pll phase noise figures of merit
TRANSCRIPT
PLL Phase Noise Figures of Merit TI Precision Labs – Clocks and Timing
Presented by Dean Banerjee
Prepared by Liam Keese
1
Phase lock loop (PLL) overview
2
R Divider
1/D
Output DividerReference
Oscillator
1/N
1/R
VCOLoopFilterPhase
Detector/Charge Pump
N Divider
Z(s)s
KVCO
+KPD
-
Phase lock loop (PLL) transfer functions
3
fVCO
1/N
1/RZ(s)
sKVCO
+KPD
-
Block Transfer Function
OSC 1
𝑅∙
𝐺(𝑠)
1 + 𝐺(𝑠) ∙ 𝐻
R/N
Dividers
𝐺(𝑠)
1 + 𝐺(𝑠) ∙ 𝐻
Phase
Detector
1
𝐾𝑃𝐷∙
𝐺(𝑠)
1 + 𝐺(𝑠) ∙ 𝐻
VCO 1
1 + 𝐺(𝑠) ∙ 𝐻
𝑯 𝒔 = 𝑯 = 𝟏/𝑵
𝑮 𝒔 = 𝑲𝑷𝑫 × 𝒁(𝒔) × 𝑲𝑽𝑪𝑶
𝒔
𝒔 = 𝒋 ∙ 𝟐𝝅𝒇
Reference oscillator noise transfer function
4
Block Transfer
Function
Response Low Frequency
Response
High Frequency
Response
OSC 1
𝑅∙
𝐺(𝑠)
1 + 𝐺(𝑠) ∙ 𝐻
Low Pass 20 ∙ log (𝑁/𝑅) 20 ∙ log (𝐺(𝑠)/𝑅)
BW
20 x log(N/R)G
ain
(d
B)
Offset Frequency (Hz)
| G(s) |
1 R ∙
G(s)1 + G(s) ∙ H
1 R ∙
Feedback N divider noise transfer function
5
Block Transfer
Function
Response Low Frequency
Response
High Frequency
Response
N (or R)
Divider
𝐺(𝑠)
1 + 𝐺(𝑠) ∙ 𝐻
Low Pass 20 ∙ log (𝑁) 20 ∙ log (𝐺(𝑠))
BW
20 x log(N)G
ain
(d
B)
Offset Frequency (Hz)
| G(s) |
G(s)1 + G(s) ∙ H
Phase det./charge pump noise transfer function
6
Block Transfer
Function
Response Low Frequency
Response
High Frequency
Response
Phase Det./
CP gain
1
𝐾𝑃𝐷∙
𝐺(𝑠)
1 + 𝐺(𝑠) ∙ 𝐻
Low Pass 20 ∙ log (𝑁/𝐾𝑃𝐷) 20 ∙ log (𝐺(𝑠)/𝐾𝑃𝐷)
BW
20 x log(N/KPD)G
ain
(d
B)
Offset Frequency (Hz)
| G(s) |
1 KPD
∙
G(s)1 + G(s) ∙ H
1 KPD
∙
VCO noise transfer function
7
Block Transfer
Function
Response Low Frequency
Response
High Frequency
Response
VCO 1
1 + 𝐺(𝑠) ∙ 𝐻
High Pass −20 ∙ log (𝐺(𝑠)) 1
BW
1G
ain
(d
B)
Offset Frequency (Hz)
1
1 + G(s) ∙ H
1
G(s)
N ∙
-180
-170
-160
-150
-140
-130
-120
-110
-100
-90
-80
-70
-60
0.1 1 10 100 1000 10000 100000
Ph
ase N
ois
e (
dB
c/H
z)
Offset (kHz)
PLL VCO Reference Total
PLL closed loop noise sources
The size of these regions will change depending on loop bandwidth
Ref OSC PLL VCO
PLL normalized phase noise
9
Log(Frequency)
BW
Everything Except VCO
VCO
Ga
in (
dB
)
0
20∙log(N)
• PLL flat noise FOM (PN1Hz)
– PLL noise floor normalized to1 Hz
– N-counter added noise = 20 log (N)
• PLL flicker noise (PN10kHz)
– Usually dominates at offset below 1 kHz
– PLL 1/f normalized to 1 GHz output and 10kHz offset
PLL noise simulation
10
• PLL FOM
– PN1Hz = PN – 20 ∙ log(N) – 10 ∙ log(fPD)
• PLL Flicker
– PN10kHz = PN(Δf) -20 ∙ log(fout/1GHz)
-10 ∙ log(10kHz/ Δf)
PLL phase noise shaping levers
PLL Functional Block To minimize Noise
contribution…
Why?
Phase Detector/Charge
Pump
Maximize charge pump gain
(KPD) (up to a certain point)
The phase detector noise
contribution is proportional to
1/(KPD)2
R-counter and N-counter
divide ratios
Maximize phase detector
compare frequency this
minimizes N
The noise contribution of the
R and N dividers is
proportional to N2.
Reference oscillator Use highest frequency practical
and use R > 1 if possible. If
deciding between maximizing R
and minimizing N, minimize N.
The noise contribution from
the reference oscillator is
proportional to (N/R)2
To find more technical resources and search products, visit ti.com/clocks
12
Quiz
• True or false: Reducing N-divider value will decrease PLL noise by 20log(N)
• True or false: The reference oscillator does not contribute to PLL in band noise
• True or false: Increasing the charge pump current setting will reduce PLL noise
• True or false: The VCO tuning constant KVCO only has an effect on noise
outside of the PLL loop bandwidth
13
Quiz
• True or false: Reducing N-divider value will decrease PLL noise by 20log(N)
• True or false: The reference oscillator does not contribute to PLL in band noise
• True or false: Increasing the charge pump current setting will reduce PLL noise
• True or false: The VCO tuning constant KVCO only has an effect on noise
outside of the PLL loop bandwidth
14
PLL Transient Response Quiz TI Precision Labs – Clocks and Timing
Presented by Dean Banerjee
Prepared by Vibhu Vanjari
1
Quiz
• True or False: The phase margin is the phase of the open loop transfer function
when the gain of the PLL is equal to 0 dB.
• True or False: Phase margins under 30º should be avoided to enhance the
stability of the PLL and minimize ringing.
• True or False: Larger bandwidths lead to shorter lock times.
2
Quiz
• True or False: The phase margin is the phase of the open loop transfer function
when the gain of the PLL is equal to 0 dB.
– The phase margin is the distance of the phase from -180 degrees when the gain of
the PLL is equal to 0 dB.
• True or False: Phase margins under 30º should be avoided to enhance the
stability of the PLL, and minimize ringing.
– Phase margins under 30º can lead to instability, peaking in the closed loop filter
response, and ringing in the transient response.
• True or False: Larger bandwidths lead to shorter lock times.
– Wider loop bandwidths allow the PLL to track changes in frequency faster.
3