6.976
High Speed Communication Circuits and Systems
Lecture 16
Noise in Integer-N Frequency Synthesizers
Michael Perrott
Massachusetts Institute of Technology
Copyright © 2003 by Michael H. Perrott
Frequency Synthesizer Noise in Wireless Systems
Zin
From Antenna
and Bandpass
Filter
PC board
trace
Zo
Package
Interface
RF in
Mixer
IF out
To Filter
LNA
LO signal
VCO
Reference
Frequency
Frequency
Synthesizer
Phase
Noise
fo
ƒ
ƒ
f
Synthesizer noise has a negative impact on system
- Receiver – lower sensitivity, poorer blocking performance
- Transmitter – increased spectral emissions (output spectrum
must meet a mask requirement)
Noise is characterized in frequency domain
M.H. Perrott
MIT OCW
Noise Modeling for Frequency Synthesizers
Sout(f)
PLL dynamics
set VCO
carrier frequency
vc(t)
Phase
Noise
Extrinsic noise
(from PLL)
Intrinsic
noise
vn(t)
vin(t)
fo
f
out(t)
To PLL
ƒ
PLL has an impact on VCO noise in two ways
ƒ
Focus on modeling the above based on phase deviations
- Adds extrinsic noise from various PLL circuits
- Highpass filters VCO noise through PLL feedback dynamics
- Simpler than dealing directly with PLL sine wave output
M.H. Perrott
MIT OCW
Phase Deviation Model for Noise Analysis
Note: Kv units are Hz/V
PLL dynamics
set VCO
carrier frequency
vc(t)
Frequency-domain view
Sout(f)
Phase
Noise
Extrinsic noise
(from PLL)
Intrinsic
noise
vn(t)
vin(t)
2πKv
s
Φvn(t)
f
fo
Φout
2cos(2πfot+Φout(t))
out(t)
To PLL
ƒ
Model the impact of noise on instantaneous phase
- Relationship between PLL output and instantaneous phase
- Output spectrum (from Lecture 12)
M.H. Perrott
MIT OCW
Phase Noise Versus Spurious Noise
ƒ
Phase noise is non-periodic
Sout(f)
1
dBc/Hz
fo
-fo
SΦout(f)
f
- Described as a spectral density relative to carrier power
ƒ
Spurious noise is periodic
dBc
-fo
fspur
Sout(f)
1 dspur 2
2 fspur
1
fo
f
- Described as tone power relative to carrier power
M.H. Perrott
MIT OCW
Sources of Noise in Frequency Synthesizers
Reference
Jitter
Reference
Feedthrough
Charge Pump
Noise
VCO Noise
-20 dB/dec
f
T
ref(t)
1/T
f
f
e(t) Charge
PFD
Pump
Loop
Filter
f
v(t)
VCO
div(t)
Divider
Divider
Jitter
N
f
ƒ
Extrinsic noise sources to VCO
- Reference/divider jitter and reference feedthrough
- Charge pump noise
M.H. Perrott
MIT OCW
Modeling the Impact of Noise on Output Phase of PLL
Divider/Reference
Jitter
Reference
Feedthrough
Charge Pump
Noise
S Espur(f)
S Φjit(f)
VCO Noise
S Φvn(f)
S Ιcpn(f)
-20 dB/dec
f
0
0
Φjit[k]
Φref [k]
f
π
PFD
f
0
e(t)
f
0
Ιcpn(t)
espur(t)
α
Φdiv[k]
1/T
Φvn(t)
Icp
H(f)
Charge
Pump
Loop
Filter
v(t)
KV
Φout(t)
jf
VCO
1
N
Divider
ƒ
Determine impact on output phase by deriving
transfer function from each noise source to PLL
output phase
- There are a lot of transfer functions to keep track of!
