Hameg HM303-6 Manual

Also see for HM303-6: Operator's manual Service manual

Contents

13Subject to change without noticedisplay shows the sum or difference is dependent on thephase relationship or the polarity of the signals and on thepositions of the INV. (invert) button.In-phase input voltages:INV. button released = sum.INV. button depressed = difference.Antiphase input voltages:INV. button released = difference.INV. button depressed = sum.In the ADD mode the vertical display position is dependentupon the Y-POS. setting of both channels. The same attenuatorswitch position is normally used for both channels withalgebraic addition.Please note that the Y-POS. settings are added too but are notaffected by the INV. pushbutton.Differential measurement techniques allow direct measure-ment of the voltage drop across floating components (bothends above ground). Two identical probes should be used forboth vertical inputs. In order to avoid ground loops, use aseparate ground connection and do not use the probe groundleads or cable shields.X-Y OperationFor X-Y operation, the pushbutton underneath the TIME/DIV.-knob marked XY must be depressed. The X signal is thenderived from the INPUT CH I (X). The calibration of the Xsignal during X-Y operation is determined by the settingof the Channel I input attenuator and variable control.This means that the sensitivity ranges and input impedancesare identical for both the X and Y axes. However, the Y-POS.Icontrol is disconnected in this mode. Its function is taken overby the X-POS. control. It is important to note that the X-MAG.x10 facility, normally used for expanding the sweep, isinoperative in the X-Y mode.The 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 ”Specifications”).The inversion of the Y-input signal (CH II) using the INV.button is possible.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. Thisalso applies for whole number multiples or fractions of theone 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 theX and Y input voltages (after measuring the distances a and bon the screen) is quite simple with the following formula, anda pocket calculator with trigonometric functions. Apart fromthe reading accuracy, the signal height has no influence on theresult.The following must be noted here:- Because of the periodic nature of the trigonometricfunctions, the calculation should be limited to angles ≤90°.However here is the advantage of the method.- Do not use a too high test frequency. The phase shift of thetwo oscilloscope amplifiers of the HM303-6 in the X-Ymode can exceed an angle of 3° above120 kHz.- It cannot be seen as a matter of course from the screendisplay if the test voltage leads or lags the referencevoltage. A CR network before the test voltage input of theoscilloscope can help here. The 1 MΩ input resistance canequally serve as R here, so that only a suitable capacitor Cneeds to be connected in series. If the aperture width ofthe ellipse is increased (compared with C short-circuited),then the test voltage leads the reference voltage and viceversa. This applies only in the region up to 90° phase shift.Therefore C should be sufficiently large and produce onlya relatively small just observable phase shift.Should both input voltages be missing or fail in the X-Y mode,a very bright light dot is displayed on the screen. This dot canburn into the phosphor at a too high brightness setting(INTENS knob) which causes either a lasting loss of brightness,or in the extreme case, complete destruction of the phosphorat this point.Phase difference measurementin DUAL modeA larger phase difference between two input signals of thesame frequency and shape can be measured very simply onthe screen in Dual mode (DUAL button depressed). The timebase should be triggered by the reference signal (phaseposition 0).The other signal can then have a leading or lagging phaseangle. Alternate mode should be selected for frequencies ≥1kHz; the Chop mode is more suitable for frequencies <1 kHz(less flickering).For greatest accuracy adjust not much more than one periodand approximately the same height of both signals on thescreen. The variable controls for amplitude and time base andthe LEVEL knob can also be used for this adjustment withoutinfluence on the result.Both base lines are set onto the horizontal graticule center linewith the Y-POS knobs before the measurement. With sinusoidalsignals, observe the zero (crossover point) transitions; the sinepeaks are less accurate. If a sine signal is noticeably distortedby even harmonics, or if a D.C. voltage is present, AC couplingis recommended for both channels. If it is a question of pulsesof the same shape, read off at steep edges.Operating modes of the vertical amplifiers