchronizing pulse, the display can also beexpanded 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 offset 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 be compensated using the Y-POS. controlso that the signal display lies in the graticule area. Then thecomposite video signal should not exceed a vertical height of6div.Line triggering (~)The instrument specific information regarding this mode ispart of the section “Controls and readout” paragraph TRIG.MODE (26).This trigger mode is present if the ~ LED is lit (26). 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 of theY 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 trigger modethe slope direction pushbutton selects the positive or negativeportion of the line/mains sinewave. The trigger level controlcan be used for trigger point adjustment.Magnetic leakage (e.g. from a power transformer) can beinvestigated for direction and amplitude using a search or pick-up coil. The coil should be wound on a small former with amaximum 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 the measuringpoint by turning and shifting the coil.Alternate triggeringThis trigger mode can be selected in DUAL mode by pressingand holding the TRIG. pushbutton (23) 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. Ifthe trigger coupling is set to TV-L or TV-F, selecting alternatetriggering automatically results in AC coupling conditions.Under line/mains triggering condition alternate triggering cannot be chosen. Thus only the following trigger coupling modesare available in alternate trigger mode: AC, DC, HF and LF. Thetrigger point symbol is internally inactivated.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 isswitched in alternate trigger mode in the same way as thechannel switching system in DUAL alternate mode, i.e. aftereach time base sweep. Phase difference measurement is notpossible in this trigger mode as the trigger level and slopesetting are equal for both signals. Even with 180° phasedifference between both signals, they appear with the sameslope 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.(23) pushbutton (see “Controls and readout”), if the triggerTriggering and time base