Hameg HM304 Manual

Hameg HM304 Manual Manual pdf 24 page image

Subject to change without notice24Of course, the quality of the transmission performance isnot only dependent on the vertical amplifier. The inputattenuators, located in the front of the amplifier, arefrequency-compensated in each position. Even smallcapacitive changes can reduce the transmission performance.Faults of this kind are as a rule most easily detected with asquare-wave signal with a low repetition rate (e.g. 1kHz). If asuitable generator with max. output of 40Vpp is available, itis advisable to check at regular intervals the deflectioncoefficients on all positions of the input attenuators andreadjust them as necessary. A compensated 2:1 seriesattenuator is also necessary, and this must be matched tothe input impedance of the oscilloscope. This attenuator canbe made up locally. It is important that this attenuator isshielded. For local manufacture, the electrical componentsrequired are a 1MΩ ±1% resistor and, in parallel with it, atrimmer 3-15pF in parallel with approx. 12pF. One side ofthis parallel circuit is connected directly to the input connectorof CH.I or CH.II and the other side is connected to thegenerator, if possible via a low-capacitance coaxial cable. Theseries attenuator must be matched to the input impedanceof the oscilloscope in the 5mV/div. position (variable controlto CAL., DC input coupling; square tops exactly horizontal;no ramp-off is permitted). This is achieved by adjusting thetrimmer located in the 2:1 attenuator. The shape of the square-wave should then be the same in each input attenuatorposition.Operating Modes: CH.I/II, DUAL, ADD,CHOP., INVERT and X-Y OperationIn DUAL mode two traces must appear immediately. Onactuation of the Y-POS. controls, the trace positions shouldhave minimal effect on each other. Nevertheless, this cannotbe entirely avoided, even in fully serviceable instruments.When one trace is shifted vertically across the entire screen,the position of the other trace must not vary by more than0.5mm.A criterion in chopped operation is trace widening andshadowing around and within the two traces in the upper orlower region of the screen. Set TIME/DIV. switch to 0.5ms/div., set input coupling of both channels to GD and advancethe INTENS. control fully clockwise. Adjust FOCUS for a sharpdisplay. With the Y-POS. controls shift one of the traces to a+2div., the other to a -2div. vertical position from the hori-zontal center line of the graticule.Do not try to synchronize (with the time variable control) thechop frequency (0.5MHz)! Check for negligible trace wideningand periodic shadowing when switching between 0.5ms/divand 0.2ms/div.It is important to note that in the I+II add mode (ADD LEDlights) or the I-II difference mode (one INV LED lights) thevertical position of the trace can be adjusted by using boththe Channel I and Channel II Y-POS. controls.In X-Y Operation (XY LED in TIME/DIV sector lights), thesensitivity in both deflection directions will be the same. Whenthe signal from the built-in square-wave generator is appliedto the input of Channel II, then, as with Channel I in thevertical direction, there must be a horizontal deflection of4div. when the deflection coefficient is set to 50mV/div.position (variable control set to its CAL. position). The checkof the mono channel display is unnecessary; it is containedindirectly in the tests above stated.Triggering ChecksThe internal trigger threshold is important as it determinesthe display height from which a signal will be stably displayed.It should be approx. 0.3-0.5div. for the HM304. An increasedtrigger sensitivity creates the risk of response to the noiselevel in the trigger circuit. This can produce double-triggeringwith two out-of-phase traces.Alteration of the trigger threshold is only possible internally.Checks can be made with any sine-wave voltage between50Hz and 1MHz. The instrument should be in automatic peak(value) triggering (NM LED dark) and the TRIG. LEVEL knobin midrange position. It should be ascertained whether thesame trigger sensitivity is also present with Normal Triggering(NM LED lights). In this trigger mode, TRIG. LEVELadjustment is absolutely necessary.The checks should show the same trigger threshold withthe same frequency. On depressing the ± (SLOPE) button,the start of the sweep changes from the positive-going tothe negative-going edge of the trigger signal.As described in the Operating Instructions, the triggerfrequency range is dependent on the trigger couplingselected. For lower frequencies the LF coupling mode canbe selected. In this mode, triggering up to at least 1.5kHz(sine-wave) is possible. Internally the HM304 should triggerperfectly at a display height of approx. 0.5div., when theappropriate trigger coupling mode is set.For external triggering (EXT LED lights), the TRIG. INP.connector requires a signal voltage of at least 0.3Vpp, whichis in synchronism with the Y input signal. The voltage valueis dependent on the frequency and the trigger coupling mode(AC-DC-HF-LF).Checking of the TV triggering is possible with a video signalof any given polarity.Use the TV-L or TV-F setting for video sync pulse separation.With the ± (SLOPE) button the correct slope of the syncpulse (front edge) must be selected and a suited TIME/DIVsetting must be chosen. The slope is then valid for both syncfrequencies.Perfect TV triggering is achieved, when in both display modesthe amplitude of the complete TV signal (from white level tothe top of the line sync pulse) is limited between 0.8 and6div and sync pulses of more then 0.5 div. height. The displayshould not shift horizontally during a change of the triggercoupling from AC to DC when displaying a sine-wave signalwithout DC offset.If both vertical inputs are AC coupled to the same signal andboth traces are brought to coincide exactly on the screen,when working in the alternate dual channel mode, then nochange in display should be noticeable, when switching fromTRIG I to TRIG II or when the trigger coupling is changedfrom AC to DC.Checking of the line/mains frequency triggering (50-60Hz) ispossible, when the input signal is time-related (multiple orsubmultiple) to the power line frequency ( ~ LED lights). Thereis no trigger threshold visible in this trigger mode. Even verysmall input signals are triggered stably (e.g. ripple voltage).For this check, use an input of approx. 1V. The displayedsignal height can then be varied by turning the respectiveinput attenuator switch and its variable control.TimebaseBefore checking the timebase it should be ascertained thatthe trace length is approx. 10div. in all time ranges. If not, itcan be corrected with the potentiometer X x1. Thisadjustment should be made with the TIME/DIV. switch in a