Hameg HM2005-2 User Manual

Contents

Hameg HM2005-2 User Manual Manual pdf 15 page image

15Subject to change without notice5 3ϕ° = — · 360° = — · 360° = 108°T 10or in angular units:t 3arc ϕ° = — · 2π = — · 2π = 1,885 radT 10Very small phase differences with moderately high frequenciesmay yield better results with Lissajous figures.However, in order to get higher precision it is possible to switchto higher sensitivities after accurately positioning at graticulecentre, thus overdriving the inputs resulting in sharper zerocrossings. Also, it is possible to use half a period over the full10 cm. As the time base is quite accurate, increasing the timebase speed after adjusting for e.g. one period = 10 cm andpositioning the fi rst crossing on the first graticule line will alsogive better resolution.Measurement of amplitude modulationThe momentary amplitude at time t of an hf carrier frequencymodulated by a sinusoidal low frequency is given by:u = UT · sinΩt + 0,5 m · UT · cos (Ω - ω) t - 0,5 m · UT · cos (Ω - ω) twhere: UT = amplitude of the unmodulated carrierΩ = 2πF = angular carrier frequencyω = 2πf = modulation angular frequencym = modulation degree (≤1 v100%)In addition to the carrier a lower side band F – f and an upperside band F + f will be generated by the modulation.Picture 1: Amplitudes and frequencies with AM (m = 50 %) ofthe spectraAs long as the frequencies involved remain within the scope’sbandwidth the amplitude-modulated hf can be displayed. Pre-ferably the time base is adjusted so that several signal periodswill be displayed. Triggering is best done from the modulationfrequency. Sometimes a stable displayed can be achieved bytwiddling with the time base variable.Picture 2: Amplitude modulated hf. F = 1 MHz, f = 1 kHz,m = 50 %, UT = 28,3 mVrmsSet the scope controls as follows in order to display the picture2 signal:CH1 only, 20 mV/cm, ACTIME/DIV: 0.2 ms/cmTriggering: NORMAL, AC, internal.Use the time base variable or external triggering.Reading a and b off the screen the modulation degree willresult:a – b a – bm = —— or m = —— · 100 [%]a + b a + ba = U T (1 + m) and b = UT (1 – m)When measuring the modulation degree the amplitude and timevariables can be used without any influence on the result.Triggering and time baseThe most important controls and displays for these functionsare to be found in the shaded TRIGGER area, they are describedin „Controls and Readout“.-In YT mode the signal will deflect the trace vertically while thetime will deflect it horizontally, the speed can be selected.In general periodic voltage signals are displayed with a peri-odically repeating time base. In order to have a stable display,successive periods must trigger the time base at exactly thesame time position of the signal (amplitude and slope).STOPPure DC can not trigger the time base, a voltagechange is necessary.Triggering may be internal from any of the input signals orexternally from a time related signal.For triggering a minimum signal amplitude is required whichcan be determined with a sine wave signal. With internal trigge-ring the trigger take off within the vertical amplifiers is directlyfollowing the attenuators. The minimum amplitude is specifiedin mm on the screen. Thus it is not necessary to give a minimumvoltage for each setting of the attenuator.For external triggering the appropriate input connector is used,thus the amplitude necessary is given in Vpp. The voltage fortriggering may be much higher than the minimum, however, itshould be limited to 20 times the minimum. Please note thatfor good triggering the external voltage should be a good dealabove the minimum. The scope features two trigger modes tobe described in the following:Automatic peak triggering (MODE menu)Consult the chapters MODE 16 > AUTO, LEVEL A/B 15 , FILTER17 and SOURCE 18 in ”Controls and Readout“. Using AUTOSETthis trigger mode will be automatically selected. With DC coup-ling and with alternate trigger this mode will be left while theautomatic triggering will remain.T r i g g e r i n g a n d t i m e b a s eF – f F F + f0,5 m · U T0,5 m · UTUTbam · U TUT