Hameg HM 407A Manual

36 Subject to change without noticewave signal of 10-100kHz is applied to the amplifier input.When the Y-POS. control is then turned fully in both directionsfrom stop to stop with a display height of approximately 8div,the upper and lower positions of the trace that are visibleshould be approximately of the same height. Differences of upto 1div are permissible (input coupling should be set to AC).Checking the drift is relatively simple. 20minutes after switchingon the instrument, set the baseline exactly on the horizontalcenter line of the graticule. The beam position must notchange by more than 0.5div during the following hour.Calibration of the Vertical AmplifierA square-wave voltage of 0.2Vpp ±1% is present at the outputsocket of the calibrator (CAL.) If a direct connection is madebetween the 0.2V output and the input of the vertical amplifier(e.g. using a x1 probe), the displayed signal in the 50mV/divposition (variable control to CAL.) should be 4div high (DC inputcoupling).Maximum deviations of 0.12div (3%) are permissible. If a x10probe (1%) is connected between the 0.2V output and Y input,the deflection coefficient should be set to 5mV/div. Then themaximum deviation is 0.16div.With higher tolerances it should first be investigated whetherthe cause lies, within the amplifier or in the amplitude of thesquare-wave signal. On occasions it is possible that the probeis faulty or incorrectly compensated. If necessary the measuringamplifier can be calibrated with an accurately known DCvoltage (DC input coupling). The trace position should thenvary in accordance with the deflection coefficient set.With variable control in the attenuator sector fully counter-clockwise, the input sensitivity is decreased at least by thefactor 2.5 in each position. In the 50mV/div position, thedisplayed calibrator signal height should vary from 4div to atleast 1.6div.Transmission Performanceof the Vertical AmplifierThe transient response and the delay distortion correction canonly be checked with the aid of a square-wave generator witha fast risetime (max. 5ns). The signal coaxial cable (e.g. HZ34)must be terminated at the vertical input of the oscilloscopewith a resistor equal to the characteristic impedance of thecable (e.g. with HZ22). Checks should be made at 100Hz,1kHz, 10kHz, 100kHz and 1MHz, the deflection coefficientshould be set at 5mV/div with DC input coupling. In so doing,the square pulses must have a flat top without ramp-off,spikes and glitches; no overshoot is permitted, especially at1MHz and a display height of 4-5div. At the same time, theleading top corner of the pulse must not be rounded. Ingeneral, no great changes occur after the instrument has leftthe factory, and it is left to the operators discretion whetherthis test is undertaken or not. A suited generator for this testis HZ60 from HAMEG.Of course, the quality of the transmission performance is notonly dependent on the vertical amplifier. The input attenuators,located in the front of the amplifier, are frequency-compensatedin each position. Even small capacitive changes can reduce thetransmission performance. Faults of this kind are as a rule mosteasily detected with a square-wave signal with a low repetitionrate (e.g. 1kHz). If a suitable generator with max. output of40Vpp is available, it is advisable to check at regular intervals thedeflection coefficients on all positions of the input attenuatorsand readjust them as necessary. A compensated 2:1 seriesattenuator is also necessary, and this must be matched to theinput impedance of the oscilloscope. This attenuator can bemade up locally. It is important that this attenuator is shielded.For local manufacture, the electrical components required area 1MΩ ±1% resistor and, in parallel with it, a trimmer 3-15pF inparallel with approx. 10pF. One side of this parallel circuit isconnected directly to the input connector of CH I or CH II andthe other side is connected to the generator, if possible via alow-capacitance coaxial cable. The series attenuator must bematched to the input impedance of the oscilloscope in the 5mV/div position (variable control to CAL., DC input coupling; squaretops exactly horizontal; no ramp-off is permitted). This is achievedby adjusting the trimmer located in the 2:1 attenuator. Theshape of the square-wave should then be the same in each inputattenuator position.Operating Modes:CH.I/II, DUAL, ADD, CHOP.,INVERT and X-Y OperationIn DUAL mode two traces must appear immediately. Onactuation of the Y-POS. controls, the trace positions shouldhave minimal effect on each other. Nevertheless, this cannotbe entirely avoided, even in fully serviceable instruments.When one trace is shifted vertically across the entire screen,the position of the other trace must not vary by more than0.5mm.A criterion in chopped operation is trace widening andshadowing around and within the two traces in the upper orlower region of the screen. Set time coefficient to 0.5ms/div,set input coupling of both channels to GD and advance theINTENS. control fully clockwise. Adjust FOCUS for a sharpdisplay. With the Y-POS. controls shift one of the traces to a+2div, the other to a -2div vertical position from the horizontalcenter line of the graticule.Do not try to synchronize (with the time variablecontrol) the chop frequency (0.5MHz)! Check for negli-gible trace widening and periodic shadowing whenswitching between 0.5ms/div and 0.2ms/div.It is important to note that in the I+II add mode or the I-IIdifference mode the vertical position of the trace can be adjustedby using both the Channel I and Channel II Y-POS. controls.In X-Y Operation, the sensitivity in both deflection directionswill be the same. When the signal from the built-in square-wave generator is applied to the input of Channel II, then, aswith Channel I in the vertical direction, there must be ahorizontal deflection of 4div when the deflection coefficient isset to 50mV/div position. The check of the mono channeldisplay is unnecessary; it is contained indirectly in the testsabove stated.Triggering ChecksThe internal trigger threshold is important as it determines thedisplay height from which a signal will be stably displayed. Itshould be approx. 0.3-0.5div for the instrument. An increasedtrigger sensitivity creates the risk of response to the noiselevel in the trigger circuit. This can produce double-triggeringwith two out-of-phase traces.Alteration of the trigger threshold is not required. Checks canbe made with any sine-wave voltage between 50Hz and1MHz. The instrument should be in automatic peak (value)triggering (NM LED dark) and the LEVEL knob in electricalmidrange position. It should be ascertained whether the sametrigger sensitivity is also present with Normal Triggering (NMLED lights). In this trigger mode, LEVEL adjustment is absolutelynecessary.Test Instructions