2017 Exam Flashcards
Knee Frequency Definition (2)
cut-off freq. value above which the energy spectral density of a digital waveform falls off rapidly (-40dB/dec)
Most of energy is digital pulses and is concentrated below the knee frequency
Freq Spectrum of unequal duty cycle
Freq Spectrum of 50% duty cycle
Freq. Spectrum of trapezoidal waveform
WHy are large # of GND via holes used on a PCB (4)
- @ high freq, return current follows the path of least inductance
- lowest ind. return path lies directly underneath T.L, minimising the total loop area between the outgoing + returning current paths
- If signal jumps from 1 PCB layer to another through via hole, there is no wat for return current path to make the jump
- Return current then follows a path other than directly underneath signal trace
- more inductance = EMI = crosstalk
- Return current then follows a path other than directly underneath signal trace
- Plenty of via holes used to minimise inductances
Why is there ISI in high speed tranmission system
Causes - reflection, crosstalk, any other source
Last transition may have not have died away by the time the next transition occurs causing ISI
Degreades performance power factor + is important when
period < 2x delay time
Eye Diagram + Uses (3)
- A superposition of observations of sequential bit periods, generated as pseudo-random bit sequence
- Induction of the impact of ISI
Wide Open = good performance
Closed = BER increases
Common Impedance Coupling Definition
causes crosstalk when a single conductor is used for more than 1 signal
Common Path Noise Voltage
product of the returning signal current + impedance of teh common conductor
Two ways to reduce common impedance coupling (2)
- // power + ground planes
- provides low impedance path between pwr + GND pins
- decoupling capacitors between GND + pwr planes to provide a low impedance path
90 angle bend effects on propagating signal
- angle bend causes a discontinuity between 2 T.Ls with different field patterns leadingto stored energy
- can be accounted for by adding an excess capacitance to the capacitors in equivalent circuit (same for inductance) - capacitance dominates
- reduce by champfering the corner reducing line width + total capacitance
Characteristics + differences of near + far-end crosstalk in T.L (5)
- Near-end (backward) - closest, Far-end (forward) - furthest
- Initial wave will generate two crosstalk pulses
- near-end pulse that flows into the near-end termination
- far-end pulse that propagates towards the far-end
- Contributions to these pulses
- M.C - creates current forward + backward
- M.I - creates current backward only
- As waves propagates along the line, continuously induces new pulses, arriving simultaneously as a single pulse (w/ initial wave) at the far end
- Arrive sequentially @ near-end, beginning w/ launch of initial wave + ending when the final contribute (created when the initial wave reached the far-end) arrives back @ near-end
- i.e. 2x propagation delay of the line
Max Jitter early or late arrival time
Early - when line carries opposite bit w.r.t L1 + L3
Late - carries same as L1 + L3
USB protocol structure (3)
- Top of Pyramid is Host
- Connected to a node of hub
- Max of 127 devices can be connected to host
-
responsible for all comms on the bus incl
- device addr
- bus b/w
- pwr requirements
-
responsible for all comms on the bus incl
USB 2.0 Diagram
- on-Twisted Power Pair
- Red - VBUB
- Black - Pwr GND
- PVC Jacket
- Outer Shield
- Inner Shield
- Twisted Signalling Pair
- White - D+
- Green - D-
- AWG Tined Copper Drain Wire
USB 3.0 Diagram
- UTP
- Shield Braid
- STP
- STP
2x Techniques USB 3 employs to mitigate attentuation + distortion
De-emphasis Amplitude of transmitted signal voltage is arranged to be higher than normal after a logic-level transition. Gives more drive when high freq. components of the signal are significant which mitigate the greater cable loss that is found @ high freq
Equalisation Perceived signal is multiplied by the inverse of teh channel transfer function so recover the transmitted s(w) using continuous time linear equalisation to flatten the freq response.
