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Class Representation For Advanced VLSI Course

Class Representation For Advanced VLSI Course. Instructor : Dr S.M.Fakhraie Student : Mohammad Ali Sedaghat Major Reference : An Analog GFSK Modulator in 0.35-m CMOS Hooman Darabi, Brima Ibrahim, and Ahmadreza Rofougaran IEEE International Solid-State Circuits Conference 2004.

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Class Representation For Advanced VLSI Course

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  1. Class Representation For Advanced VLSI Course Instructor : Dr S.M.Fakhraie Student : Mohammad Ali Sedaghat Major Reference : An Analog GFSK Modulator in 0.35-m CMOS Hooman Darabi, Brima Ibrahim, and Ahmadreza Rofougaran IEEE International Solid-State Circuits Conference 2004

  2. Outline • Introduction • Analog modulation Concept • Modulator circuit implementation • Experimental results

  3. Transmitter Architectures • Direct I-Q Up-Conversion2 • Better performance • More robust • Up-conversion mixers needed VCO/PLL Modulation1 Lower power Sensitive to KVCO variations Compromises PLL noise & settling time Potential frequency drift issues

  4. Conventional Modulator Implementation existing direct-conversion transmitters use a digital modulator at baseband consists of direct-digital frequency synthesizer (DDFS) followed by a digital-to-analog converter (DAC) • DAC is power consuming • DDFS/DAC switching degrades VCO clockphase noise • Direct analog implementation eliminates the DAC

  5. FM Differential Equation General FM Signal: Instantaneous Frequency: FM Differential Equation:

  6. GFSK Implementation • input data is first shaped (limit it’s spectral width) by a Gaussian filter in the time domain and then applied to the FM modulator • the modulator take the digital data at input, and a Gaussian filter with a bandwidth of half of the data rate will produce the desired Gaussian spectrum in the frequency domain

  7. Analog Modulator Circuit Implementation • Active-RC integrators are employed due to their superior dynamic range • RC calibration circuit adjusts the integrator gain by tuning the capacitors in the opamps’ feedback path • The opamps are two-stage differential amplifiers to drive the resistive load • multipliers, implemented as 4-FET switches • data edges are aligned with the 12-MHz system clock and a logic circuit produces complementary clock signals to drive the 4-FET switches • analog Gaussian low-pass filter following the modulator

  8. Amplitude Control Loop • To meet the spectrum mask requirements, the modulator • output swing must be well • defined • amplifier A followed by CA result in a CEQ=A*CA in parallel with the first integrator resistor that then creates a zero in the integrator transfer function Resulting in a phase shift in the integrator output that Depends on the sign of A is negative or positive, which in turn causes the modulator amplitude to grow or decay • Amplifier gain is a function of the difference between the modulator output swing and a reference voltage • Zero once the loop reaches the steady state

  9. Amplifier Circuit • The amplifier is a Gilbert-type current-steering circuit • Assuming the input transistors are square-law, in the vicinity of lock, the amplifier gain is linearly proportional to the difference between VR and VP Then the slope is a function of the amplifier device sizes and the total output resistance

  10. Amplitude Control Loop Dynamics • If VP-VR is small, the modulator amplitude is: where VPO is the initial amplitude • Loop behavior modeled by an integrator with gain of ωI : • Critically damped settling requires:

  11. Amplitude Modulation • Baseband implementation results in random amplitude modulation • in I-Q outputs. • Thus, a conventional peak detector which finds the highest voltage • fails to determine the output swing properly • Nonetheless, Complex power, , remains constant and is • equal to the peak amplitude

  12. Power Detector Circuit • the square-law property of a long-channel MOSFET is utilized to realize the power detector • Transistors M1–M4 are identical and are connected to the • differential I and Q signals The sum of their currents consists of following two terms squared 1-constant dc term 2-variable component proportional to the input amplitude The dc term is cancelled by the current source IB, and transistor M5

  13. Modulator Die Photograph 0.35 µm CMOS process Active area: 0.31 mm2

  14. Summary of Performance

  15. Conclusions • Direct up-conversion is robust and low-power • Analog implementation of modulator saves power • FM differential equation realizes the GFSK modulator in the analog domain • 􀀹Amplitude control loop is essential for low distortion • 􀀹Complex power detector is accurate and fast

  16. Refrences: [1]Hooman Darabi, Brima Ibrahim, and Ahmadreza Rofougaran “An Analog GFSK Modulator in 0.35-m CMOS” 2004 IEEE International Solid-State Circuits Conference [2] Pilar Parra, Antonio Acosta, and Manuel Valencia “Reduction of Switching Noise in Digital CMOS Circuits by Pin Swapping of Library Cells1” Instituto de Microelectrónica de Sevilla-CNM / Universidad de Sevilla Avda. Reina Mercedes s/n, 41012-Sevilla, SPAIN [3] Alistair McEwan and Steve Collins “ANALOGUE INTERPOLATION BASED DIRECT DIGITAL FREQUENCY SYNTHESIS” Department of Engineering Science University of Oxford Parks Road Oxford UK

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