M.H. Perrott
MIT OCW
Simplified Noise Model
PFD-referred
Noise
S En(f)
VCO-referred
Noise
S Φvn(f)
-20 dB/dec
0
1/T
f
Φvn(t)
en(t)
Φref [k]
α
π
Φdiv[k]
PFD
f
0
e(t)
Icp
H(f)
Charge
Pump
Loop
Filter
v(t)
KV
Φout(t)
jf
VCO
1
N
Divider
ƒ
Refer all PLL noise sources (other than the VCO) to
the PFD output
- PFD-referred noise corresponds to the sum of these
noise sources referred to the PFD output
M.H. Perrott
MIT OCW
Impact of PFD-referred Noise on Synthesizer Output
PFD-referred
Noise
S En(f)
VCO-referred
Noise
S Φvn(f)
-20 dB/dec
0
1/T
f
Φvn(t)
en(t)
Φref [k]
α
π
Φdiv[k]
PFD
f
0
e(t)
Icp
H(f)
Charge
Pump
Loop
Filter
v(t)
KV
Φout(t)
jf
VCO
1
N
Divider
ƒ
Transfer function derived using Black’s formula
M.H. Perrott
MIT OCW
Impact of VCO-referred Noise on Synthesizer Output
PFD-referred
Noise
S En(f)
VCO-referred
Noise
S Φvn(f)
-20 dB/dec
0
1/T
f
Φvn(t)
en(t)
Φref [k]
α
π
Φdiv[k]
PFD
f
0
e(t)
Icp
H(f)
Charge
Pump
Loop
Filter
v(t)
KV
Φout(t)
jf
VCO
1
N
Divider
ƒ
Transfer function again derived from Black’s formula
M.H. Perrott
MIT OCW
A Simpler Parameterization for PLL Transfer Functions
PFD-referred
Noise
S En(f)
VCO-referred
Noise
S Φvn(f)
-20 dB/dec
0
1/T
f
Φvn(t)
en(t)
Φref [k]
α
π
Φdiv[k]
PFD
f
0
e(t)
Icp
H(f)
Charge
Pump
Loop
Filter
v(t)
KV
Φout(t)
jf
VCO
1
N
Divider
ƒ
Define G(f) as
Always has a gain
of one at DC
- A(f) is the open loop transfer function of the PLL
M.H. Perrott
MIT OCW
Parameterize Noise Transfer Functions in Terms of G(f)
ƒ
PFD-referred noise
ƒ
VCO-referred noise
M.H. Perrott
MIT OCW
Parameterized PLL Noise Model
PFD-referred
Noise
S En(f)
VCO-referred
Noise
S Φvn(f)
-20 dB/dec
1/T
0
f
0
en(t)
π
α N G(f)
fo
f
Φvn(t)
fo
1-G(f)
Φnvco(t)
Φnpfd(t)
Φn(t)
Divider Control
Φc(t)
of Frequency Setting
(assume noiseless for now)
ƒ
ƒ
ƒ
Φout(t)
PFD-referred noise is lowpass filtered
VCO-referred noise is highpass filtered
Both filters have the same transition frequency values
Defined as f
M.H. Perrott
o
MIT OCW
Impact of PLL Parameters on Noise Scaling
VCO-referred
Noise
S Φvn(f)
-20 dB/dec
1/T
0
f
0
en(t)
f
2
π
α N S e n (f)
Radians2/Hz
PFD-referred
Noise
S En(f)
S Φvn(f)
Φvn(t)
f
0
fo
π
α N G(f)
fo
1-G(f)
Φnvco(t)
Φnpfd(t)
Φn(t)
Divider Control
Φc(t)
of Frequency Setting
(assume noiseless for now)
ƒ
Φout(t)
PFD-referred noise is scaled by square of divide value
and inverse of PFD gain
- High divide values lead to large multiplication of this noise
ƒ
VCO-referred noise is not scaled (only filtered)
M.H. Perrott
MIT OCW
Optimal Bandwidth Setting for Minimum Noise
VCO-referred
Noise
S Φvn(f)
-20 dB/dec
1/T
0
f
0
en(t)
f
2
π
α N S e n (f)
Radians2/Hz
PFD-referred
Noise
S En(f)
S Φvn(f)
Φvn(t)
0
fo
π
α N G(f)
fo
(fo)opt
f
1-G(f)
Φnvco(t)
Φnpfd(t)
Φn(t)
Divider Control
Φc(t)
of Frequency Setting
(assume noiseless for now)
ƒ
Φout(t)
Optimal bandwidth is where scaled noise sources meet
- Higher bandwidth will pass more PFD-referred noise
