Hameg HM 407A Manual

35Subject to change without noticeWith the start of each recording, the signal voltage at theinput(s) of the analog-to-digital converters is briefly measured(sampled), converted to an 8-bit value and written to anaddress in RAM. The next sampled value is converted in thesame way, but stored at the next RAM address.The maximum sampling rate is 100MS/s. This yields a samplinginterval of 10ns. Assuming that 10 measurements (samples)per signal period are sufficient for a sinewave signal, themaximum frequency to be captured is 10MHz (100ns perperiod).Alias signal displayIf, due to the time base setting, the sampling rate is too low,the display of an alias signal may occur.For example:A sinewave signal may be sampled only once per period. Ifthe sinewave signal frequency is accidentally in phase withthe sampling frequency and each sample is taken at thepositive peak value, a straight line is displayed in this position.Alias signal display may also occur in the form of an apparentuntriggered waveform display of different frequency from thetrue signal. Another aliasing condition is the display of signalsseeming to be amplitude modulated.The easiest way to recognize alias signals is to switch to analogmode, where the true waveform is displayed. Transfer fromanalog to store mode without changing time base range mustproduce the same frequency display.Operating modes of the vertical amplifiersIn principle, the instrument can operate in digital storage modewith the same operating modes as in analog mode. Thus, thefollowing can be displayed:- Channel I by itself- Channel II by itself- Channel I and II simultaneously- The sum or difference of both channels- XY modeStorage mode differs from analog operation of the scope in thefollowing respects:- In DUAL mode (for simultaneous display of both channels),both input signals are also simultaneously captured, sinceeach channel has its own A/D converter. In contrast toanalog mode therefore, it is not necessary to switch betweenchopped and alternating mode.- Because of the high display rate (approx. 80Hz) of the screen,no flicker can occur.- The storage time base is also active in XY storage mode.Test InstructionsGeneralThese Test Instructions are intended as an aid for checking themost important characteristics of the instrument at regularintervals without the need for expensive test equipment.Resulting corrections and readjustments inside the instrument,indicated by the following tests, are described in the “ServiceManual”. They should only be undertaken by qualified personnel.A chargeable “Service Manual” is available from HAMEG. The“Service Manual” describes the complete adjustment of theinstrument in English language and contains the circuit diagramsas well as the component locations.As with the First Time Operation instructions, care should betaken that all variable functions are set to their calibratedpositions. This is the case if AUTO SET had been pressed. Itis recommended to switch on the instrument for about 20minutes prior to the commencement of any check.Cathode-Ray Tube: Brightness and Focus,Linearity, Raster DistortionNormally, the CRT of the instrument has very good brightness.Any reduction of this brightness can only be judged visually.Decreased brightness may be the result of reduced highvoltage. In this case the sensitivity of the vertical amplifiers issignificantly increased .The control range for maximum and minimum brightness(intensity) must be such that the beam just disappears beforereaching the left hand stop of the INTENS. control (particularlywhen in XY mode), while with the control at the right hand stopthe focus and the line width are just acceptable.With maximum intensity the time base fly-back must on noaccount be visible. Visible trace fault without input signal:bright dot on the left side or decreasing brightness from left toright or shortening of the baseline. (Cause: Incorrect UnblankingPulse.) It should be noted that with wide variations in brightness,refocusing is always necessary. Moreover, with maximumbrightness, no “pumping” of the display must occur. If pumpingdoes occur, it is normally due to a fault in the regulationcircuitry for the high voltage supply. The presetting pots for thehigh voltage circuit, minimum and maximum intensity, are onlyaccessible inside the instrument.A certain out-of-focus condition in the edge zone of the screenmust be accepted. It is limited by standards of the CRTmanufacturer. The same is valid for tolerances of theorthogonality, the undeflected spot position, the non-linearityand the raster distortion in the marginal zone of the screen inaccordance with international standards (see CRT data book).These limit values are strictly supervised by HAMEG. Theselection of a cathode-ray tube without any tolerances ispractically impossible.Astigmatism CheckCheck whether the horizontal and vertical sharpness of thedisplay are equal. This is best seen by displaying a square-wavesignal with the repetition rate of approximately 1MHz. Focusthe horizontal tops of the square-wave signal at normal intensity,then check the sharpness of the vertical edges. If it is possibleto improve this vertical sharpness by turning the FOCUScontrol, then an adjustment of the astigmatism control isnecessary. A potentiometer of 47k Ohm is provided inside theinstrument for the correction of astigmatism. A certain loss ofmarginal sharpness of the CRT is unavoidable; this is due to themanufacturing process of the CRT.Symmetry and Drift of the Vertical AmplifierBoth of these characteristics are substantially determined bythe input stages of the amplifiers.The symmetry of both channels and the vertical final amplifiercan be checked by inverting Channel I and II (depress thecorresponding INV pushbutton). The vertical position of thetrace should not change by more than 0.5div. However, achange of 1div is just permissible. Larger deviations indicatethat changes have occurred in the amplifier.A further check of the vertical amplifier symmetry is possibleby checking the control range of the Y-POS. controls. A sine-Test Instructions