Frequency synthesis and clock generation are now key elements in all aspects of high speed data acquisition and RF design.The primary types of frequency synthesizers—phase-locked loops (PLL) and direct digital synthesizers (DDS)—are discussed along with the applications for which each is appropriate. Also covered are detailed aspects of synthesizer design. Other applications, such as clock distribution and translation are addressed, and problems associated with poor clocking are identified. Examples of poor clocking are shown along with the results of doing it properly.
3. 3
Today’s Agenda
Applications areas for clocks and frequency synthesis
Design and application of phase-locked loops (PLLs)
Design and application of direct digital synthesis (DDS)
Clock generation and distribution
Issues of clocking data converters
4. 4
Five Types of Clocking Chips
Analog PLLs
Uses an analog multiplier as the phase detector
Not in Wide Use
Digital PLLs
Use a digital phase frequency detector (PFD), analog loop filter, voltage controlled oscillator (VCO)
Simple architecture
Very high performance and low noise
All-Digital PLLs
Use a digital phase frequency detector (PFD), digital loop filter, NCO
Increased flexibility for faster locking
Excellent jitter cleaning
Extremely flexible
Direct Digital Synthesis
Extremely flexible frequency generation
Very fast frequency sweeping and hopping
Very popular in military and instrumentation applications
General oscillators
Crystal oscillators
Voltage-controlled oscillators (VCO)
5. 5
What is a clock and what are
the common frequencies?
Unlike a data waveform, a clock signal is a square wave whose
frequency is usually constant.
Common frequencies include:
1 pps (pulse per second) used by GPS
8 kHz (commonly used in wired communcations) and is commonly referred to
as a BITS clock
19.44 MHz is a common reference clock in synchronous optical (SONET)
networks, and is still used in OTU (Optical Transport Unit) networks that are
replacing SONET
122.88 MHz is commonly used in wireless communications
125 and 156.25 MHz are common Ethernet reference clocks
32.768 kHz is the common watch crystal oscillator
6. 3.6
Basic Phase Locked Loop (PLL) Model
(B) STANDARD NEGATIVE FEEDBACK
CONTROL SYSTEM MODEL
(A) PLL MODEL
ERROR DETECTOR LOOP FILTER VCO
FEEDBACK DIVIDER
PHASE
DETECTOR
CHARGE
PUMP FO = N FREF
7. August 2006 ADI Confidential Information7
/2REFA /R1
Phase
Freq
Det
(PFD)
32 MHz< FPFD <44 MHz
Charge
Pump
Loop Filter
(External)
VCO
/4 or 5/B
OUT
Feedback Divider
(N Divider)
/P/2 /R2
Reference
Monitor and
control Logic
REFB
REF
FLAG
VCO
div
Digital PLL Block Diagram
8. 3.8
Phase/Frequency Detector (PFD)
Driving a Charge Pump (CP)
D1 Q1
CLR1
CLR2
D2 Q2
V+
V−
HI
HI
+IN
−IN
DELAY
UP
DOWN
CP OUT
I
I
U1
U2
U3
PFD
CP
D1 Q1
CLR1
