Hameg HM 2005 Manual 

Hameg HM 2005 Manual  Manual pdf 28 page image

Subject to change without notice28met (please note “Controls and Readout”). In the case ofchopped DUAL mode, selecting alternate trigger modeautomatically sets the instrument to alternate DUAL mode. Thetrigger point symbol and the peak value detection (in automatictrigger mode) are internally deactivated. Only the following triggercoupling modes are available in alternate trigger mode: AC, DC,NR, HF and LF.With alternate triggering it is possible to trigger two signals whichare different in frequency (asynchronous). Each input signal mustbe of sufficient height to enable trigger. To avoid trigger problemsdue to different DC voltage components, AC input coupling forboth channels is recommended.The internal trigger source is switched in alternate trigger modein the same way as the channel switching system in DUALalternate mode, i.e. after each time base sweep. Phase differencemeasurement is not possible in this trigger mode as the triggerlevel and slope setting are equal for both signals. Even with 180°phase difference between both signals, they appear with thesame slope direction. If signals are applied with a high frequencyratio (difference), the trace intensity then becomes reduced ifthe time base is set to smaller time coefficients (faster sweep).This happens as the number of sweeps does not increasebecause it depends on the lower frequency signal, but with afaster sweep the phosphor becomes less activated.External triggeringThe external trigger input is activated with the aid of the TRIG.(20) pushbutton (see “Controls and Readout”), if the triggercoupling is not set to line/mains trigger coupling. Then the internaltrigger source is deactivated.As the external trigger signal applied at the TRIG. EXT socketnormally has no relation to the signal height of the displayedsignal, the trigger point symbol is switched off. The externaltrigger voltage must have a minimum amplitude of 0.3V pp andshould not increase above 3V pp. The input impedance of theTRIG. 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 a completely different formfrom the test signal voltage, but must be synchronous with thetest signal. Triggering is even possible in certain limits with wholenumber multiples or fractions 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.Trigger indicator “TR”The following description applies to the “TR” LED. Please noteitem (14) under “Controls and Readout”.An LED on condition indicates that the trigger signal has asufficient amplitude and the trigger level control setting is correct.This is valid with automatic and with normal triggering. Byobserving the trigger LED, sensitive trigger level adjustment ispossible when normal triggering is used, particularly at very lowsignal frequencies. The indication pulses are of only 100msduration.Thus for fast signals the LED appears to glow continuously, forlow repetition rate signals, the LED flashes at the repetition rateor at a display of several signal periods not only at the start ofthe sweep at the left screen edge, but also at each signal period.In automatic triggering mode the sweep generator startsrepeatedly without test signal or external trigger voltage. If thetrigger signal frequency decreases the trigger automaticrepetition frequency the sweep generator starts without awaitingthe trigger pulse. This causes an untriggered display and a flashingtrigger LED.HOLD OFF-time adjustmentFor instrument specific information please note DEL.POS. - HO(24) in section “Controls and Readout”.If it is found that a trigger point cannot be found on extremelycomplex signals, even after careful adjustment of the trigger levelcontrol, a stable display may often be obtained using the holdoffcontrol. This facility varies the holdoff time between two sweepperiods approx. up to the ratio 10:1. Pulses or other signalwaveforms appearing during this off period cannot trigger thetime base. Particularly with burst signals or aperiodic pulse trainsof the same amplitude, the start of the sweep can be delayeduntil the optimum or required time.A very noisy signal or a signal with a higher interfering frequencyis at times displayed double. It is possible that trigger leveladjustment only controls the mutual phase shift, but not thedouble display. The stable single display of the signal, requiredfor evaluation, is easily obtainable by expanding the hold off timeuntil one signal is displayed.A double display is possible with certain pulse signals, wherethe pulses alternately show a small difference of the peakTriggering and Time BaseFig. 1 shows a case where the holdoff control is in the minimumposition and various different waveforms are overlapped onthe screen, making the signal observation unsuccessful.Fig. 2 shows a case where only the desired parts of the signal arestably displayed.