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RF Microelectronics

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  1. RF Microelectronics RFIC Professor: Dr. Ali Fotowat Ahmady Date: July 16, 2007 Lectures: 1-10 (sections 1-5)

  2. Contents • Introduction • Basic concepts • Digital modulation, Spectral control, Detection • Multiple access standards, TDM, CDM, OFDM • TRx architecture • LNA and Mixer • Oscillator • Frequency Synthesizer • Power Amplifier

  3. Section One • Introduction • Complexities • Goals • Technology

  4. Complexities Simple FM Radio

  5. Complexities Philips GSM phone

  6. IS-19 cellular telephone RF section block diagram A 45 MHz offset frequency oscillator generates the required receiver and transmitter local oscillator frequency.

  7. IS-55 block diagram A narrowband IF filter is required for digital operation, as well as an ADC in the baseband.

  8. Why…? • IF frequency • 50-ohm impedance matching • Shielding • All-band filtering • I&Q receiver • Gain and phase setting in receiver • I&Q transmitter • LC filter for transmitter • SSB mixer • Out-of-band noise • Output power control • …

  9. Disciplines required in RF design As the industry moves toward higher integration and lower cost, RF and wireless design demands increasingly more “concurrent engineering”.

  10. RF design hexagon The trade-offs involved in the design of such circuits can be summarized in the “RF design hexagon”.

  11. Then … • In chip • Increase frequency • Decrease power • Change architecture

  12. Technology • Three critical factors influencing the choice of technologies in the competitive RF industry: • Performance • Cost • Time to market • Issues play an important role in the decisions made by the designers: • Level of integration • Form factor • Prior successful experience

  13. Technology • The technologies constitute the major section of the RF market: • GaAs • Silicon Bipolar • BiCMOS • SiGe • CMOS CMOS technology must resolve a number of practical issues: Substrate coupling of signals that differ in amplitude by 100dB, parameter variation with temperature and process, and devices modeling for RF operation.

  14. Contents • Introduction • Basic concepts • Digital modulation, Spectral control, Detection • Multiple access standards, TDM, CDM, OFDM • TRx architecture • LNA and Mixer • Oscillator • Frequency Synthesizer • Power Amplifier

  15. Section Two • Basic concepts • Cascaded stage nonlinearity • 1 dB compensation point • IP3 • Intersymbol interference • Noise figure • Sensitivity and dynamic range

  16. Effects of nonlinearity Model nonlinearity as a Taylor series expansion up to its third order term: If a sinusoid is applied to a nonlinear system: The term with input frequency is called the fundamental and the higher-order terms the harmonics.

  17. 1 dB Compression Point The small-signal gain of a circuit is usually obtained with the assumption that harmonics are negligible. In RF circuit, 1-dB Compression point defined as: The input signal level that causes the small-signal gain to drop by 1 dB.

  18. 1 dB Compression Point To calculate the 1-dB compression point, we can write from gain equivalent: That is, In typical front-end RF amplifiers: The 1-dB compression point occurs around -20 to -25 dBm (63.2 to 35.6 mVpp in a 50 Ohm system).

  19. Desensitization and Blocking If a small signal and a large interferer are applied to a compressive system, the “average” gain for the small signal is reduced: Assume, The output is Which, for , reduces to

  20. Desensitization and Blocking The gain for the desired signal is equal to • A decreasing function of A2 if a3<0 . • For sufficiently large A2, the gain drops to zero, and we say the signal is “blocked”. • The interferer is called a blocking signal. Many RF receivers must be able to with stand blocking signals 60 to 70 dB greater than the wanted signal.

  21. Cross Modulation When a weak signal and a strong interferer pass through a nonlinear system, Weak signal: Strong interferer: Then, Cross modulation is the transfer of modulation on the amplitude of the interferer to the amplitude of the weak signal.

  22. Cross Modulation • If two signals experience nonlinearity, amplitude modulation in one appears in the other. • Most important in “multi-carrier” systems. Example include cable TV transmitters and base station transmitters.

  23. Intermodulation • If the input sinusoid frequency is chosen such that its harmonics fall out of the passband, • The output distortion appears quite small even if the input stage of the filter introduces substantial nonlinearity.