CLR2
D2 Q2
V+
V−
HI
HI
+IN
−IN
DELAY
UP
DOWN
CP OUT
I
I
U1
U2
U3
PFD
CP
(A) OUT OF FREQUENCY LOCK AND PHASE LOCK
(B) IN FREQUENCY LOCK, BUT
SLIGHTLY OUT OF PHASE LOCK
0
+I
+I
0
(A) OUT OF FREQUENCY LOCK AND PHASE LOCK
(B) IN FREQUENCY LOCK, BUT
SLIGHTLY OUT OF PHASE LOCK
0
+I
+I
0
UP
1
0
0
DOWN
0
1
0
CP OUT
+ I
−I
0
UP
1
0
0
DOWN
0
1
0
CP OUT
+ I
−I
0
(C) IN FREQUENCY LOCK AND PHASE LOCK
9. 3.9
Adding an Input Reference Divider
and a Prescaler to the Basic PLL
(A)
(B)
REFERENCE
DIVIDER
R
REFERENCE
DIVIDER
R
PRESCALER
P
10. All-Digital PLL Detailed Block Diagram
(AD9557 Shown)
10
SPI/I2C
SERIAL PORT
EEPROM
REF MONITORING
AUTOMATIC
SWITCHING
÷N1 ÷N2
÷N3
÷2 ÷M0
OUT0
OUT0
OUT1
OUT1
10-BIT
INTEGER
DIVIDERS
MAX 1.25GHz
÷M1
×2
×2
LF
PFD/CP
RF DIVIDER 1
÷3 TO ÷11
XO OR XTAL XO FREQUENCIES
10MHz TO 180MHz
XTAL: 10MHz TO 50MHz
RF DIVIDER 2
÷3 TO ÷11
FOUT = 360kHz TO 1.25GHz
INTEGER DIVIDER
OUTPUT PLL (APLL)
FRAC1/
MOD1
17-BIT
INTEGER
24b/24b
RESOLUTION DIGITAL PLL (DPLL)
÷2
REGISTER
SPACE
2kHzTO1.25GHz
R DIVIDER (20-BIT)
SYNC RESET PINCONTROL M0 M1 M2 M3 IRQ
SPI/I2C
DIGITAL
LOOP
FILTER
TUNING
WORD
CLAMP AND
HISTORY
FREERUN
TW
PLL2
STATUS
LF CAP
PFD/CP LF
3.34GHz
TO
4.05GHz
DPFD
30-BITNCO
ROM
AND
FSM
MULTI-FUNCTION I/O PINS
(CONTROL AND STATUS
READ BACK)
SYSTEM
CLOCK
MULTIPLIER
÷2
AD9557
REFA
REFA
REFB
REFB
09197-135
All-Digital PLL Core
Digital PLL
11. 3.11
Key PLL Specifications
RF Input Frequency (Minimum/Maximum)
Phase Noise and Phase Jitter
Reference Spurs
Frequency Lock Time
Output Frequency Error
Phase Lock Time
Output Phase Error
Loop Bandwidth and Phase Margin
12. 3.12
Integer-N Compared to
Fractional-N Synthesizer
REF
DIVIDER
R
PFD FILTER VCO
N COUNTER
FREF
F1
FOUT
10MHz
R =50
0.2MHz
N = 4501
900.2MHz
REF
DIVIDER
R
PFD FILTER VCO
"N" COUNTER
FREF
F1
FOUT
10MHz
R =10
1MHz
900.2MHz
N =900.2
"N" = NINTEGER +
NFRACTION
NMODULUS
= 900 +
NFRACTION
5
FOUT = FREF (N/R)
(A) INTEGER N
(B) FRACTIONAL N
13. 13
Common Uses for PLLs
Frequency translation
Jitter Cleanup
Redundant clocking
Holdover
Clock Distribution
14. /2
REFA
19.44
MHz
/R1
Phase
Freq
Det
(PFD)
10 kHz< FPFD < 50MHz
Charge
Pump
Loop Filter
(External)
VCO
/4 or 5/B
156.25
MHz
Feedback Divider
(N Divider)
/P/2 /R2
Reference
Monitor and
control Logic
REFB
REF
FLAG
VCO
div
14
Frequency Translation Example:
19.44 MHz (SONET) to 156.25 MHz (10 Gb/s Ethernet):
R divider=162, B=15625, VCO divider = 3, P divider = 4
Phase detector frequency: 120 kHz
VCO frequency: 1875 MHz
15. 15
Jitter Clean-up
Clean signal from main
clock board
Backplane
has lots of
noise
sources Clock received by line
card is contaminated
Clock received from back plane is
used to establish phase and
frequency of the output
Signal purity of the output is
dependent upon the Local
oscillator (Crystal, TCXO, or
OCXO) used HOW?