- Lower bandwidth will pass more VCO-referred noise
M.H. Perrott
MIT OCW
Resulting Output Noise with Optimal Bandwidth
VCO-referred
Noise
S Φvn(f)
-20 dB/dec
1/T
0
f
0
en(t)
f
2
π
α N S e n (f)
Radians2/Hz
PFD-referred
Noise
S En(f)
S Φvn(f)
Φvn(t)
0
fo
Φnvco(t)
Φnpfd(t)
Φn(t)
Divider Control
Φc(t)
of Frequency Setting
(assume noiseless for now)
(fo)opt
1-G(f)
Φout(t)
S Φnpfd(f)
Radians2/Hz
fo
π
α N G(f)
f
S Φnvco(f)
0
(fo)opt
f
ƒ
PFD-referred noise dominates at low frequencies
ƒ
VCO-referred noise dominates at high frequencies
- Corresponds to close-in phase noise of synthesizer
Corresponds to far-away phase noise of synthesizer
M.H. Perrott
MIT OCW
Analysis of Charge Pump Noise Impact
Charge Pump
Noise
VCO Noise
S Φvn(f)
S Ιcpn(f)
-20 dB/dec
PFD-referred
Noise
α
π
Φdiv[k]
PFD
e(t)
f
0
Ιcpn(t)
en(t)
Φref [k]
f
0
Φvn(t)
Icp
H(f)
Charge
Pump
Loop
Filter
v(t)
KV
Φout(t)
jf
VCO
1
N
Divider
ƒ
We can refer charge pump noise to PFD output by
simply scaling it by 1/Icp
M.H. Perrott
MIT OCW
Calculation of Charge Pump Noise Impact
Charge Pump
Noise
VCO Noise
S Φvn(f)
S Ιcpn(f)
-20 dB/dec
PFD-referred
Noise
α
π
Φdiv[k]
PFD
e(t)
f
0
Ιcpn(t)
en(t)
Φref [k]
f
0
Φvn(t)
Icp
H(f)
Charge
Pump
Loop
Filter
v(t)
KV
Φout(t)
jf
VCO
1
N
Divider
ƒ
Contribution of charge pump noise to overall output noise
- Need to determine impact of I
cp
M.H. Perrott
on SIcpn(f)
MIT OCW
Impact of Transistor Current Value on its Noise
Id
Ibias
id2bias
current
bias
M1
id2 W
L
M2
Cbig
current
source
ƒ
Charge pump noise will be related to the current it
creates as
ƒ
Recall that gdo is the channel resistance at zero Vds
- At a fixed current density, we have
M.H. Perrott
MIT OCW
Impact of Charge Pump Current Value on Output Noise
ƒ
Recall
ƒ
Given previous slide, we can say
- Assumes a fixed current density for the key transistors
ƒ
in the charge pump as Icp is varied
Therefore
- Want high charge pump current to achieve low noise
- Limitation set by power and area considerations
M.H. Perrott
MIT OCW
Impact of Synthesizer Noise on Transmitters
Sx(f)
Sy(f)
reduction
of SNR
out-of-band
emission
f
fIF
x(t)
f
fRF
y(t)
out(t)
Sout(f)
close-in
phase noise
far-away
phase noise
Synthesizer
fLO
ƒ
f
Synthesizer noise can be lumped into two categories
- Close-in phase noise: reduces SNR of modulated signal
- Far-away phase noise: creates spectral emissions outside
the desired transmit channel
ƒ This is the critical issue for transmitters
M.H. Perrott
MIT OCW
Impact of Remaining Portion of Transmitter
Sx(f)
Sy(f)
reduction
of SNR
out-of-band
emission
f
fIF
x(t)
f
fRF
y(t)
Band Select
Filter
PA
out(t)
Sout(f)
close-in
phase noise
far-away
phase noise
Synthesizer
fLO
ƒ
To
Antenna
f
Power amplifier
- Nonlinearity will increase out-of-band emission and create
harmonic content
ƒ
Band select filter
- Removes harmonic content, but not out-of-band emission
M.H. Perrott
MIT OCW
Why is Out-of-Band Emission A Problem?
