Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Resistive opto-isolator wikipedia , lookup
Fault tolerance wikipedia , lookup
Opto-isolator wikipedia , lookup
Mains electricity wikipedia , lookup
Ground loop (electricity) wikipedia , lookup
Ground (electricity) wikipedia , lookup
Immunity-aware programming wikipedia , lookup
Tektronix analog oscilloscopes wikipedia , lookup
Oscilloscope wikipedia , lookup
High-Speed Digital Test & Measurement Chris Allen ([email protected]) Course website URL people.eecs.ku.edu/~callen/713/EECS713.htm 1 Topics Test equipment Oscilloscope characteristics Oscilloscope probes • Passive probes • Active probes • Probe inductance from ground lead Design for test Test procedures Measuring noise margin Measuring timing margin Sensitivity to supply voltage variations Sensitivity to temperature variations Test vectors Future trends 2 Test equipment Oscilloscope characteristics Note: to relate BW3dB to T10-90 T10-90 = 0.338 / BW3dB Vertical amplifier bandwidth (typically specified as 3-dB BW, BW3dB) Limits the observed risetime of the measured signal The risetime of the vertical amplifier adds in root-sum-square (RSS) fashion to the circuit’s actual Tr to yield a measured Tr Tr measured Tr2 circuit Tr2 vertical amplifier 3 Test equipment Scope probe characteristics The probe connecting the oscilloscope to the circuit under test also has a frequency response, characterized by Tr(probe), that affects the measurement. Including the contribution of the scope probe yields Tr measured Tr2 circuit Tr2 vertical amplifier Tr2 probe Consequently, the circuit’s actual risetime may be shorter than what measurements indicate. Example: Using a 500-MHz oscilloscope with a 1-GHz probe an 820-ps risetime is measured. What is the circuit’s actual risetime? 500 MHz Tr(vert amp) = 676 ps, 1 GHz Tr(probe) = 338 ps Tr circuit 820 ps2 676 ps2 338 ps2 318 ps 4 Test equipment Scope probe characteristics The model for the probe includes an input capacitance in parallel with a high-value resistance, R1 (9 MΩ) and an inductive ground lead. The scope is modeled as a high-value resistor, R2, in parallel with an input capacitance, C2. 5 Test equipment Scope probe characteristics At DC this arrangement produces a 10x attenuation by the voltage divider R2/(R1 + R2). At DC the impedance of the probe/scope is |Zmeas| = 10 MΩ. Simplified equivalent circuit At 100 MHz the impedance is (ignoring XLGround Lead) C1 (12 pF) |XC1| = 133 C2 (20 pF) |XC2| = 80 C3 (55 pF) |XC3| = 29 |Zmeas| |XC1 + (XC2 // XC3)| = 154 At higher frequencies, the probe/scope impedance decreases further. 6 Test equipment Other measurement considerations The relatively low probe/scope impedance at high frequencies can load the circuit under test causing: • changes in circuit performance • corrupt measurements To avoid the loading problem we can use probes with less capacitance. Another option is to use active probes that use FET amplifiers to isolate measurement capacitance from the circuit under test. 7 Test equipment Active probes The FET in the active probe acts as a signal buffer. The probes’s input impedance is now the FET’s input impedance • typical FET input capacitance < 1 pF • high input resistance Consequently, active probes • reduce circuit loading • have a wide operating bandwidth (Tektronix active probe 500 MHz to 4 GHz) • requires a bias voltage to power the FET amplifier Active probes can present a bias voltage to the probe tip • useful for probing unterminated outputs 8 Test equipment Probe inductance from ground lead Oscilloscope measurements generally require a ground connection as a voltage reference. Typical scope probes have a ground lead wire for signal reference. This ground lead has significant inductance (100s of nH) and can act as an antenna (both radiating signals as well as coupling ambient RF signals into the measurement). A long ground lead presents a number of measurement problems • increased Tr(measured) • crosstalk • electromagnetic interference (EMI) 9 Test equipment Probe inductance from ground lead To avoid these problems we must reduce Lgnd Special probe tip attachments provide a ground connection with reduced lead inductance (few nH) These can improve the measurement-induced errors affecting Tr, crosstalk, and EMI However to effectively use these tip attachments all signals to be probed must have a nearby ground pad for the probe. Therefore generous use of ground metal on the board surface facilitates probing. 