Digital PLL w/ a
Programmable
Digital loop
Filter capable of
<1 Hz BW
16. Switchover and Holdover
Holdover:
Holdover is the ability to provide output signals even when the reference input disappears.
Holdover can be initiated either as directed by controller/processor elements in a system, or
via a provided monitoring function which will automatically switch into holdover mode when
the reference input goes quiet.
Switchover:
Switchover provides additional security beyond the holdover function. If one of the references
fails, the clock device will use one of the alternate references instead. An important aspect of
all the switchover functions provided in ADI clock devices is that no runt pulses and no extra
long pulses result from this change. Downstream PLLs will not lose lock as a result, of or
during, switchover - even when no predefined relationship exists between the phases of the
various reference input signals. Switchover can be initiated either as directed by
controller/processor elements in a system, or via a provided monitoring function which will
automatically implement switchover when the active reference input goes quiet.
17. Switchover, Synchronization, and Holdover
NOTE
output is synchronized to primary
reference
But what happens when the
primary reference disappears?
The PLL will maintain the output clock in holdover until another
reference input is available. The output phase may or may not slew
(depending on the application) so that either the input-output phase
is the maintained or there is no output clock phase slewing.
AD9548
18. 3.18
TOOLS – Design, Simulation, Evaluation
Full Range of Evaluation Boards for DDS, Clock Generation and Distribution, PLLs Available.
Full suite of Windows-Compatible Software Available
http://www.analog.com/en/evaluation-boards-kits/resources/index.html
http://www.analog.com/en/rf-tools/topic.html
http://ez.analog.com/welcome
22. Forums in
22
Get fast answers to
new questions
Search existing content
for immediate answers
http://ez.analog.com/community/dds
http://ez.analog.com/community/clock_and_timing
http://ez.analog.com/community/rf
23. 3.23
Eval Software Frequency Planning Wizard:
Enter your desired input and outputs…
Input Frequency Window Output Frequency Window
27. 27
AD9516 Family 1.5 -3.0 GHz, 8/5-Channel Clock
Distribution ICs
Clock Outputs
1.2 GHz LVPECL
800 MHz LVDS
250 MHz CMOS
PLL Core
250 MHz REFIN
1.6 GHz PLL
Jitter Clean-up
Programmable Dividers
Any integer 1 to 32
Phase offset control
Each divider independent
Programmable Delay Adjust
Fullscale from 1ns to 10ns
32 delay steps
64-LFCSP
typically replaces
Five(5) discrete ICs
AD9510 Shown Below, Broadband RMS Jitter <1ps
28. 28
Application – Wireless Transceiver Card
ADC
TRX
Clock Distribution IC
ADC
ADC
ADC
DDC or
ASIC
DAC
DUC or
FPGA
DAC
User’s
Reference
Clock
Clock to A-D Converters
Clock to D-A Converters
Clock to
Digital Chips
Critical Clock Functions on Transceiver Card:
• clean-up jitter on user’s input reference
• up-convert user reference frequency to highest
frequency needed, usually driven by DAC clock
requirements
• generate multiple frequencies for RX & TX
• provide low jitter clocks for converters
• generate mix of LVPECL, LVDS, CMOS clocks
• adjust phase or delay between clock channels
• offer isolation between clock channels
TRX Cards
30. SyncE / IEEE1588 Hybrid
(with Hooks for Pure IEEE1588)
Backplane
Line Card
AD9557
AD9547
TCXO /
OCXO Recovered clocks
from Line cards
BITS
GPS
Timing Card
XO AD9553/7
(Optional)
Tx
Rx
CPU / FPGA / DSP
IEEE1588
Protocol / Algorithm
SPI/I2C
MAC/PHY
SyncE Clock Recovering
+
IEEE1588 Time Stamp
Time Stamps
Frequency
Synchronization
1 PPS
Timing Card 2
Line Card n
Time of Day Offset Adjustment
1 PPS
Time of Day
Clock/Frequency Control
AD9548
32. 32
Generating Clocks using DDS
Limiter
Reconstruction
Filter
Fsysclock(fc) DAC out Filter out
Clock out
Ideal Time
Domain
Response
Ideal
Frequency
Domain
Response
"Real World"
Frequency
Response
t
0
1 1 3 5 7
Odd harmonic series
1 3 5 7
t t
f ff
fff
fc
fc 2fc
2fc
DDS
The DDS chip can synchronize to a user’s reference. An on-chip clock multiplier can
generate the fast clock needed to clock the NCO/DAC. A frequency tuning word may be
written to set the output clock rate. External filtering removes unwanted images.
