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Chapter Two: Radio-Frequency Circuits

Chapter Two: Radio-Frequency Circuits

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Chapter Two: Radio-Frequency Circuits

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  1. Chapter Two:Radio-Frequency Circuits

  2. Introduction • There is a need to modulate a signal using an information signal • This signal is referred to as a baseband signal • The carrier needs to be a higher frequency than the baseband • RF Amplifiers, Oscillators, Mixers, and frequency synthesizers are used to meet these conditions

  3. High-Frequency Effects • At very low frequencies, capacitors and other components behave in very straightforward ways • A capacitor is considered an open circuit to DC voltages and a short circuit for AC at low frequencies • As frequencies become higher, component interaction becomes more critical both directly and as “stray” reactances, inductances, and capacitances

  4. Effect of Frequency on Device Characteristics • All electronic devices have capacitances and inductances • As frequency increases, so does inductive reactance • As frequency increases, capacitive reactance decreases • At some point, the two reactances will be equal and the circuit will self-resonate

  5. Lumped & Distributed Constants • At low frequencies, the capacitances and inductances found between the traces on a printed circuit board are negligible • As frequency increases, the stray capacitances and inductances are considered as distributed along the length of the pc board • They are said to be distributed constants

  6. High-Frequency Construction Techniques • Circuits are designed to reduce the “stray” capacitances and inductances resulting from the wiring and circuit board • Traces and wires are kept short and well separated • Conductors and inductors in close proximity are kept at right angles • Toroidal cores for inductors are used to reduce stray magnetic fields • Shielding is used • A gimmick is used in some circuits

  7. Radio-Frequency Amplifiers • RF amplifiers differ from audio amplifiers in that wide bandwidth may or may not be required • Linearity of the output may or may not be required • Efficiency can be improved through the use of Class C amplifiers

  8. Narrowband Amplifiers • Many RF amplifiers are required to operate only within a narrow range of frequencies • Filters are used to reduce the bandwidth • The tuned amplifier is set according to the formula:

  9. Miller Effect • Inter-electrode capacitance and inductance is a problem in RF circuits • This problem is especially severe for the collector-base capacitance in a common-emitter amplifier • The multiplication of the effect of capacitance in this configuration is called the Miller Effect

  10. Common-Base Amplifier • One solution to the Miller Effect is to use a common-base amplifier configuration as shown at the right

  11. Wideband Amplifiers • Baseband parts of RF systems may make use of wideband amplifiers • Wideband amplifiers typically use transformer coupling • Typical wideband amplifiers need negative feedback to compensate for higher low-frequency gain: as frequency increases, negative feedback decreases

  12. Amplifier Classes • Amplifiers are classified according to the portion of the input cycle the active device conducts current • This is referred to as the conduction angle and is expressed in degrees • Single-ended audio amps are operated in Class A where the device conducts for 360° • Push-pull amps can be a Class B if one of the two devices is conducting at all times • Most audio power amps operate in Class AB - a compromise between Class A and Class B

  13. Class B RF Amplifier • A simple Class B amplifier is shown at the right • It uses transformer coupling • Both transistors are biased near cutoff

  14. Class C Amplifiers • Class C amplifiers conduct for less than 180° of the input cycle • Class C amplifiers can be single-ended or push-pull • Class C amplifiers are very efficient in RF applications but inherently induce severe distortion

  15. Neutralization • Transistors or tubes may introduce sufficient feedback to cause the circuit to oscillate and become unstable • Neutralization can cancel this type of feedback by feeding back a portion of the output signal to the input in such a way that it has the same amplitude as the unwanted signal but the opposite phase

  16. Frequency Multipliers • Sometimes it is useful to use harmonic operation to generate a frequency higher than is conveniently generated by using a frequency multiplier

  17. Radio-Frequency Oscillators • RF oscillators do not differ in principle than other oscillators but practical circuits are quite different • Any amplifier can be made to oscillate if a portion of the output signal is fed back to the input • The Barkhausen criteria establishes the requirements for a circuit to oscillate

  18. LC Oscillators • Practical RF circuits whose frequency is controlled by a resonant LC circuit are: • Hartley Oscillator • Colpitts Oscillator • Clapp Oscillator

  19. Hartley Oscillator • Common configurations for a Hartley Oscillator

  20. Colpitts Oscillator • Common configurations for a Colpitts Oscillator

  21. Clapp Oscillator • Common configuration for a Clapp Oscillator

  22. Varactor-Tuned Oscillator • The frequency of an oscillator may be tuned by varying the inductance or capacitance of the circuit • Varactors are more convenient substitutes than variable capacitors in many circumstances

  23. Crystal-Controlled Oscillators • Crystal-controlled oscillators are more stable than LC oscillators • Crystal oscillators utilize the piezoelectric effect to generate a frequency-variable signal

  24. Mixers • Mixers are nonlinear circuits that combine two signals to produce the sum and difference of of the two input frequencies

  25. Types of Mixers • Square-law mixers: output is derived by the formula: • Diode Mixers use a diode operated in the forward bias mode • Transistor Mixers use bipolar and FET transistors • Balanced Mixers are mixers where the input frequencies do not appear at the output

  26. Frequency Synthesizers • Conventional LC oscillators tend to be unstable because of: • Vibration • Temperature changes • Voltage changes • Component aging • Crystal oscillators are more stable but are are limited to a narrow range of operating frequencies • Frequency Synthesizers overcome these limitations and may end up being more cost effective

  27. Phase-Locked Loops • The phase-locked loop is the basis of nearly all modern synthesizer designs • The loop consists of a: • Phase detector • Voltage-controlled oscillator (VCO) • Low-pass filter • The purpose of the PLL is lock the VCO to the reference signal

  28. Simple Frequency Synthesizer • In addition to the phase detector, VCO, and filter, a programmable divider is necessary for frequency synthesis using a PLL as shown below

  29. Prescaling • Because programmable dividers are unavailable at frequencies above 100MHz, fixed- and two-modulus prescalers are used • Two-modulus prescalers can be programmed to divide by two consecutive integers