Transmitter
1
Interfering
Channel
( Desired
Channel)
Relative
Power
Difference
(dB)
Transmitter
2
( Interfering
Channel )
ƒ
Base
Station
Desired
Channel
Near-far problem
- Interfering transmitter closer to receiver than desired
transmitter
- Out-of-emission requirements must be stringent to
prevent complete corruption of desired signal
M.H. Perrott
MIT OCW
Specification of Out-of-Band Emissions
Maximum
RF Output
Emission
(dBm)
M0 dBm
M1 dBm
M3 dBm
Integration
Bandwidth
= R Hz
M2 dBm
f
fRF
Channel Spacing
= W Hz
ƒ
Maximum radiated power is specified in desired and
adjacent channels
- Desired channel power: maximum is M dBm
- Out-of-band emission: maximum power defined as
0
integration of transmitted spectral density over
bandwidth R centered at midpoint of each channel offset
M.H. Perrott
MIT OCW
Calculation of Transmitted Power in a Given Channel
R Hz
Sx(fmid)
Sx(fmid)
fmid
ƒ
R Hz
fmid
For simplicity, assume that the spectral density is flat
over the channel bandwidth
- Actual spectral density of signal often varies with
frequency over the bandwidth of a given channel
ƒ
Resulting power calculation (single-sided Sx(f))
ƒ
Express in dB ( Note: dB(x) = 10log(x) )
M.H. Perrott
MIT OCW
Transmitter Output Versus Emission Specification
Piecewise Constant Approximation
of Transmitter Output Spectrum
RF Output
(dBm)
Y0 dBm
Y0+X2 dBm
Y0+X3 dBm
fRF
Channel Spacing
= W Hz
ƒ
Maximum
RF Output
Emission
(dBm)
Channel
Spacing
= W Hz
Y0+X1 dBm
ƒ
Emission Specification
M0 dBm
M1 dBm
f
M3 dBm
Integration
Bandwidth
= R Hz
M2 dBm
f
fRF
Channel Spacing
= W Hz
Assume a piecewise constant spectral density profile
for transmitter
- Simplifies calculations
Issue: emission specification is measured over a
narrower band than channel spacing
- Need to account for bandwidth discrepancy when doing
calculations
M.H. Perrott
MIT OCW
Correction Factor for Bandwidth Mismatch
Piecewise Constant Approximation
of Transmitter Output Spectrum
RF Output
(dBm)
Y0 dBm
Maximum
RF Output
Emission
(dBm)
Channel
Spacing
= W Hz
Y0+X1 dBm
Y0+X2 dBm
Y0+X3 dBm
fRF
Channel Spacing
= W Hz
ƒ
Emission Specification
M0 dBm
M1 dBm
f
M3 dBm
Integration
Bandwidth
= R Hz
M2 dBm
fRF
f
Channel Spacing
= W Hz
Calculation of maximum emission in offset channel 1
M.H. Perrott
MIT OCW
Condition for Most Stringent Emission Requirement
Piecewise Constant Approximation
of Transmitter Output Spectrum
RF Output
(dBm)
Y0 dBm
Emission Specification
Maximum
RF Output
Emission
(dBm)
Channel
Spacing
= W Hz
Y0+X1 dBm
Y0+X2 dBm
Y0+X3 dBm
fRF
M0 dBm
M1 dBm
M3 dBm
f
M2 dBm
fRF
Channel Spacing