A squaring function then converts sine wave to square wave.
33. 3.33
A Flexible DDS System
fc
SERIAL
OR BYTE
LOAD
REGISTER
nn
FREQUENCY CONTROL
PHASE
REGISTER
LPF
DAC
PARALLEL
DELTA
PHASE
REGISTER
M CLOCK
n n
PHASE ACCUMULATOR
n
PHASE
TRUNCATION
12-19 BITS
AMPLITUDE
TRUNCATION
2n
=fo
M • fc
N-BITS
n = 24 - 48 BITS
PHASE-TO
AMPLITUDE
CONVERTER
M = TUNING WORD
SYSTEM CLOCK
(10-14)
6-bit
phase
wheel
0
1
2
3
4
63
0
2
4
31
29
……
5-bit
amplitude
resolution
fo
vector data
raw DDS-DAC output
filtered output
compared output
34. 3.34
Signal Flow Through the DDS Architecture
REFERENCE
CLOCK
PHASE
ACCUMULATOR
(n-BITS)
PHASE-TO-AMPLITUDE
CONVERTER
DAC
M
TUNING WORD SPECIFIES
OUTPUT FREQUENCY AS A
FRACTION OF REFERENCE
CLOCK FREQUENCY
IN DIGITAL DOMAIN ANALOG
N
DDS CIRCUITRY (NCO)
TO
FILTER
2n
=fo
M • fc
2n
=fo
M • fc
fc
2n
=fo
M • fc
M = JUMP SIZE
36. 3.36
DDS Single Loop Upconversion
Using the AD9858
DDS
1GHz
DAC
1032
LPF
DIVIDER
1/2/4
PHASE/
FREQUENCY
DETECTOR
150MHz
CHARGE PUMP
0.5mA-2mA
0.5mA STEPS
LOOP
FILTER ~
DIVIDER
K
DC - 400MHz
VCO
f = K fREF
DDS/DAC
CLOCK
FREQUENCY
TUNING WORD
PART OF
AD9858:
fREF
DC - 150MHz
37. DDS vs. PLL
Comparing: Advantage The rest of the story
Freq. Resolution DDS Fractional N PLLs shrink the gap, Programmable Modulus
improves DDS precision
Freq. Agility DDS Fast hopping PLLs shrink the gap
Phase Resolution &
Agility
DDS Digital PLLs can provide some level of phase control.
Amplitude Resolution
& Agility
DDS
Power Consumption PLL Gap shrinks with geometry; interleaved cores
Output Frequency
Range
PLL
Price PLL* Gap shrinks with geometry; in no small part this is due to
the breadth of adoption of PLL technology,
Broad Spectral Purity PLL
Ancillary circuitry PLL
Freq. Up-conversion PLL Super Nyquist operation and hybrids
37
38. Hybrid configurations
DDSRefCLK PLL Upconverting PLL
DDSRefCLK PLL RefCLK multipying PLL
PLL
DDS
RefCLK DDS in feedback path
PLL
DDS
RefCLK DDS as a DCO
38
40. Effective Aperture Delay Time
Measured with Respect to ADC Input
SAMPLING
CLOCK
ANALOG INPUT
SINEWAVE
ZERO CROSSING
+FS
-FS
0V
+te
–te
te
' '
'
41. 41
Jitter – common noise source introduced at
SHA in A-D Converter
Clock jitter is the sample to sample variation
in the encode clock (both the external jitter as well
as the internal jitter).