10 Test equipment Special probing fixtures The author discusses a low-cost, shop-built 21:1 probe built from a length of coaxial cable and a leaded 1-kΩ resistor. Division ratio 50 50 1000 Division ratio 0.048 1: 21 Benefits of this approach include • high DC resistance of 1050 Ω (vs. 50 Ω of cable alone) • low circuit loading, fast rise time (Tr) minimal capacitance (~ 0.5 pF) 1-kΩ series resistance • low cost Assumptions scope is terminated with 50 Ω (to reduce reflections) ground pad is available near the signal to be probed (short lead length) 11 Test equipment Special probing fixtures Low-cost, shop-built 21:1 probe (photo essay) from http://paulorenato.com/joomla/index.php?option=com_content&view=article&id=93&Itemid=4 testing with passive probes circuit under test: 125-MHz oscillator testing with shop-built, wide bandwidth probe 12 Design for test Special probing fixtures Special test points can also be incorporated into the board design to ease the probing of high-speed signals. 13 Test procedures Once a digital circuit is functioning, it is useful to determine its operating margin Consider the variety of factors that contribute to this margin • noise margin • reflections • timing margin • temperature effects • supply voltage effect Several tests are recommended to ensure reliable operation 14 Test procedures Noise margin / reflection testing By introducing additional noise at various nodes in the system, signals with noise sensitivity can be identified. Useful in locating the source of intermittent errors. The Noise Source is composed of resistor, R, that develops thermal noise (broadband, random) which is then amplified to the desired level. Other noise generations approaches are available (e.g., communication noise sources). 15 Test procedures Timing testing By varying the relative timing of the clock and data signals, the timing margin can be estimated. Use of a coaxial delay line select a clock or remove a portion data line to of the line and be tested expose the copper insert a short segment of coaxial line Demonstrates usefulness of ground area on surface The opened trace can be readily repaired after testing Another method for making this measurement is to use an independent clock source, synchronized with variable phase control. 16 Test procedures Testing tolerance to varying supply voltages, VEE, VTT Test to find out how sensitive your design is to voltage variations, to establish the supply voltage tolerances. This test demonstrates another advantage of separate VEE and VTT supplies, as opposed to These voltage variations can change threshold voltage and signal DC bias levels resulting in small timing changes. 17 Test procedures Testing tolerance to varying temperature Variations in ambient temperature can change the device temperature which may result in significant propagation delay changes. The effect may be localized by selective temperature adjustment by applying local heat/cooling at the chip level to isolate the effect. Heat gun 18 Test procedures Test vectors At the device level, board level, or system level comprehensive testing requires exercising all circuit functions at speed, simultaneously. Testing functions independently is useful but not conclusive. Such testing requires that all inputs and expected outputs be specified. These are known as test vectors The inputs are used to stimulate the circuit and the responses are observed. The test vectors bring the circuit or system to a known state (e.g., reset, or all 0s) and then steps through all states of interest. 19 Test procedures Test vectors Such testing brings together all factors affecting performance including timing, crosstalk, EMI, noise on the power and ground. Such testing is often expensive and limited. It typically involves an extensive variety of test equipment, programming, and planning. 20 Future trends Application-driven advances Evolutionary and revolutionary advances require ever-increasing data rates and computational throughput in smaller packages while consuming less power. Leading applications include: • computing (gaming consoles, super computers) • sensors (fine resolution video, radar, RFID) • wireless communication Emerging technologies • energy harvesting (power derived from local environment) • optical interconnects die-to-die (optical transmitters/receivers integrated into die) – may involve holographic reflectors or optically transparent substrates 21