= W Hz
ƒ
Integration
Bandwidth
= R Hz
f
Channel Spacing
= W Hz
Out-of-band emission requirements are function of
the power of the signal in the desired channel
- For offset channel 1 (as calculated on previous slide)
- Most stringent case is when Y
M.H. Perrott
0
maximum
MIT OCW
Table of Most Stringent Emission Requirements
Piecewise Constant Approximation
of Transmitter Output Spectrum
RF Output
(dBm)
Y0 dBm
Emission Specification
Maximum
RF Output
Emission
(dBm)
Channel
Spacing
= W Hz
Y0+X1 dBm
Y0+X2 dBm
Y0+X3 dBm
M1 dBm
f
fRF
M0 dBm
M3 dBm
M2 dBm
Channel Spacing
= W Hz
M.H. Perrott
fRF
f
Channel Spacing
= W Hz
Channel
Offset
Mask
Power
0
M0 dBm
Y0 = M0 (for most stringent case)
1
M1 dBm
X1 = M1-M0 + dB(W/R) dB
2
M2 dBm
M3 dBm
X2 = M2-M0 + dB(W/R) dB
X3 = M3-M0 + dB(W/R) dB
3
Integration
Bandwidth
= R Hz
Emission Requirements
(Most Stringent)
MIT OCW
Impact of Synthesizer Noise on Transmitter Output
IF Input
(dBm)
RF Output
(dBm)
M0 dBm
M0+X2 dBm
fIF
f
f
fRF
foffset
Synthesizer
Spectrum
(dBc)
0 dBc
IF
RF
LO
X2 dBc
fLO
To
Antenna
PA
Band Select
Filter
f
foffset
ƒ
Consider a spurious tone at a given offset frequency
- Convolution with IF signal produces a replica of the
desired signal at the given offset frequency
M.H. Perrott
MIT OCW
Impact of Synthesizer Phase Noise (Isolated Channel)
IF Input
(dBm)
RF Output
(dBm)
M0 dBm
M0+X2 dBm
fIF
f
f
fRF
foffset
Synthesizer
Spectrum
(dBc)
0 dBc
IF
RF
LO
X2 dBc
fLO
To
Antenna
PA
Band Select
Filter
f
foffset
ƒ
Consider phase noise at a given offset frequency
- Convolution with IF signal produces a smeared version
of the desired signal at the given offset frequency
ƒ For simplicity, approximate smeared signal as shown
M.H. Perrott
MIT OCW
Impact of Synthesizer Phase Noise (All Channels)
IF Input
(dBm)
RF Output
(dBm)
fIF
M0 dBm
M0+X1 dBm
M0+X2 dBm
M0+X3 dBm
f
f
fRF
Channel Spacing
= W Hz
Synthesizer
Spectrum
(dBc)
X3 dBc
0 dBc
IF
RF
X0 dBc
X1 dBc
X2 dBc
fLO
LO
To
Antenna
PA
Band Select
Filter
f
Channel Spacing
= W Hz
ƒ
Partition synthesizer phase noise into channels
- Required phase noise power (dBc) in each channel is
M.H. Perrott
related directly to spectral mask requirements
ƒ Exception is X0 – set by transmit SNR requirements
MIT OCW
Synthesizer Phase Noise Requirements
Synthesizer
Spectrum
(dBc)
X3 dBc
0 dBc
IF
RF
X0 dBc
X1 dBc
X2 dBc
fLO
LO
To
Antenna
PA
Band Select
Filter
f
Channel Spacing
= W Hz
ƒ
Impact of channel bandwidth (offset channel 1)
ƒ
Overall requirements (most stringent, i.e., Y0 = M0)
Channel
Offset
Emission Requirements
(Most Stringent)
Maximum Synth. Phase Noise
