Data Receivers Digital data receivers Equalization Data detection Timing recovery NRZ data spectra Eye diagrams Transmission line response Think of it as another example for a 247 project A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 1 Digital Data Receivers Clock Data Input Input Way back in Lecture 1 we looked briefly at the digital communication problem Everyone wants to send bits as far as they can as fast as they can through the cheapest possible media until recovery of those bits is a complex signal processing problem A D DSP EECS 247 Lecture 25 Digital Data Receivers Data Transmitter Cheap noisy Channel Data Receiver Clock Data Output Output 2002 B Boser 2 Digital Data Receivers Also since nobody wants to invest in a separate channel to send a clock alongside the data timing recovery is a second key responsibility of digital data receivers Today data detection timing recovery is the biggest mixed signal processing market there is A D DSP EECS 247 Lecture 25 Digital Data Receivers Clock Data Input Input Data Transmitter Cheap noisy Channel Data Receiver Clock Data Output Output 2002 B Boser 3 Digital Data Receivers We ll examine digital communications using high speed digital video over coaxial cable as our underlying example 300Mb s over distances of 200m It illustrates many key principles of data detection and timing recovery 2 3 A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 4 NRZ Data Spectrum NRZ Non Return to Zero data is a complicated sounding name for a very simple two level transmission scheme The data transmitter produces two output levels and holds the appropriate binary level for a full bit period We ll assume the two levels are 1 and 1 What s the spectrum of random NRZ data A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 5 Ideal NRZ Data Spectrum We looked at random 1b sequences in Lecture 15 slides 15 4 15 5 Random sequences yield white noise An ideal NRZ data transmitter convolves in time digital data impulses with a zero order hold function slides 12 11 12 12 The resulting spectrum is the product of the digital data s white spectrum and the zero order hold s sinx x response H f Te j fT A D DSP EECS 247 Lecture 25 Digital Data Receivers sin fT fT 2002 B Boser 6 Ideal NRZ Data Spectrum Am dBWN 30 0 30 60 90 0 A D DSP 100 200 300 EECS 247 Lecture 25 Digital Data Receivers 400 500 2002 B Boser 7 Ideal NRZ Data Spectrum Communication channels are not sampled data systems The digital data is passed through a ZOH Let s look at 300Mb s NRZ data Expect nulls at multiples of 300MHz from sinc A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 8 Ideal NRZ Data Spectrum 40 Amplitude dBWN Log frequency scale 20 0 20 zeroes at m 300MHz 40 107 A D DSP 108 109 1010 EECS 247 Lecture 25 Digital Data Receivers 1011 Hz 2002 B Boser 9 Ideal NRZ Data Spectrum 40 Amplitude dBWN H f Te j fT 20 0 20dB decade slope 20 40 107 A D DSP sin fT fT 108 EECS 247 Lecture 25 Digital Data Receivers 109 1010 1011 Hz 2002 B Boser 10 Ideal NRZ Data Spectrum Averaging can provide a better indication of long term bin amplitudes 30 averages here produce a DFT plot based on 900 unique transmitted bits Results conform much more closely to the sinx x response We ll return to our more customary linear frequency scale for DFT plots at 1GHz and below A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 11 Ideal NRZ Data Spectrum Amplitude dBWN 40 20 0 300Mb s NRZ data 30000 point 300GHz DFT 30 averages 20 40 0 A D DSP 250 EECS 247 Lecture 25 Digital Data Receivers 500 750 1000 MHz 2002 B Boser 12 Non Zero Transition Times The zero order hold NRZ spectrum assumes zero transition times between binary levels All real world NRZ data drivers take some time to switch from one level to the other How does this change the ideal NRZ data spectrum A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 13 Non Zero Transition Times We ll shape the edge rate of the ideal NRZ signal via a low pass filter The low pass filter we ll use is a Gaussian LPF commonly used in digital signal analysis applications 4 A Gaussian filter s magnitude response is given by H f e A D DSP f2 2 2 EECS 247 Lecture 25 Digital Data Receivers trise f s 2 56 2002 B Boser 14 Non Zero Transition Times Let s check the effect of a Gaussian Filter Set the 10 to 90 rise times and fall times of the NRZ signal to 100psec 100psec transitions times are still very fast relative to our 3 3nsec data period The filtered NRZ data spectrum appears in red on the following slide A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 15 Ideal vs Filtered Data Spectra Amplitude dBWN 40 300Mb s NRZ data 30000 point 300GHz DFT 30 averages 20 0 20 0 risetime 100psec risetime 40 107 A D DSP 108 109 EECS 247 Lecture 25 Digital Data Receivers 1010 1011 Hz 2002 B Boser 16 Ideal vs Filtered Data Spectra The frequency at which the ideal and filtered NRZ spectra begin to diverge by 6 8dB in fact is called the knee frequency fknee Knee frequencies depend only on transition times not NRZ data rates There s not enough energy above fknee to have much effect on even the simplest data receiver a CMOS inverter For digital signals with 10 90 transition times f knee A D DSP 1 2trise EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 17 Ideal vs Filtered Data Spectra 40 Amplitude dBWN tR 100psec fknee 5GHz 20 0 20 0 risetime 100psec risetime 40 107 A D DSP 108 109 EECS 247 Lecture 25 Digital Data Receivers 1010 1011 Hz 2002 B Boser 18 0 8 V div NRZ Data in the Time Domain tR 100psec 20 nsec div A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 19 0 8 V div Isolated 1 Data Bit tR 100psec 1 nsec div A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 20 Filtered NRZ Data In high speed communications applications the transition times are usually comparable to the bit period Then filtered outputs reach the full 1 and 1 levels only if 2 consecutive data bits are identical Isolated 1 and 1 pulses yield smaller swings Let s see what happens when tR 3nsec fknee 167MHz A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 21 0 8 V div 30 Bit Periods tR 3nsec 20 nsec div A D DSP EECS 247 Lecture 25 Digital Data Receivers 2002 B Boser 22 Eye Diagrams Random …
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