Hameg HM504-2 Manual

Hameg HM504-2 Manual Manual pdf 28 page image

Subject to change without notice26The bandwidth of the X amplifier, is lower than the Y amplifierand the phase angle which increases with higher frequencies,must be taken into account (please note data sheet).The Y signal applied at INPUT CHII can be inverted.Lissajous figures can be displayed in the X-Y mode for certainmeasuring tasks: Comparing two signals of different frequency or bringingone frequency up to the frequency of the other signal.This also applies for whole number multiples or fractionsof the one signal frequency. Phase comparison between two signals of the samefrequency.Phase comparison with Lissajous figuresThe following diagrams show two sine signals of the samefrequency and amplitude with different phase angles.Calculation of the phase angle or the phase shift between the Xand Y input voltages (after measuring the distances a and b on thescreen) is quite simple with the following formula, and a pocketcalculator with trigonometric functions. Apart from the readingaccuracy, the signal height has no influence on the result.The following must be noted here: Because of the periodic nature of the trigonometric functions,the calculation should be limited to angles ≤90° Howeverhere is the advantage of the method. Due to phase shift, do not use too high a test frequency. It cannot be seen as a matter of course from the screen displayif the test voltage leads or lags the reference voltage. A CRnetwork before the test voltage input of the oscilloscope canhelp here. The 1MOhm input resistance can equally serve asR here, so that only a suitable capacitor C needs to beconnected in series. If the aperture width of the ellipse isincreased (compared with C short-circuited), then the testvoltage leads the reference voltage and vice versa. This appliesonly in the region up to 90° phase shift. Therefore C shouldbe sufficiently large and produce only a relatively small, justobservable phase shift.Should both input voltages be missing or fail in the XY mode, avery bright light dot is displayed on the screen. This dot can burninto the phosphor at too high a brightness setting (INTENS.setting) which causes either a lasting loss of brightness, or in theextreme case, complete destruction of the phosphor at this point.Phase difference measurementin DUAL mode (Yt)Phase differences between two input signals of the samefrequency and shape can be measured very simply on the screenin Dual mode. The time base should be triggered by the referencesignal (phase position 0). The other signal can then have a leadingor lagging phase angle. In alternate triggering condition, phasedifference measurement is not possible.For greatest accuracy, adjust the time base for slightly over oneperiod and set approximately the same height of both signals onthe screen. The Y-deflection coefficients, the time base coefficientand the trigger level setting can be used for this adjustment,without influence on the result. Both base lines are set onto thehorizontal graticule center line using the Y-POS.-knobs before themeasurement. With sinusoidal signals, use the zero (crossoverpoint) transitions; the sine peaks are less accurate. If a sine signalis noticeably distorted by even harmonics, or if a DC voltage ispresent, AC coupling is recommended for both channels. If it isa question of pulses of the same shape, read off at steep edges.It must be noted that the phase difference cannot be determinedif alternate triggering is selected.Phase difference measurement in DUAL modet = horizontal spacing of the zero transitions in divT = horizontal spacing for one period in divIn the example illustrated, t = 3 div and T = 10 div, the phasedifference in degrees is calculated fromor expressed in radiansRelatively small phase angles at not too high frequencies can bemeasured more accurately in the X-Y mode with Lissajousfigures.Measurement of an amplitude modulationThe momentary amplitude u at time t of a HF carrier voltage,which is amplitude modulated without distortion by a sinusoidalAF voltage, is in accordance with the equationwhere: UT = unmodulated carrier amplitudeΩ = 2 pF = angular carrier frequencyω = 2 pf = modulation angular frequencym = modulation factor.As well as the carrier frequency F, a lower side frequency F-f andupper side frequency F+f arise because of the modulation.Operating modes oft the Y amplifiers in Yt mode