(Most Stringent)
0
1
Y0 = M0
X1 = M1-M0 + dB(W/R) dB
X2 = M2-M0 + dB(W/R) dB
X3 = M3-M0 + dB(W/R) dB
set by required transmit SNR
X1 - dB(W) dBc/Hz
X2 - dB(W) dBc/Hz
X3 - dB(W) dBc/Hz
2
M.H. Perrott
3
MIT OCW
Example – DECT Cordless Telephone Standard
ƒ
ƒ
Standard for many cordless phones operating at 1.8 GHz
Transmitter Specifications
- Channel spacing: W = 1.728 MHz
- Maximum output power: M = 250 mW (24 dBm)
- Integration bandwidth: R = 1 MHz
- Emission mask requirements
o
M.H. Perrott
MIT OCW
Synthesizer Phase Noise Requirements for DECT
ƒ
Using previous calculations with DECT values
Channel
Offset
0
1.728 MHz
Mask
Power
24 dBm
-8 dBm
3.456 MHz -30 dBm
5.184 MHz -44 dBm
ƒ
Maximum Synth. Noise
Power in Integration BW
Maximum Synth. Phase Noise
at Channel Offset
set by required transmit SNR
X1 = -29.6 dBc
-92 dBc/Hz
X2 = -51.6 dBc
X3 = -65.6 dBc
-114 dBc/Hz
-128 dBc/Hz
Graphical display of phase noise mask
Synthesizer
Spectrum
(dBc)
-92 dBc/Hz
-114 dBc/Hz
-128 dBc/Hz
fLO
f
Channel Spacing = 1.728 MHz
M.H. Perrott
MIT OCW
Critical Specification for Phase Noise
ƒ
Critical specification is defined to be the one that is
hardest to meet with an assumed phase noise rolloff
- Assume synthesizer phase noise rolls off at -20
dB/decade
ƒ Corresponds to VCO phase noise characteristic
ƒ
For DECT transmitter synthesizer
- Critical specification is -128 dBc/Hz at 5.184 MHz offset
Synthesizer
Spectrum
(dBc)
0 dBc
Phase Noise
Rolloff: -20 dB/dec
-92 dBc/Hz
-114 dBc/Hz
Critical
Spec.
-128 dBc/Hz
fLO
f
Channel Spacing = 1.728 MHz
M.H. Perrott
MIT OCW
Receiver Blocking Performance
RF Input
(dBm)
Band Select Filter Must
Pass All Channels
IF Output
(dBm)
-39
-58
-73
fRF
f
Channel Filter
Bandwidth
f
fIF
Band Select
Filter
Channel
Filter
LNA
Synthesizer
Spectrum
(dBc/Hz)
To
IF Processing
Stage
0 dBc
Synthesizer
fLO
ƒ
ƒ
f
Radio receivers must operate in the presence of large
interferers (called blockers)
Channel filter plays critical role in removing blockers
M.H. Perrott
ƒ Passes desired signal channel, rejects interferers
MIT OCW
Impact of Nonidealities on Blocking Performance
RF Input
(dBm)
Band Select Filter Must
Pass All Channels
IF Output
(dBm)
-39
-58
-73
Channel Filter
Bandwidth
Synthesizer Noise
and Mixer/LNA Distortion
Produce Inband Interference
fRF
f
f
fIF
Band Select
Filter
Channel
Filter
LNA
Synthesizer
Spectrum
(dBc/Hz)
0 dBc
Synthesizer
fLO
To
IF Processing
Stage
Phase Noise
(dBc/Hz)
Spurious Noise
(dBc)
f
ƒ
Blockers leak into desired band due to
ƒ
In-band interference cannot be removed by channel filter!
