Txoj kev tsom xam ntawm oscilloscope passive probes
Cov kws tshaj lij feem ntau siv 10 npaug ntawm qhov kev sojntsuam voltage Rp (9MΩ) thiab Cp nyob rau hauv qhov kev sojntsuam taub, Rp rau qhov kev sojntsuam input impedance, Cp rau qhov kev sojntsuam input capacitance, R1 (1MΩ) sawv cev rau cov input impedance ntawm oscilloscope, C1 sawv cev. cov input capacitance ntawm oscilloscope thiab qhov sib npaug capacitance ntawm lub coaxial cable nrog rau kev sojntsuam them nyiaj lub thawv capacitance ntawm kev sib xyaw ntawm cov nqi. Txhawm rau ntsuas qhov tseeb, ob lub sijhawm RC lub sijhawm (RpCp thiab R1C1) yuav tsum sib npaug; ib qho kev tsis txaus ntseeg yuav ua rau muaj kev cuam tshuam ntawm qhov ntsuas waveform, thiab tsis txhob ua rau qhov ntsuas tsis raug ntawm qee qhov tsis xws li lub sijhawm nce thiab amplitude. Yog li ntawd, nws yog ib qho tsim nyog los ntsuas qhov ua haujlwm ntawm oscilloscope sojntsuam ua ntej kev ntsuas kom ntseeg tau qhov tseeb ntawm qhov ntsuas ntsuas. Los ntawm cov qauv teeb liab ntawm qhov kev sojntsuam peb tuaj yeem txheeb xyuas tau tias rau DC ntsuas ntawm lub teeb liab, qhov input capacitance Cp thiab C1 sib npaug rau qhov qhib Circuit Court. Lub teeb liab muab faib los ntawm Rp thiab R1, thiab qhov kawg oscilloscope input yog: Vout=[R1/Rp+R1]*Vin=1/10*Vin
Oscilloscope Lub tswv yim teeb liab rau oscilloscope yog attenuated rau 1/10 ntawm lub teeb liab input yuav ntsuas. rau ntau zaus input signals, cov nyhuv ntawm capacitive reactance ntawm lub teeb liab yuav ntau dua qhov impedance. Piv txwv li, tus qauv 1MΩ ~ 10pF passive voltage sojntsuam, input teeb liab zaus ntawm 100MHz, lub sij hawm no, qhov kev sojntsuam input capacitive impedance Xc (Cp)=1 / (2 × π × f × C) {{7 }}Ω, capacitive impedance yog ntau me dua qhov kev sojntsuam impedance ntawm 9MΩ, lub teeb liab tam sim no yuav ntau dua los ntawm cov input capacitance muab los ntawm cov hluav taws xob tsis tshua muaj hluav taws xob, qhov hluav taws xob tiv thaiv siab ntawm 9MΩ impedance yog sib npaug rau bypass. Kuj tseem tuaj yeem nkag siab zoo li 159Ω thiab 9MΩ kev sib txuas ua ke tom qab qhov sib npaug impedance ntawm 159Ω. Lub sijhawm no, qhov kev tawm tswv yim tiag tiag rau oscilloscope teeb liab amplitude (AC / siab zaus) yog txiav txim siab los ntawm qhov input capacitance ntawm kev sojntsuam nrog rau qhov sib piv ntawm tag nrho cov capacitance ntawm lub voj, qhov sib npaug ntawm: Vout=[Cp/Cp+C1]*Vin
Feem ntau, ib qho kev sojntsuam passive muaj lub peev xwm thauj khoom ntawm 8-10pF / ko taw (1 ko taw ko taw=12 ntiv tes ntiv tes=0.3048meter meter) thiab lub sijhawm nce ntawm 1.5nS / ko taw. Rau ib tug 6feet cable muaj 60pF ntawm capacitance, ntxiv rau 20pF input capacitance ntawm ib tug raug oscilloscope thiab ib co stray, roughly 90pF los yog li ntawd. Raws li 1:10 voltage division, input capacitance ntawm kev sojntsuam yuav tsum yog li ntawm 10pF kom los siav tus yam ntxwv ntawm Vout/Vin{14}}[10/10+90]=1/10 input attenuation los ntawm qhov tseem ceeb ntawm 10. Muab rau hauv tus account qhov kev sojntsuam thiab cable capacitance ntawm qee qhov yuam kev, yuav tsum tau siv qhov kev sojntsuam them nyiaj capacitance lub thawv los ua ib qho kev them nqi hluav taws xob, vim qhov yuam kev, passive voltage probe input capacitance feem ntau ntawm 8 ~ 12pF . Tam sim no lub mainstream 10 zaug passive voltage sojntsuam input load qauv yog feem ntau input capacitance 8 ~ 12pF, input kuj 9M ohm.
