RF Design Basic Tutorial

In this tutorial we are going to learn about RF Design Basic Tutorial

Here Below is the RF Design Basic:

  • Distinguishing features of RF design
  • Specialized methods and CAD
  • Parts limitations and models (what to look out for)
  • Matching
  • Board layout techniques specialized to RF
  • Basic RF subsystem and blocks (in transceivers)
  • IC trends in RF design
  • RF system design and why it is critical
  • Link budget and link behavior
  • A basic RF tools
  • Possible future presentations
  • Conclusions

Specialized analysis and CAD ½

  • In the past “RF design” was regarded as specialized analog circuit design
  • By “specialized” it is meant that it included the methods needed to extend low frequency analog to much higher frequency (example: “stability”)
  • Today it also includes system design, software , and lots of digital processing also

A large fraction of all electronic engineering is included in a cell phone , requiring design teams of dozens of engineers

Specialized analysis and CAD 2/2

  • Specialized circuit CAD includes scattering parameters , or “s-parameters”
  • This is a linear two port set describing reflected and transmitted waves through a circuit block
  • Motivation: There is no such thing as perfect opens and shorts in RF
  • A 1 nh inductance typical of a via to ground is not a short – at 1 GHz it is 6.28 ohms
  • A 1 PF parasitic capacitance to ground is not open – at 1 GHz t is 159 ohms

These are the parameters measured by “network analyzers”

  • Spice can be used up to several GHz , but must be used with care (realistic models for frequency range simulated)

Parts models: capacitance in

o=1

Q=1/ῳR C=ῳR C          Low Z at resonance

Ceff = C/1-ῳ2LC                 effective capacitance below resonance

Inside Trick: Making power RF capacitors

Parallel them for better Q and power handling! High quality RF caps show smaller Q with larger capacitance , generally about half the Q with twice the capacitance (see below) example , a 15 pF murata 0402 high Q capacitor has Q of 85 at GHz , but a 30 pf only has Q of 45 . but paralleling two capacitors of half the desired size will maintain the same Q as each of the smaller capacitors , allowing both lower losses and higher power dissipation.

Micro phonics

High dielectric constant capacitors (X5R,X7R, Y5V) are piezoelectric and generate Noise proportional to vibration

  • A particular danger on voltage controlled oscillators (such as PLL loop filters) and mixers , where their baseband noise will mix around RF signals
  • Use COG/NPO , plastic film, or tantalum can reject microphonic noise to a high degree , but not perfectly
  • High BW synthesizers and regulators can reject microphonic noise to  high degree , but not perfectly

Parts Models: Inductors in Real life

Q=ῳL/RSER=RP/ῳL                              Effective inductance below resonance

            LD=L/1-ῳL C=L/1-ῳ202

Example RF Transmit Filter with Parasitics

  • 500 MHz land mobile low pass 7th order Chebyshev 0.5 DB insertion loss, 10 watt capable
  • Suppresses 2nd harmonic well below 65 DBC , but note “bounce “ due to parasitic

Smith Chart: A graph of reflection or “s11 or s22”

Matching on the smith chart:

Amplifier with capacitive high impedance input converted to 50 ohms

RF Board layout Rules to Live by

  • Keep it tight
  • One part should generally flow right in to the next
  • When using multiple decoupling capacitors to ground , the smaller (higher self-resonant frequency) capacitor should generally get preference
  • But, beware magnetic coupling loops that can pick up noise or cause instability. keep them small but separated
  • This  is analytic , but often mistake for a ground plane problem
  • Ground Planes
  • This more common professional opinion is that one highly common ground plane is superior to separated analog and digital ground planes

Key Transceiver Concepts

  • The super heterodyne principle
  • Using a “mixer” to convert an incoming signal over a range of frequencies to a fixed “intermediate frequency” (IF)
  • The frequency synthesizer principle
  • A feedback control system allows RF signals to be generated at arbitrary frequency but with the high accuracy of low frequency crystals Transceiver subsystems (using the superheat Principle)

What’s so great about Frequency Synthesis?

  • Allows translating the high accuracy of a low fixed frequency crystal to a higher “voltage controlled oscillator” (VCO)
  • As a bonus can suppress VCO “phase noise” within “loop bandwidth”
  • Can be firmware controlled
  • Can be implemented as a small system, in some cases a signal IC

The Frequency Synthesizer Principle

Synthesizer Noise Performance

IC Trends

  • It tooks a long time for radios to become mostly integrated due to high noise and the lack of “Q” on-die
  • It has now arrived with direct conversion , low IF fractional N synthesis, and DSP
  • Rather good if  not great radios can be fully integrated
  • Including RF power up to about 1 watt
  • Low Inter mediate frequency  allows active filters that avoid crystal filtering
  • Wide band fractional N synthesizers can suppers poor on-die VCO phase noise

About EEE

We have designing Experience for the last 40 years.

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