1. Summary Thus far we have: ECE 4710: Lecture #39
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 #39

2. 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
Output S / N sometimes called Carrier to Noise (C/N) ratio
Carrier implies RF/IF  prior to demodulation to baseband
Must perform noise analysis for entire RF / IF system
Develop system noise characteristics
Tes and Fs
Sole purpose is to accurately estimate No
So is simply Si + device gains - device losses
ECE 4710: Lecture #39

3. ˜ S / N @ Rx Output ECE 4710: Lecture #39 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
Output
Data
DSP
Baseband
Amplifier
ECE 4710: Lecture #39

4. Cascaded Linear Devices
 System Noise Figure
ECE 4710: Lecture #39

5. System Noise Figure System Noise Figure
We want to solve for Fs in terms of F1 and F2
Must determine relationship between device noise terms (ND1 and ND2) and device noise figures (F1 and F2)
ECE 4710: Lecture #39

6. Device Noise Model #1
Thermal Noise Source
ECE 4710: Lecture #39

7. System Noise Figure System Noise Figure Using & then
Extending this for many cascaded devices
ECE 4710: Lecture #39

8. System Noise Figure
Key Observation: if first stage in cascaded system has high gain then F1 will dominate overall system noise figure
Low Noise Amplifiers (LNAs)
Special class of RF amplifiers with high gain and low noise figure
Used to ensure that system noise figure will be small so that overall S / N performance in Rx is very good
ECE 4710: Lecture #39

9. Noise Figure Example ECE 4710: Lecture #39 G2 = 15 dB L3 = 1 dB
Antenna
G2 = 15 dB
L3 = 1 dB
L4 = 2 dB
G5 = 40 dB
Cable IF
Filter IF
AMP
Low Noise
RF Amp
90 ft.
loss = 1 dB / 30 ft
F2 = 2 dB
F5 = 10 dB
ECE 4710: Lecture #39

10. Noise Figure Example ECE 4710: Lecture #39
Antenna
G1 = 15 dB
L2 = 3 dB
L3 = 1 dB
L4 = 2 dB
G5 = 40 dB
Cable IF
Filter IF
AMP
Low Noise
RF Amp
90 ft.
loss = 1 dB / 30 ft
F5 = 10 dB
F1 = 2 dB
For low noise performance amplification is required at
or near antenna before the signal is passed via
transmission line to rest of system
ECE 4710: Lecture #39

11. System Temperature
For cascaded linear devices the effective system temperature can be shown to be
Same observation as Fs  low noise (small Te1) and high gain first stage mean overall system noise performance is very good
Relationship between Fs and Tes 
ECE 4710: Lecture #39

12. System Parameters ECE 4710: Lecture #39 GS FS Tes G1 F1 Te1 G2 F2 Te2
Antenna
Device #1
G1 F1 Te1
Device #2
G2 F2 Te2
Device #3
G3 F3 Te3
Device #4
G4 F4 Te4
ECE 4710: Lecture #39

13. System Output Noise System output noise formula :
Ideal case is Tes = 0  output noise is only from amplified input noise
For Tes >> Ta  system noise dominates Rx
Any improvement in Fs or Tes has significant impact on overall system noise performance
For Ta  Tes  input noise & system noise ~ same
Largest possible improvement in S / N is only factor of 2  3 dB!!
For Ta >> Tes  input noise dominates Rx
LNA should NOT be used  no benefit
ECE 4710: Lecture #39

14. Link Budget Equation
All communication systems designed for certain baseband S / N performance
Analog TV  S / N = dB
Landline Voice Telephony  S / N = dB
Analog Cellular  S / N = dB
Digital Cellular  S / N = 7-10 dB for BER < 10-3
Satellite Digital TV  S / N = dB for BER < 10-9
In digital communications desired BER determines required Eb / No
ECE 4710: Lecture #39

15. Link Budget Equation In communication system design
System analysis works backward from required (S / N)o
Link analysis works forward from transmitter and PTx
Two meet at Rx front end  PRx
What is the minimum detectable signal, Smin, at Rx front end (antenna output) that is needed to produce required output signal to noise ratio?
ECE 4710: Lecture #39

16. Link Budget Equation Two key formulas :
Rewriting S / N formula and solve for Si = Smin
 S/N required prior to detector / demodulator
for desired baseband performance
ECE 4710: Lecture #39

17. Link Budget Equation In link formula let
Complete end to end description of most important parameters affecting communication system
End-to-end  Tx + Channel + Rx
Example: What is Tx power required to achieve desired performance?
ECE 4710: Lecture #39