- Nonlinearity of LNA and mixer (IIP3)
- Synthesizer phase and spurious noise
M.H. Perrott
MIT OCW
Quantifying Tolerable In-Band Interference Levels
RF Input
(dBm)
Band Select Filter Must
Pass All Channels
IF Output
(dBm)
-39
-58
-73
Channel Filter
Bandwidth
Synthesizer Noise
and Mixer/LNA Distortion
Produce Inband Interference
Min SNR: 15-20 dB
fRF
f
f
fIF
Band Select
Filter
Channel
Filter
LNA
Synthesizer
Spectrum
(dBc/Hz)
0 dBc
Synthesizer
fLO
To
IF Processing
Stage
Phase Noise
(dBc/Hz)
Spurious Noise
(dBc)
f
ƒ
Digital radios quantify performance with bit error rate (BER)
ƒ
Goal: design so that SNR with interferers is above SNRmin
- Minimum BER often set at 1e-3 for many radio systems
- There is a corresponding minimum SNR that must be achieved
M.H. Perrott
MIT OCW
Impact of Synthesizer on Blockers
RF Input
(dBm)
IF Output
(dBm)
Y dB
fRF
f
fIF
foffset
Synthesizer
Spectrum
(dBc)
0 dBc
RF
f
IF
LO
fLO
ƒ
f
Synthesizer passes desired signal and blocker
- Assume blocker is Y dB higher in signal power than
desired signal
M.H. Perrott
MIT OCW
Impact of Synthesizer Spurious Noise on Blockers
RF Input
(dBm)
IF Output
(dBm)
Y dB
X dB
SNR: -X-Y dB
fRF
f
fIF
foffset
Synthesizer
Spectrum
(dBc)
Spurious
Tone
0 dBc
RF
f
IF
X dBc
LO
foffset
ƒ
fLO
f
Spurious tones cause the blocker (Y dB) (and desired)
signals to “leak” into other frequency bands
- In-band interference occurs when spurious tone offset
frequency is same as blocker offset frequency
Resulting SNR = -X-Y dB with spurious tone (X dBc)
M.H. Perrott
MIT OCW
Impact of Synthesizer Phase Noise on Blockers
RF Input
(dBm)
IF Output
(dBm)
Y dB
X dB
SNR: -X-Y dB
fRF
f
fIF
foffset
Synthesizer
Spectrum
(dBc)
0 dBc
RF
ƒ
IF
LO
X dBc
foffset
f
fLO
f
Same impact as spurious tone, but blocker signal is
“smeared” by convolution with phase noise
- For simplicity, ignore “smearing” and approximate as
shown above
M.H. Perrott
MIT OCW
Blocking Performance Analysis (Part 1)
RF Input
(dBm)
In-Channel
IF Output
(dBm)
Y dB
SNR: -X-Y dB
fRF
f
fIF
foffset
Synthesizer
Spectrum
(dBc)
0 dBc
RF
ƒ
IF
LO
X dBc
foffset
f
fLO
f
Ignore all out-of-band energy at the IF output
- Assume that channel filter removes it
- Motivation: simplifies analysis
M.H. Perrott
MIT OCW
Blocking Performance Analysis (Part 2)
RF Input
(dBm)
Y1 dB
In-Channel
IF Output
(dBm)
Channel Filter
Bandwidth
= W Hz
Y2 dB
Inband Interference
Produced by
Synth. Phase Noise
SNRmin: 15-20 dB
fRF
Synthesizer
Spectrum
(dBc)
f
fIF
f
Channel Spacing
RF
IF
0 dBc
X0 dBc
X1 dBc
X2 dBc
fLO
LO
f
Channel Spacing
ƒ
Consider the impact of blockers surrounding the
desired signal with a given phase noise profile
- SNR must be maintained
- Evaluate impact on SNR one blocker at a time
min
M.H. Perrott
MIT OCW
Blocking Performance Analysis (Part 3)
RF Input
(dBm)
Y1 dB
