Hameg HM304 Manual

Hameg HM304 Manual Manual pdf 10 page image

Subject to change without notice10sources are only slightly loaded (approx. 10MΩ II 16pF or100MΩ II 9pF with HZ53). Therefore, if the voltage loss dueto the attenuation of the probe can be compensated by a higheramplitude setting, the probe should always be used. The seriesimpedance of the probe provides a certain amount of protectionfor the input of the vertical amplifier. Because of their separa-te manufacture, all attenuator probes are only partiallycompensated, therefore accurate compensation must beperformed on the oscilloscope (see Probe compensation ).Standard attenuator probes on the oscilloscope normallyreduce its bandwidth and increase the rise time. In all caseswhere the oscilloscope bandwidth must be fully utilized (e.g.for pulses with steep edges) we strongly advise using theprobes HZ51 (x10) HZ52 (x10 HF) and HZ54 (x1 and x10).This can save the purchase of an oscilloscope with largerbandwidth.The probes mentioned have a HF-calibration in addition tolow frequency calibration adjustment. Thus a group delaycorrection to the upper limit frequency of the oscilloscope ispossible with the aid of an 1MHz calibrator, e.g. HZ60.In fact the bandwidth and rise time of the oscilloscope arenot noticably changed with these probe types and thewaveform reproduction fidelity can even be improved becausethe probe can be matched to the oscilloscopes individual pulseresponse.If a x10 or x100 attenuator probe is used, DC inputcoupling must always be used at voltages above 400V.With AC coupling of low frequency signals, theattenuation is no longer independent of frequency,pulses can show pulse tilts. Direct voltages aresuppressed but load the oscilloscope input couplingcapacitor concerned. Its voltage rating is max. 400 V(DC + peak AC). DC input coupling is therefore of quitespecial importance with a x100 attenuation probewhich usually has a voltage rating of max. 1200 V (DC+ peak AC). A capacitor of corresponding capacitanceand voltage rating may be connected in series withthe attenuator probe input for blocking DC voltage (e.g.for hum voltage measurement).With all attenuator probes, the maximum AC input voltagemust be derated with frequency usually above 20kHz.Therefore the derating curve of the attenuator probe typeconcerned must be taken into account.The selection of the ground point on the test object isimportant when displaying small signal voltages. It shouldalways be as close as possible to the measuring point. If thisis not done, serious signal distortion may result from spuriouscurrents through the ground leads or chassis parts. Theground leads on attenuator probes are also particularly critical.They should be as short and thick as possible. When theattenuator probe is connected to a BNC-socket, a BNC-adapter, should be used. In this way ground and matchingproblems are eliminated. Hum or interference appearing inthe measuring circuit (especially when a small deflectioncoefficient is used) is possibly caused by multiple groundingbecause equalizing currents can flow in the shielding of thetest cables (voltage drop between the protective conductorconnections, caused by external equipment connected to themains/line, e.g. signal generators with interference protectioncapacitors).First Time OperationSwitch on the oscilloscope by depressing the red POWERpushbutton. The instrument will revert to its last usedoperating mode. Except in the case of COMP. TESTER mode,where a trace appears on the screen if the INTENS. knob isin center position, all LED‘s should remain unlit. The trace,displaying one baseline or the shorter COMP TESTERbaseline, should be visible after a short warm-up period ofapprox. 10 seconds. If the COMP TESTER mode is active,depress the COMP TESTER pushbutton once to switch toXY or Yt mode. In XY mode the XY LED in the TIME/DIVsection is lit, in this case depress the XY pushbutton once toswitch over to Yt mode. Adjust Y-POS.I and X-POS. controlsto center the baseline. Adjust INTENS. (intensity) and FOCUScontrols for medium brightness and optimum sharpness ofthe trace. The oscilloscope is now ready for use.• Rotate the variable controls with arrows, i.e. TIME/DIV.variable control, CH.I and CH.II attenuator variablecontrols, and HOLD OFF control to their calibrated detent.• Set all controls with marker lines to their midrange position(marker lines pointing vertically).••••• Depress the upper NORM. pushbutton until the ACsymbol on the trigger coupling scale is lit.• Both GD input coupling pushbutton switches for CH.I andCH.II in the Y-field should be set to the GD position (GD lit).If only a spot appears (CAUTION! CRT phosphor can bedamaged), reduce the intensity immediately and check thatthe XY mode is not selected (XY LED dark). If the trace isnot visible, check the correct positions of all knobs and modes(particularly NM LED - normal triggering - LED on).To obtain the maximum life from the cathode-ray tube, theminimum intensity setting necessary for the measurementin hand and the ambient light conditions should be used.Particular care is required when a single spot is displayed, asa very high intensity setting may cause damage to thefluorescent screen of the CRT. Switching the oscilloscopeoff and on at short intervals stresses the cathode of the CRTand should therefore be avoided.The instrument is so designed that even incorrect operationwill not cause serious damage.The HM304 accepts all signals from DC (direct voltage) up toa frequency of at least 35MHz (-3dB). For sinewave voltagesthe upper frequency limit will be 50MHz (-6dB).However, in this higher frequency range the vertical displayheight on the screen is limited to approx. 4-5div. The timeresolution poses no problem. For example, with 50MHz andthe fastest adjustable sweep rate (10ns/div.), one cycle willbe displayed every 2div. The tolerance on indicated valuesamounts to ±3% in both deflection directions. All values tobe measured can therefore be determined relativelyaccurately.However, from approximately 10MHz upwards the measuringerror will increase as a result of loss of gain. At 18MHz thisreduction is about 10%. Thus, approximately 11% should beadded to the measured voltage at this frequency. As thebandwidth of the amplifiers may differ slightly (normallybetween 35 and 38MHz), the measured values in the upperfrequency range cannot be defined exactly. Additionally, asalready mentioned, for frequencies above 35MHz the dynamicrange of the display height steadily decreases. The verticalamplifier is designed so that the transmission performanceis not affected by its own overshoot.Trace Rotation TRIn spite of Mumetal-shielding of the CRT, effects of the earthsmagnetic field on the horizontal trace position cannot becompletely avoided. This is dependent upon the orientation