Hameg HM 2005 Manual 

Hameg HM 2005 Manual  Manual pdf 8 page image

Subject to change without notice8Basics of signal voltageType of signal voltageThe oscilloscope HM2005 allows examination of DC voltages andmost repetitive signals in the frequency range up to at least200MHz (-3dB). The vertical amplifiers have been designed forminimum overshoot and therefore permit a true signal display.The display of sinusoidal signals within the bandwidth limitscauses no problems, but an increasing error in measurementdue to gain reduction must be taken into account when measuringhigh frequency signals. These errors become noticeable at approx.100MHz. At approx. 120MHz the reduction is approx. 10% andthe real voltage value is 11% higher. The gain reduction error cannot be defined exactly as the -3dB bandwidth of the amplifiersdiffer between 200MHz and 220MHz.For sine wave signals the -6dB limits are approx. 280MHz.When examining square or pulse type waveforms, attention mustbe paid to the harmonic content of such signals. The repetitionfrequency (fundamental frequency) of the signal must thereforebe significantly smaller than the upper limit frequency of thevertical amplifier.Displaying composite signals can be difficult, especially if theycontain no repetitive higher amplitude content which can be usedfor triggering. This is the case with bursts, for instance. To obtaina well triggered display in this case, the assistance of the variableholdoff function or the second time base may be required.Television video signals are relatively easy to trigger using thebuilt in TV-Sync-Separator (TV).For optional operation as a DC or AC voltage amplifier, each verticalamplifier input is provided with a DC/AC switch. DC couplingshould only be used with a series connected attenuator probe orat very low frequencies or if the measurement of the DC voltagecontent of the signal is absolutely necessary.When displaying very low frequency pulses, the flat tops may besloping with AC coupling of the vertical amplifier (AC limit frequencyapprox. 1.6 Hz for 3dB). In this case, DC operation is preferred,provided the signal voltage is not superimposed on a too high DClevel. Otherwise a capacitor of adequate capacitance must beconnected to the input of the vertical amplifier with DC coupling.This capacitor must have a sufficiently high breakdown voltage rating.DC coupling is also recommended for the display of logic and pulsesignals, especially if the pulse duty factor changes constantly.Otherwise the display will move upwards or downwards at eachchange. Pure direct voltages can only be measured with DC-coupling.The input coupling is selectable by the AC/DC pushbutton. Theactual setting is displayed in the readout with the “ = “ symbolfor DC- and the “ ~ “ symbol for AC coupling.Amplitude MeasurementsIn general electrical engineering, alternating voltage data normallyrefers to effective values (rms = root-mean-square value).However, for signal magnitudes and voltage designations inoscilloscope measurements, the peak-to-peak voltage (V pp ) valueis applied. The latter corresponds to the real potential differencebetween the most positive and most negative points of a signalwaveform.If a sinusoidal waveform, displayed on the oscilloscope screen,is to be converted into an effective (rms) value, the resulting peak-to-peak value must be divided by 2x√2 = 2.83. Conversely, itshould be observed that sinusoidal voltages indicated in Vrms(Veff) have 2.83 times the potential difference in Vpp. Therelationship between the different voltage magnitudes can beseen from the following figure.Voltage values of a sine curveV rms = effective value; V p = simple peak or crest value;V pp = peak-to-peak value; Vmom = momentary value.The minimum signal voltage which must be applied to the Y inputfor a trace of 1div height is 1mV pp (± 5%) when this deflectioncoefficient is displayed on the screen (readout) and the vernier isswitched off (VAR-LED dark). However, smaller signals than thismay also be displayed. The deflection coefficients are indicatedin mV/div or V/div (peak-to-peak value).The magnitude of the applied voltage is ascertained by multiplyingthe selected deflection coefficient by the vertical display heightin div. If an attenuator probe x10 is used, a further multiplicationby a factor of 10 is required to ascertain the correct voltage value.For exact amplitude measurements, the variable control (VAR)must be set to its calibrated detent CAL position.With the variable control activated the deflection sensitivity canbe reduced up to a ratio of 2.5 to 1 (please note “Controls andReadout”). Therefore any intermediate value is possible withinthe 1-2-5 sequence of the attenuator(s).With direct connection to the vertical input, signals up to100Vpp may be displayed (attenuator set to 5V/div,variable control to 2.5:1).With the designationsH = display height in div,U = signal voltage in Vpp at the vertical input,D = deflection coefficient in V/div at attenuator switch,the required value can be calculated from the two given quantities:However, these three values are not freely selectable. They haveto be within the following limits (trigger threshold, accuracy ofreading):H between 0.5 and 8div, if possible 3.2 to 8div,U between 1mV pp and 40V pp ,D between 1mV/div and 5V/div in 1-2-5 sequence.Basics of signal voltage