Digital Systems Measurement Techniques I CMPE 6501 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Scope LimitationsThe oscilloscope has three primary limitations:• Inadequate sensitivity• Insufficient range• Limited bandwidthLimited bandwidth is the most significant limitation.Higher bandwidthLower bandwidthBoth probesmeasure thesame inputsignal(higher freq components filtered)Probe Oscilloscope vertical ampt2t3Digital Systems Measurement Techniques I CMPE 6502 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Scope LimitationsBoth the probe and vertical amp degrade the rise time.The degradation in rise time of the combination is given by:Oscilloscope manufacturers commonly quote the 3-dB bandwidth, F3dB, ofprobes and vertical amplifiers instead of rise time.Trise compositeT12T22… TN2+ + +( )=(when impulse responseis gaussian)t1input response of probet12t22+ t12t22t32+ +composite responseDigital Systems Measurement Techniques I CMPE 6503 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Scope LimitationsThis conversion technique between 3-dB bandwidth and the 10-90% rise timeassumes the frequency response of the probe is gaussian:If you are analyzing low-pass filters (which do not have a gaussian frequencyresponse), the relationship is given as:For example, a 300 MHz probe and scope degrades a 2 ns signal:T10-90%0.338F3dB-------------= T10-90%0.361FRMS---------------=if RMS bandwidthoris givenT10-90%2.2LR---=T10-90%2.2RC=(for two-pole RLC filter near critically damped)T10-90%3.4 LC=Trscope0.338 300MHz⁄ 1.1ns= =Trprobe0.338 300MHz⁄ 1.1ns= =Tdisp1.121.1222+ + 2.5ns= =Digital Systems Measurement Techniques I CMPE 6504 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Probe Self-InductanceThe self-inductance of the ground loop in standard 10:1 probes is the primaryfactor degrading their performance.Beware that the manufacturer rates performance with the probe tip andground connected directly to the circuit (no ground wire).We more commonly use them as shown:The 10 pF and 10 MΩ are typical values for scope probes.L1 impedes the current on its return to the source.It adds to the impedance of the probe input and increases the measuredrise time.+-V1 in.3 in.+- -+Toscope10MΩ10pFL1RSRSVDigital Systems Measurement Techniques I CMPE 6505 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Probe Self-InductanceThe dimensions of the ground loop are 1 in. + 3 in., made of 24 AWG wirewith diameter 0.02 in.Appendix C of text gives the inductance of a rectangular loop as:The LC time constant of this circuit is:The 10-90% rise time for a critically damped two-pole circuit of this type:The original 300 MHz-rated probe has a rise time of 1.1 ns.The 3 in. ground wire degrades this to 4.8 ns!L 10.16 12 3×0.02------------  ln 32 1×0.02------------  ln+ 200nH≈ ≈TLCLC 10 pF 200nH× 1.4ns= = =T10-903.4TLC4.8ns= =Digital Systems Measurement Techniques I CMPE 6506 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Probe Self-InductanceThe resistor, RS, in the previous circuit in series with the source models theoutput impedance of the driving gate.For TTL and high performance CMOS, it’s about 30 Ω, while ECL (siliconor GaAs) it’s about 10 Ω.The Q (resonance) of the LC circuit is dramatically affected by this resistance:Here, Q is the ratio of energy stored in the loop/energy lost per radian dur-ing resonant decay, i.e. high Q circuits ring for a long period after excitation.QL C⁄RS---------------≈110 100100040200-20-40RS = 5ΩRS = 25ΩRS = 125ΩMagnitudeof freq. response(dB)freq (MHz)29 dB resonanceresonance almost eliminatedgreatly distortsdigital signals w/Fknee > 100MHzDigital Systems Measurement Techniques I CMPE 6507 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Probe Self-InductanceThis graph also shows that signals with Fknee < 100MHz will exhibit no artifi-cial ringing/overshoot under this probe configuration.With a 100 MHz limitation, the rise time is constrained:Note that Q and rise time of the probe are separate issues.Rise time performance depends only on L and C while Q also incorpo-rates RS, the output resistance of the driver.There is no way of curing the probe inductance problem by using a biggerwire.In general, inductance is roughly proportional to loop area and wirelength.Attachment to the CUT without the ground wire and plastic clip can signifi-cantly improve the results (see text for an example).Rise time0.5100MHz---------------------> 5ns=Digital Systems Measurement Techniques I CMPE 6508 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Inductive CouplingAny ground wire loop also picks up noise that masquerades as noise presentin the signal itself.Mutual inductance using formula from Appendix C:This generates, in this case, only a small noise voltage in loop B:Loop A~0.3 in. X 0.3 inLoop BSeparationbetween loopsTo/from 50 pF load~1 in. X 3 in.dIdt----- 7.0 107× A s⁄=r = 2 in.LM5.08A1A2r3--------------5.080.3 0.3×( ) 1 3×( )23-------------------------------------------0.17nH= = =VnoiseLMdIdt-----0.17nH 7.0 107V s⁄×× 12mV= = =Digital Systems Measurement Techniques I CMPE 6509 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Probe LoadingAnother experiment to evaluate inductive coupling:Turning the loop perpendicular to the magnetic field lines reduces thecoupled signal.Probes load a circuit and change the generated signal.Their effect depends on the relative values of the circuit’s source impedancewith the scope’s input impedance at the knee frequency.Higher probe shunt capacitances reduce the impedance (adds more load),under a given impedance mode, e.g., 1 MΩ.Digital Systems Measurement Techniques I CMPE 65010 (2/21/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Probe LoadingIf we want to probe to have no more than a 10% effect on the CUT’s signal,then probe impedance needs to be 10X larger than the CUT’s src impedance.Probe input impedance for some common probe types:Under the assumption that the source impedance is between 10 - 75 Ω, it’s clearthat the 10 pF probe fails for any rise time less than 5 ns.Text gives example.10,000Magnitudeof probereactance (Ω)100010010500 50 5 0.51 10 100 100010-90% Tr (ns)Fknee (MHz)0.5 pF/1000 Ω (pass.)10 pF/10 MΩ (pass.)1.7 pF/10 MΩ