In-Channel
IF Output
(dBm)
Channel Filter
Bandwidth
= W Hz
Y2 dB
Inband Interference
Produced by
Synth. Phase Noise
SNRmin: 15-20 dB
fRF
Synthesizer
Spectrum
(dBc)
f
fIF
Channel Spacing
RF
IF
0 dBc
X0 dBc
X1 dBc
X2 dBc
fLO
LO
f
Channel Spacing
Channel Relative Blocking Maximum Synth. Noise
Offset
Power
Power at Channel Offset
0
0 dB
1
Y1 dB
X1 = -SNRmin-Y1 dBc
2
Y2 dB
Y3 dB
X2 = -SNRmin-Y2 dBc
X3 = -SNRmin-Y3 dBc
3
M.H. Perrott
f
Derive using
the relationship
SNR = -X-Y dB >= SNRmin
X0 = -SNRmin dBc
MIT OCW
Blocking Performance Analysis (Part 4)
RF Input
(dBm)
Y1 dB
In-Channel
IF Output
(dBm)
Channel Filter
Bandwidth
= W Hz
Y2 dB
Inband Interference
Produced by
Synth. Phase Noise
SNRmin: 15-20 dB
fRF
Synthesizer
Spectrum
(dBc)
f
fIF
f
Channel Spacing
RF
IF
0 dBc
X0 dBc
X1 dBc
X2 dBc
fLO
LO
f
Convert power to
spectral density
Channel Spacing
Channel Relative Blocking Maximum Synth. Noise
Offset
Power
Power at Channel Offset
M.H. Perrott
X0 = -SNRmin dBc
X0 - dB(W) dBc/Hz
Y1 dB
X1 = -SNRmin-Y1 dBc
X1 - dB(W) dBc/Hz
Y2 dB
Y3 dB
X2 = -SNRmin-Y2 dBc
X3 = -SNRmin-Y3 dBc
X2 - dB(W) dBc/Hz
X3 - dB(W) dBc/Hz
0
0 dB
1
2
3
Maximum Synth. Phase
Noise at Channel Offset
MIT OCW
Example – DECT Cordless Telephone Standard
ƒ
Receiver blocking specifications
- Channel spacing: W = 1.728 MHz
- Power of desired signal for blocking test: -73 dBm
- Minimum bit error rate (BER) with blockers: 1e-3
-
M.H. Perrott
ƒ Sets the value of SNRmin
ƒ Perform receiver simulations to determine SNRmin
ƒ Assume SNRmin = 15 dB for calculations to follow
Strength of interferers for blocking test
MIT OCW
Synthesizer Phase Noise Requirements for DECT
RF Input
(dBm)
-33 dBm
-39 dBm
-58 dBm
-73 dBm
In-Channel
IF Output
(dBm)
Channel Filter
Bandwidth
W = 1.73 MHz
Inband Interference
Produced by
Synth. Phase Noise
SNRmin: 15 dB
fRF
Channel Spacing
= 1.73 MHz
Synthesizer
Spectrum
(dBc)
X3 dBc
f
fIF
RF
f
IF
0 dBc
X0 dBc
X1 dBc
X2 dBc
fLO
LO
f
Channel Spacing
= 1.73 MHz
Channel Relative Blocking Maximum Synth. Noise
Offset
Power
Power at Channel Offset
0
1.728 MHz
3.456 MHz
5.184 MHz
M.H. Perrott
Maximum Synth. Phase
Noise at Channel Offset
0 dB
Y1 = 15 dB
X0 = -15 dBc
X1 = -30 dBc
-77 dBc/Hz
-92 dBc/Hz
Y2 = 34 dB
Y3 = 40 dB
X2 = -49 dBc
X3 = -55 dBc
-111 dBc/Hz
-117 dBc/Hz
MIT OCW
Graphical Display of Required Phase Noise Performance
ƒ
Mark phase noise requirements at each offset frequency
Synthesizer
Spectrum
(dBc)
0 dBc
Phase Noise
Rolloff: -20 dB/dec
-92 dBc/Hz
Critical
Spec.
-111 dBc/Hz
-117 dBc/Hz
fLO
f
Channel Spacing = 1.728 MHz
ƒ
Calculate critical specification for receive synthesizer
- Critical specification is -117 dBc/Hz at 5.184 MHz offset
M.H. Perrott
ƒ Lower performance demanded of receiver synthesizer than
transmitter synthesizer in DECT applications!
MIT OCW