Hameg HM1008-2 Manual

Hameg HM1008-2 Manual Manual pdf 10 page image

10 Subject to change without noticeBasic signal measurementSignals which can be measuredThe following description pertains to analog and digital ope-ration. The different specifications in both operating modesshould be kept in mind.The oscilloscope HM1008-2 can display all repetitive signalswith a fundamental repetition frequency of at least 100 MHz.The frequency response is 0 to 100 MHz (-3 dB). The verticalamplifiers will not distort signals by overshoots, undershoots,ringing etc.Simple electrical signals like sine waves from line frequencyripple to hf will be displayed without problems. However, whenmeasuring sine waves, the amplitudes will be displayed with anerror increasing with frequency. At 80 MHz the amplitude errorwill be around –10 %. As the bandwidths of individual instru-ments will show a certain spread (the 100 MHz is a guaranteedminimum) the actual measurement error for sine waves cannotbe exactly determined.Pulse signals contain harmonics of their fundamental fre-quency which must be represented, so the maximum usefulrepetition frequency of non sinusoidal signals is much lowerthan 100 MHz (5 to 10 times). The criterion is the relationshipbetween the rise times of the signal and the scope; the scope’srise time should be <1/3 of the signal’s rise time if a faithfulreproduction without too much rounding of the signal shapeis to be preserved.The display of a mixture of signals is especially difficult if itcontains no single frequency with a higher amplitude as thescope’s trigger system normally discriminates by amplitude.This is typical of burst signals for example. Display of suchsignals may require using the HOLD OFF control.Composite video signals may be displayed easily as the instru-ment has a tv sync separator.The maximum sweep speed of 5 ns/cm allows sufficient timeresolution, e.g. a 100 MHz sine wave will be displayed one periodper 2 cm.The vertical amplifier inputs may be DC or AC coupled. Use DCcoupling only if necessary and preferably with a probe.Low frequency signals when AC coupled will show tilt (AC lowfrequency – 3 dB point is 1.6 Hz), so if possible use DC coupling.Using a probe with 10:1 or higher attenuation will lower the–3 dB point by the probe factor. If a probe cannot be used dueto the loss of sensitivity, DC coupling the scope and an externallarge capacitor may help which, of course, must have a sufficientDC rating. Care must be taken, however, when charging anddischarging a large capacitor.DC coupling is preferable with all signals of varying duty cycle,otherwise the display will move up and down depending on theduty cycle. Of course, pure DC can only be measured with DCcoupling.The readout will show which coupling was chosen: = stands forDC, ~ stands for AC.Amplitude of signalsIn contrast to the general use of rms values in electrical engi-neering oscilloscopes are calibrated in Vpp as that is what isdisplayed.To derive rms from V pp: divide by 2.84. To derive Vpp from rms:multiply by 2.84.Values of a sine wave signalVrms = rms valueV pp = pp – valueVmom = momentary value, depends on time vs. period.The minimum signal for a one cm display is 1 mVpp ±5 % provi-ded 1 mV/cm was selected and the variable is in the calibratedposition.The available sensitivities are given in mVpp or Vpp. The cursorslets you read the amplitudes of the signals immediately on thereadout as the attenuation of probes is automatically taken intoaccount. Even if the probe attenuation was selected manuallythis will be overridden if the scope identifies a probe withdifferent identification contact. The readout will always give thetrue amplitude.It is important that the variable be in its calibrated position. Thesensitivity may be continuously decreased by using the variable(see chapt. Controls and Readout). Each intermediate valuebetween the calibrated positions 1–2–5 may be selected. Thusa maximum of 400 Vpp may be displayed without using a probe(20 V/div x 8 cm screen x 2.5 variable).Amplitudes may be directly read off the screen by measuringthe height and multiplying by the V/div. setting.STOPPlease note: Without a probe the maximum permis-sible voltage at the inputs must not exceed 400 Vpirrespective of polarity.In case of signals with a DC content the peak value DC + ACpeak must not exceed + or – 400 Vp . Pure AC of up to 800 V ppis permissible.STOPIf probes are used their possibly higher ratings areonly usable if the scope is DC coupled.In case of measuring DC with a probe while the scope input isAC coupled the capacitor in the scope input will see the inputDC voltage as it is in series with the internal 1 MΩ resistor.This means that the maximum DC voltage (or DC + peak AC) isthat of the scope input, i.e. 400 VP ! With signals which containDC and AC the DC content will stress the input capacitor whilethe AC content will be divided depending on the AC impedanceB a s i c s i g n a l m e a s u r e m e n tVp VrmsV momVppTest Equipment Depot - 800.517.8431 - 99 Washington Street Melrose, MA 02176FAX 781.665.0780 - TestEquipmentDepot.com