  24. Intermodulation Assume; Thus, Expanding the left side and discarding dc terms and harmonics, obtain the following Intermodulation products:

  25. Intermodulation Intermodulation in a nonlinear system: Corruption of a signal due to Intermodulation between two interferer:

  26. Third Intercept Point “IP3” • IP3 is measured by two-tone test • A is chosen to be sufficiently small so that higher-order nonlinear terms are negligible and the gain is relatively constant and equal to a1

  27. IP3 Calculation of IP3: • IIP3|dBm=Pinput|dBm +DPdB/2 . • IIP3|dBm=Poutput|dBm -GaindB+DPdB/2 . • OIP3|dBm=Poutput|dBm +DPdB/2 .

  28. The relationship between 1-dB compression point and iip3

  29. Cascade nonlinear stages Input-output characteristics of the two stage are expressed: Then, Thus,

  30. Cascade nonlinear stages

  31. Cascade nonlinear stages • This equation readily gives a general expression for three or more stages: • Typical receiver IP3 is -15 dBm.

  32. Example • Stage 1 is a super linear amplifier, Thus AIP3,1=∞ . • AIP3,tot= AIP3,2 /a11 • AIP3,tot|dBm = AIP3,2|dBm – Gain1st satge|dB. • Thus • If stage 1 is an amplifier, IIP3tot =IIP32 – GdB • If stage 1 is an attenuator, IIP3tot =IIP32 + GdB

  33. Intersymbol Interference • Linear time-invariant systems with insufficient bandwidth distort the signal. An example of such behavior: output with exponential tail a periodic square wave Low-pass filter

  34. Intersymbol Interference • With a random sequence of ONES and ZEROS as the input : Low-pass filter Vin Vout

  35. Intersymbol Interference • Each bit level is corrupted by decaying tails created by previous bits. Called “Intersymbol Interference” (ISI). • Leads to higher error rate in the detection of random waveforms transmitted through band-limited channels • Particularly troublesome in wireless communications because of narrow bandwidth allocated to each channel

  36. Intersymbol Interference • Methods of reducing ISI: • In Transmitter: Pulse shaping (Nyquist signaling) • In Receiver: Equalization

  37. Input-Referred Noise • Representation of noise by input noise generators • The correlation between two sources must be taken into account

  38. Input-Referred Noise • An example to illustrate the idea:

  39. Noise Figure • Most of the front-end receiver blocks are characterized in terms of their “noise figure” rather the input-referred noise • Noise Figure in dB: • Noise figure is a measure of how much the SNR degrades as the signal passes through a system Signal-to-noise ratio at the input Signal-to-noise ratio at the output

  40. Calculation of Noise Figure • SNR in is the ratio of the input signal power to the noise generated by the source resistance, R s, modeled by

  41. Calculation of Noise Figure Voltage gain from Vin to the input port of the circuit (node P) Voltage gain from P to V out V n and I n R s are added before squaring to account for their correlation

  42. Calculation of Noise Figure (for the spot noise figure to emphasize the very small bandwidth ) =Total noise at the output

  43. Calculation of Noise Figure • Thus, to calculate the Noise figure, we divide the total output noise power by the square of the voltage gain from V in to V out and normalize it to the noise of R s. • As an example: consider the single resistor, R p

  44. Calculation of Noise Figure • What is the noise figure of this circuit with respect to a source resistance R s ?

  45. Sensitivity • For an RF receiver: The minimum signal level that the system can detect with acceptable signal-to-noise ratio • The overall power is distributed across the channel bandwidth, B. Thus the two sides of the equation must be integrated over the bandwidth to obtain the total mean square power for a flat channel

  46. Sensitivity Minimum input level that achieves SNR min The noise power that R s delivers to the receiver Assuming conjugate matching at the input

  47. Sensitivity At room temperature: Total integrated noise of the system called Noise Floor Since P in,min is a function of the bandwidth, a receiver may appear very sensitive because it employs a narrowband channel

  48. Dynamic Range (DR) • Generally is the ratio of the maximum input level that the circuit can tolerate to the minimum input level at which the circuit provides a reasonable signal quality • In RF design, this definition is based on the Intermodulation behavior and the sensitivity called : Spurious-free dynamic range

  49. Dynamic Range (DR) • The upper end of the dynamic range is: The maximum input level in a two-tone test for which the Third-order IM products do not exceed the noise floor And hence,

  50. Dynamic Range (DR) • The input level for which the IM products become equal to the noise floor is: