Hameg HM 2005 Manual 

Hameg HM 2005 Manual  Manual pdf 27 page image

27Subject to change without noticeHF: In this coupling mode the transmission range equals ahigh pass filter. It cuts off the DC content of the triggersignal and the lower frequency range.NR: Decreases the maximum trigger bandwidth for highfrequency signals. As in DC coupling condition, lowerfrequency signal parts are not affected if normal triggering(NM) is selected.LF: The LF coupling (low pass filter function) is often moresuited for low-frequency signals than the DC coupling,because the (white) noise in the trigger voltage is stronglysuppressed. So jitter or double-triggering of complexsignals is avoidable or at least reduced, in particular withvery low input voltages.TV-L: The built-in active TV-Sync-Separator provides theseparation of line sync pulses from the video signal.Even distorted video signals are triggered and displayedin a stable manner. This mode is described underparagraph “Triggering of video signals”.TV-F: The built-in active TV-Sync-Separator also provides theseparation of frame sync pulses from the video signal.Even distorted video signals are triggered and displayedin a stable manner.This mode is described under paragraph“Triggering ofvideo signals”.Triggering 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 or thereadout is active. Video signals are triggered in the automaticmode. The internal triggering is virtually independent of the displayheight, but the sync 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 slope setting forpositive going edges must be chosen. In the case of sync pulsesbelow the field/line, the leading edge is negative and consequentlythe slope selection must be set for falling edges. Since the invertfunction may cause a misleading display, it must not be activated.TVF: On the 2ms/div setting and field TV triggering selected, 1field is visible if a 50 fields/s signal is applied. If minimumhold off time is chosen, it triggers without line interlacingaffects caused by the consecutive field.More details in the video signal become visible if inalternate or B time base mode the B time base speed isincreased above the A time base speed. If the time baseis set to 5ms/div 2.5 frames are visible. The time delay forthe B time base can then be used to select any line(s) tobe displayed expanded on the screen. The influence ofthe integrating network which forms a trigger pulse fromthe vertical sync pulses may become visible under certainconditions.Due to the integrating network time constant not allvertical sync pulses starting the trace are visible.TVL: On the 10μs/div setting and line TV triggering selected,approx. 1½ lines are visible. Those lines originate fromthe odd and even fields at random.The sync-separator-circuit also operates with externaltriggering. It is important that the voltage range (0.3Vppto 3Vpp) for external triggering should be noted. Againthe correct slope setting is critical, because the externaltrigger signal may not have the same polarity or pulseedge as the test signal displayed on the CRT. This can bechecked, if the external trigger voltage itself is displayedfirst (with internal triggering).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.With a changing picture content (e.g. normal program),DC input coupling is recommended, because the displayvaries its vertical position on screen with AC input couplingat each change of the picture content. The DC contentcan be compensated using the Y-POS. control so that thesignal display lies in the graticule area. Then the compositevideo signal should not exceed a vertical height of 6div.Line triggering (~)The instrument specific information regarding this mode is partof the section“Controls and Readout” paragraph TRIG. MODE(23).This trigger mode is present if the trigger coupling information inthe readout displays “TR:~”. The symbol can be changed by 180°by using the trigger slope (12) pushbutton. The trigger pointsymbol is inactive in line/mains trigger mode as there is no directamplitude relationship between the trigger voltage and the signalvoltage.A voltage originating from mains/line (50 to 60Hz) is used fortriggering purposes if the trigger coupling is set to ~. This triggermode is independent of amplitude and frequency of the Y signaland is recommended for all mains/line synchronous signals. Thisalso applies within certain limits, to whole number multiples orfractions of the line frequency. Line triggering can also be usefulto display signals below the trigger threshold (less than 0.5div).It is therefore particularly suitable for measuring small ripplevoltages of mains/line rectifiers or stray magnetic field in a circuit.In this trigger mode the slope direction pushbutton selects thepositive or negative portion of the line/mains sine wave.The trigger level control can 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-upcoil. The coil should be wound on a small former with maximumturns of thin lacquered wire and connected to a BNC connector (forscope input) via a shielded cable. Between cable and BNC centerconductor a resistor of at least 100Ω should be series-connected(RF decoupling). Often it is advisable to shield the surface of thecoil. However, no shorted turns are permissible. Maximum,minimum, and direction to the magnetic source are detectable atthe measuring point by turning and shifting the coil.Alternate triggeringThis trigger mode can be selected in DUAL mode by pressingand holding the TRIG. pushbutton (20) if the preconditions areTriggering and Time Base