Frequency Synthesizers in Communication Systems

1
Frequency Synthesizers in Communication Systems

• Chinh Doan
• Robert W. Brodersen
• University of California, Berkeley
• BWRC Retreat
• January 2000

2
Why Do We Need Frequency Synthesizers?

• What we want
• GHz oscillator at exactly the carrier frequency
• What we have
• LC-tuned and ring oscillators operate up to
  several GHz, worse than 10 frequency accuracy
• Crystal oscillators operate up to 100 MHz,
  10-100 ppm frequency stability

Need to combine speed of LC-tuned or ring
oscillator with accuracy of crystal oscillators.
3
Phase-Locked Loop Frequency Synthesizer

• Phase Detector DC value of output µ fref - fdiv
• Loop Filter sets noise and transient performance
  of PLL
• VCO output frequency controlled by input voltage
• Frequency Divider fdiv fout/N

4
Frequency Synthesizer Impairments
5
Phase Noise in Receivers

• Reciprocal mixing causes additive broadband noise
  from interferer
• In-band phase noise modifies desired signal

6
Reciprocal Mixing

• Processing gain of CDMA relaxes phase noise
  requirements

7
RMS Phase Noise

• Close-in phase noise causes constellation rotation

8
Choosing Multiple Access Schemes

• DS-CDMA reduces high-frequency (fmgtfsymbol) phase
  noise

9
Choosing Receiver Architectures

• Heterodyne receiver requires agile frequency
  synthesizer

Homodyne receiver requires fixed frequency
oscillator
10
Conclusion

• Influence of phase noise on system depends upon
  multiple access technique
• Reducing phase noise requires more power
• Receiver architecture determines frequency
  synthesizer requirements
• Low power receiver requires codesign!