Fullscale SNR is jitter-limited by:
See AN-501 and AN-756
SHA = Sample & Hold Amplifier
jitterrms
rms
jitter
ftN
S
SNR
2
1
log20log20
42. 42
45.0
50.0
55.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
100 1000
50 fs
100 fs
200 fs
400 fs
800 fs
Fullscale Analog Input (sinewave)
84dB
78dB
AIN = 200 MHz
Each line shows
constant RMS
clock jitter in
femtoseconds (fs)
72dB
66dB
60dB
300
MHz
400
MHz
500
MHz
SNR of ADC @ 200 MHz
AIN varies with clock jitter
SignaltoNoiseRatio(SNR)indB
ADC
Analog
Input
Sampling Clock
SNR
Digital
Output
As analog signal increases, clock jitter limits SNR
jitter
jit
ft
SNR
2
1
log20
47. 3.47
LOOP
FILTER
VCXO
System Clock Distribution Examples
ADC FIFO
122.88 MHz
122.88 MHz
LVPECL CMOS
DELAY = 4.3ns
HIGH SPEED MEASUREMENT SUBSYSTEM
REFCLK
491.52 MHz
LVPECL
30.72 MHz
DAC
DACFPGA
LVDS
CMOS
CMOS
QUADRATURE TRANSMIT SOURCE
61.44 MHz
61.44 MHz
PHASE = 90
DELAY = 10ns
122.88
MHz
LVPECL
491.52 MHz
CLEAN_REFCLK
30.72 MHz
CALIBRATION
15.36 MHz
Clock ICs simplify board design
by integrating phase control,
delay adjust, frequency dividers,
and logic translation
PHASE = 0
TOYOCOM
491.52 MHz
AD9513/AD9514/AD9515 easy to design
in. Require only a +3.3V supply. All
functionality selected by tying input pins to
VS, GND, VREF, or NC
48. 3.48
AD9512 1.2GHz Clock Distribution IC
Delay 1-10ns
1:5 Fanout
Buffer
Divide by 1-32
LVDS OR
CMOS
LVDS OR
CMOS
225 fs rms
225 fs rms
350 fs rms
1-3 ps rms
A
rms jitter added to
signal at A
225 fs rms
Divide by 1-32
Divide by 1-32
Divide by 1-32
Divide by 1-32 LVPECL
Buffer
LVPECL
Buffer
LVPECL
Buffer
TOTAL JITTER = J1
2 + J2
2 + J3
2 +...+JN
2
49. 49
ADI’s Complete Clock Portfolio
Digital and All-Digital PLLs
Used for frequency multiplication/translation
Redundant Clocking and Holdover
Synthesizers
Used for clock generation
Clock Distribution
Used for sending the identical clock to multiple chips
Also used for logic level translation (i.e., LVPECL to LVDS)
May include frequency dividers (/2, /4, etc.)
May include skew adjustment
Voltage-controlled oscillators
50. 50
What we covered
As system complexity and performance demands increase,
frequency synthesis devices have had to keep pace with greater
performance and versatility
Design and application of phase-locked loops (PLLs)
Design and application of direct digital synthesis (DDS)
Software tools greatly simplify design and set-up of complex
frequency synthesis devices
Clocks for data converters need to have low jitter to keep distortion
at a minimum
Specialized clock generation and distribution allows precise
frequency tuning and phase control
51. Visit the DDS, PLL and CLK simulators in the
demonstration room
ADIsimCLK, ADIsimPLL and
ADIsimDDS can quickly
configure the complex registers
and settings on frequency
synthesis devices to provide
optimum performance
Image of demo/board
51
VERSION 3.5