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  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 W)
Maximum power transfer  no reflections (VSWR = 1)
For line with physical temperature TL the output noise power (matched condition) is
ECE 4710: Lecture #38

4. Source resistor is at same physical temperature as transmission line
Definition of Te
Definition of F
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)
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
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 ECE 4710: Lecture #38
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 elevation angle = 90° (pointing straight up)
With sidelobes  earth radiation (280 °K) causes Ta = °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:
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: ECE 4710: Lecture #38
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 ECE 4710: Lecture #38 G1 F1 L2 L3 G2 F2 So No
Antenna
G1 F1 L2 L3
G2 F2 So
Low Noise
RF Amp IF
FIlter IF
AMP No
Mixer
Si = PRX
Demod /
Detector
Ni = k Ta B ˜
Local
Oscillator
LPF
DSP
Baseband
Amplifier
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
ECE 4710: Lecture #38