A = 10 log10(A1 + A2 + A3 + A4) RMS PHASE JITTER (radians) ≈ PHASE NOISE (dBc/Hz) A/10 2• 10 A/10 2•10 2 π fC
RMS JITTER (seconds) fC = CLOCK FREQUENCY
A/10 2•10
The World Leader in High-Performance Signal Processing Solutions
Performance Clocks: Demystifying Jitter
Low Jitter Clocks – a Review
The first ADI Web Seminar on Low Jitter Clocks was titled:
Time Jitter on clock results in higher Bit Error Rate (BER), poor call quality Spurious signals on clock result in Adjacent Channel Interference (ACI), dropped calls Signal Path Clock
For an oscillator
Fundamental power decreases As phase noise increases
fc
17
f
foffset
Calculating Time Jitter from Phase Noise
A= AREA = INTEGRATED PHASE NOISE POWER (dBc)
Random Time Jitter is characterized by a statistical distribution. Usually the distribution is assumed to be Gaussian (Normal).
distribution of many error samples
Time Jitter on clock degrades ability to make accurate measurements Time Jitter on clock degrades image in medical equipment
Wireless Infrastructure Radios
200 MHz
INTEGRATE OVER BW OF INTEREST –155 dBc
A
10k
100k
1M
10M
100M
1G
foffset
FREQUENCY OFFSET (Hz) = –155 + 83 = –72 dBc = 355 • 10–6 radians = 565 fs
A = –155 + 10 log10 2⋅ 108 – 0.01⋅ 106 RMS PHASE JITTER (radians) ≈ RMS JITTER (seconds) =
dV
error voltage
encode dt
13
“Low Jitter” refers to Jitter < 1 picosecond RMS
130 120 110 100 SNR in dB 90 80 70 60 50 40 30 1 10 100 1000 ADC Analog Input Frequency in MHz
10
Peak-to-Peak: A statistical reminder
mean
1σ 1σ 6σ
6.18σ bounds 99.9% of measurements
11
What effect does Time Jitter have on Data Converters?
FA
Digital Input
DAC
Analog Output (after LPF)
FA
FC
Sampling Clock
SNR
ADC
Analog Input
Digital Output SNR
For data converter applications, the answer is directly related to analog frequency FA. The actual clock rate FC is not as important!
actual edges occur at different times than expected
ideal signal
actual signal
7
edges should be here if occuring evenly in time
Time Jitter is Statistical
15
Phase Noise Power Spectrum of an Oscillator
Close-in Phase Noise
dBc/Hz
1 Hz BW
Broad-Band Phase Noise
fc
16
foffset
f
Phase Noise Power Spectrum of an Oscillator
14
16 bits 14 bits 12 bits 10 bits
FA 70-300MHz, SNR 60-80dB
Learning to speak a new language…
There is another phenomenon that many engineers are perhaps more familiar with than Time Jitter. Phase Noise Phase Noise is the Frequency Domain effect of Time Jitter. Time Jitter is the Time Domain effect of Phase Noise.
2 Outputs PLL Core DDS/DAC 48 LFCSP
/clocks
3
Low Time Jitter (Low Phase Noise) Clocks are Critical to Signal Processing Applications
Instrumentation
12
FC
ter and SNR
Clock jitter is the sample to sample variation in the encode clock Fullscale SNR (signal to noise ratio) is jitter limited by:
Broadband Infrastructure
Phase Noise on clocks result in data errors, lower throughput
4
AD9510 Clock Distribution IC
ADC
AD9510 provides clocks with sub-picosecond jitter to critical signal path components.
ideal signal
actual signal
edges should be here, but usually aren’t
8
RMS Jitter 1σ (one sigma – a standard deviation) is the same as the RMS
value for a Gaussian (or Normal) distribution
2
ADI’s Low Jitter / Low Phase Noise Clock Products
Clock Distribution
Clock Generation
8 Outputs PLL Core 64 LFCSP
5 Outputs PLL Core 48 LFCSP
5 Outputs 2 Inputs 48 LFCSP
≈
A1
INTEGRATE OVER BW OF INTEREST
A2 A3 A4
10k
100k
1M
10M
100M
1G
foffset
18
FREQUENCY OFFSET (Hz)
Calculating Time Jitter from Broadband Phase Noise
PHASE NOISE (dBc/Hz) fc = 100 MHz CLOCK FREQUENCY
Clock to A-D Converters
ADC ADC ADC
Reference Clock
AD9510 Clock Distribution IC
Clock to Digital Chips
DDC or ASIC DUC or FPGA DAC
ADC, Analog-to-Digital Converters DAC, Digital-to-Analog Converters DDC, Digital Down Converters DUC, Digital Up Converters
Using Low Jitter Clocks to Enhance Converter Performance
(available at /onlineseminars) That seminar introduced the ADI low jitter clock family of ICs. The uses and advantages of these clock chips were presented. These clock chips address application spaces which require better than (less than)1ps of time jitter.