Hameg HM 407A Manual

24 Subject to change without noticeConnect the probes (Types HZ51, 52, 54, or HZ36) to the CHI input. Set the deflection coefficient to 5mV/div and the inputcoupling to DC. The time deflection coefficient should be setto 0.2ms/div. All deflection coefficients should be calibrated.Plug the probe tip into the calibrator output socket.Approximately 2 complete waveform periods are displayed onthe CRT screen. The compensation trimmer should be adjusted.The location of the low frequency compensation trimmer canbe found in the probe information sheet. Adjust the trimmerwith the insulated screw driver provided, until the tops of thesquare wave signal are exactly parallel to the horizontal graticulelines (see 1kHz diagram). The signal height should then be 4div± 0.16div (= 4% (oscilloscope 3% and probe 1%). During thisadjustment, the signal edges will remain invisible.Adjustment at 1MHzProbes HZ51, 52 and 54 can also be HF-compensated. Theyincorporate resonance de-emphasing networks (R-trimmer inconjunction with inductances and capacitors) which permitprobe compensation in the range of the upper frequency limitof the vertical oscilloscope amplifier. Only this compensativeadjustment ensures optimum utilization of the full bandwidth,together with constant group delay at the high frequency end,thereby reducing characteristic transient distortion near theleading edge (e.g. overshoot, rounding, ringing, holes orbumps) to an absolute minimum.Using the probes HZ51, 52 and 54, the full bandwidth of theoscilloscope can be utilized without risk of unwanted waveformdistortion.Prerequisite for this HF compensation is a square wavegenerator with fast risetime (typically 4ns), and low outputimpedance (approx. 50Ω), providing 0.2V at a frequency ofapprox. 1MHz. The calibrator output of this instrument meetsthese requirements when the CAL. pushbutton is depressed.Connect the probe to CH.I input. Depress the CAL. pushbuttonfor 1MHz. Operate the oscilloscope as described under 1kHzbut select for 0.2μs/div time deflection coefficient setting.Insert the probe tip into the output socket. A waveform will bedisplayed on the CRT screen, with leading and trailing edgesclearly visible. For the HF-adjustment now to be performed, itwill be necessary to observe the rising edge as well as theupper left corner of the pulse top. The location of the highfrequency compensation trimmer(s) can also be found in theprobe information sheet. These R-trimmer(s) have to beadjusted such that the beginning of the pulse is as straight aspossible. Overshoot or excessive rounding are unacceptable.The adjustment is relatively easy if only one adjusting point ispresent. In case of several adjusting points the adjustment isslightly more difficult, but causes a better result. The risingedge should be as steep as possible, with a pulse top remainingas straight and horizontal as possible.After completion of the HF-adjustment, the signal amplitudedisplayed on the CRT screen should have the same value asduring the 1kHz adjustment.Probes other than those mentioned above, normally have alarger tip diameter and may not fit into the calibrator output.Whilst it is not difficult for an experienced operator to build asuitable adapter, it should be pointed out that most of theseprobes have a slower risetime with the effect that the totalbandwidth of scope together with probe may fall far below thatof the oscilloscope. Furthermore, the HF-adjustment featureis nearly always missing so that waveform distortion can notbe entirely excluded. The adjustment sequence must befollowed in the order described, i.e. first at 1kHz, then at1MHz. The calibrator frequencies should not be used for timebase calibration. The pulse duty cycle deviates from 1:1 ratio.Prerequisites for precise and easy probe adjustments, as wellas checks of deflection coefficients, are straight horizontalpulse tops, calibrated pulse amplitude, and zero-potential atthe pulse base. Frequency and duty cycle are relatively uncritical.For interpretation of transient response, fast pulse risetimesand low-impedance generator outputs are of particularimportance.Providing these essential features, as well as switch-selectableoutput-frequencies, the calibrator of the instrument can, undercertain conditions, replace expensive squarewave generatorswhen testing or compensating wideband-attenuators or -amplifiers. In such a case, the input of an appropriate circuit willbe connected to the CAL.-output via a suitable probe.The voltage provided at a high-impedance input (1MΩ II 15-30pF) will correspond to the division ratio of the probe used(10:1 = 20mVpp output). Suitable probes are HZ51, 52, and 54.Operating modes of the verticalamplifiers in Yt modeThe most important controls regarding the operation modes ofthe vertical amplifiers are the pushbuttons: CH I (21), DUAL(22) and CH II (25).Their functions are described in the section“ Controls and readout”.In most cases oscilloscopes are used to display signals in Ytmode. Then the signal amplitude deflects the beam in verticaldirection while the time base causes an X deflection (from leftto right) at the same time. Thereafter the beam becomesblanked and fly back occurs.The following Yt operation modes are available:Single channel operation of channel I (Mono CH I).Single channel operation of channel II (Mono CH II).Two channel operation of channel I and channel II (DUAL).Two channel operation of channel I and channel II -displayingthe algebraic result as the sum or difference - (ADD).The way the channel switching is determined in DUAL modedepends on the time base setting and is described in thesection “Controls and readout”.In ADD mode the signals of both channels are algebraicallyadded and displayed as one signal. Whether the resultingdisplay shows the sum or difference is dependent on thephase relationship or the polarity of the signals and on theinvert function.In ADD mode the following combinations are possible forIn-phase input voltages:No invert function active = sum.Channel II invert function active = difference.First Time Operation