ece 4710: lecture #38 1 device noise two figures of merit for noisy devices noise figure f ...

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


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


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


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


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


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


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


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


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!!


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)


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)


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


Antenna Temperature

Low Noise Window


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


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 = ???


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


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

ece 4710: lecture #38 1 device noise two figures of merit for noisy devices noise figure f ...

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