ece 4710: lecture #38 1 device noise two figures of merit for noisy devices noise figure f ...
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ECE 4710: Lecture #38 1
Device Noise
Two figures of merit for noisy devices Noise Figure F
Effective Noise Temperature Te
One or the other is usually specified for active devices (e.g. amplifiers) F and Te are normally measured
What about passive (lossy) devices? Transmission Lines Filters Switches Mixers
ECE 4710: Lecture #38 2
Passive Devices
Noise also present in passive devices Passive devices have small but non-zero amount of
loss Usually ohmic loss (i2R)
» Motion of free electrons in conductors creates collisions» Collisions convert EM energy to thermal energy (heat)
Loss often called “insertion loss” (IL)» Typically IL 0.5-2 dB for most passive devices
Filters, mixers, etc.» For transmission lines the amount of loss depends on length
Must find F and Te for passive and/or lossy devices
ECE 4710: Lecture #38 3
Transmission Line
Source and load matched to characteristic impedance of line (normally Ro = 50 ) Maximum power transfer no reflections (VSWR = 1)
For line with physical temperature TL the output noise power (matched condition) is BTkN Lo
oN
LT 11
or 1 G
LG
oR impedance sticcharacteri
ECE 4710: Lecture #38 4
o
L
o
e
TLT
TT
F)1(
11
Transmission Line
Definition of Te
Definition of F
111
then
& since and
LTG
TG
TGTBkG
TBkGBTkT
TTBTkNBkG
TBkGNT
LLLLiL
e
LiLoio
e
Source resistor is at same physical temperature as transmission line
Line Loss
ECE 4710: Lecture #38 5
Transmission Line
If line TL = 290 K = To then
T0 = 290 K = 63°F is room temperature
Thus F = L or FdB = LdB is a good approximation under most circumstances Example: Assume TL has 3 dB loss and TL = 273 K =
32°F = 0°C (freezing point)
GL
TLT
Fo
L 1)1(1
dB 2.87or 936.1290
)110(2731
)1(1
10/3
o
L
TLT
FdB 3 dBL
ECE 4710: Lecture #38 6
Passive Devices
Many other passive and/or lossy devices with physical temperature TD have same noise characteristics as lossy transmission line
If noise characteristics are not specified by manufacturer the above formulas should be used to model noise performance of passive devices
Above formulas are always appropriate for RF/microwave transmission lines
oDo
D TTLTLT
F for )1(
1)1( LTT De
ECE 4710: Lecture #38 7
Antenna Temperature
Effective noise temperature at antenna is NOT related to physical temperature of antenna Antenna is a non-thermal noise source Effective antenna temperature, Ta, determines input noise
power (Ni) at front-end of wireless communication Rx where Ni = k Ta B
Link formula predicts received signal power Si = PRX at front-end of Rx
Together PRX and Ta allow us to estimate (S/N)i
After this then overall Rx gain and noise performance allows us to predict (S/N)o
ECE 4710: Lecture #38 8
Antenna Temperature
Effective antenna temperature will in most cases be substantially different than To = 290 K
What does Ta depend on? Frequency Antenna pointing direction Noise characteristics of materials within antenna field of
view (FOV)» FOV approximately main lobe of antenna pattern
Surrounding noise environment» Antenna sidelobes allow noise energy from directions other than
main lobe substantially attenuated but can have significant effect
ECE 4710: Lecture #38 9
Antenna Temperature
For f < 30 MHz the principle source of noise is due to lightening discharge EM waves from lightening propagate large distances
thousands of miles» Propagation of communication signals for f < 30 MHz is very good
same applies to lightening
Therefore it is NOT necessary to have lightening in the vicinity of communication system for this to dominate noise performance
Ta in the range of 103 – 107 °K VERY noisy» Larger at night time than day time thunderstorms + lightening
occur more frequently at night!!
ECE 4710: Lecture #38 10
Antenna Temperature
For 30 MHz < f < 1 GHz the principle source of noise is due to galactic or cosmic noise Time-varying EM waves from outer space due to charge motion in
stars
Ta decreases as f increases
Ta in the range from 10,000 – 100 °K for f > 100 MHz
For narrow beam antennas the cosmic noise is a function of antenna pointing direction (e.g. deep space vs. star clusters)
Our sun is important source at times when sun angle is directly aligned with antenna main lobe» DirectTV Rx affects during specific season at certain time of day
» Diurnal noise effects (more noise during day than at night)
ECE 4710: Lecture #38 11
Antenna Temperature
For f > 10 GHz the principle source of noise is due to Earth’s atmosphere Water vapor (H20) and oxygen molecules (O2) are
significant attenuators of RF energy at these frequencies Resonant absorption of EM energy by molecules causes
RF attenuation and also causes thermal noise emission» Vibration of molecules constitutes random motion of charge
» Molecules vibrate in all physical materials with T > 0° K
Ta in the range from 10 – 1000 °K for f > 10 GHz» Increases with frequency
Ta depends on elevation angle of antenna (wrt horizon)
ECE 4710: Lecture #38 12
Antenna Temperature
Frequency range from 1 GHz < f < 10 GHz is the low noise window Bounded by effects of cosmic noise ( f < 1 GHz) and
atmospheric noise ( f > 10 GHz) Preferred operating frequencies for all satellite and/or
spaceborne communication systems» Low atmospheric attenuation and low thermal noise emission
In U.S. f = 4 GHz is widely used for satellite communications
Ta in the range from only 2 – 50 °K !! Ta = 2-4 °K possible for very narrow beam antennas with
small sidelobes @ elevation angle = 90° (pointing straight up)» With sidelobes earth radiation (280 °K) causes Ta = 10-30 °K
ECE 4710: Lecture #38 13
Antenna Temperature
Low Noise Window
ECE 4710: Lecture #38 14
Antenna Temperature
Input noise power at front end (antenna output port) of communication Rx determined by effective antenna temperature and Rx signal BW Ni = k Ta B
Two important assumptions:
1) There is bandpass filter at RF or IF to restrict Rx
BW to be equal to signal BW
2) There is no interference from other sources entering antenna from channel
In some applications (cellular radio, military) the interference power >> thermal noise power
ECE 4710: Lecture #38 15
Summary
Thus far we have: Developed link formula to predict PRX for system and link
parameters (PT , GAT , d, etc.) Si = PRX
Described basic properties of thermal noise Characterized noise performance of individual devices
» F and Te
» Active and passive/lossy
Characterized effective antenna temperature Ta
» Allows us to estimate input noise power : Ni = k Ta B
One more step to complete link budget analysis» What is S/N ratio at receiver output (S/N )o = ???
ECE 4710: Lecture #38 16
S / N @ Rx Output
IFFIlter
˜
Antenna
Low NoiseRF Amp
LPF
BasebandAmplifier
LocalOscillator
Mixer
IFAMP
Demod /Detector
DSP
Si = PRX
Ni = k Ta B
G1 F1 L2 L3 G2 F2 So
No
ECE 4710: Lecture #38 17
S / N @ Rx Output
Output S / N normally specified at input to detector Baseband BER vs. Eb / No results rely upon S / N at input to
detector/demodulator Must perform noise analysis for entire RF / IF
system Develop system noise characteristics
» Tes and Fs
Sole purpose is to determine No
So is simply Si + device gains device losses