General Receivers and Synthesizers Sections: 4-15 Outline • General Receivers General Receivers • In general, receivers can be coherent or non-coherent – Coherent (synchronized) – two inputs! – Non-Coherent (asynchronized) – one input • Non-coherent receives use detection technique (Envelope Detectors) – Example: Tuned Radio Frequency Receivers (TRF Receivers) • Coherent receives use superheterodyne technique – The local oscillator locks to the incoming signal – Heterodyning: mixing two frequencies using a non-linear device OR translating frequencies using non-linear mixer Noncoherent TRF Receiver • Common in AM demodulators • They are simple and sensitive • Used primarily for single channels and low-frequency applications • Main disadvantages: – Difficult to tune accurately to different center frequencies • The tuning (tank) circuit has has constant Q-Factor • B=f/Qà As the frequency changes à BW changes – Relatively instable Superheterodyne Receivers • Uses down/up converting and synchronizes the Flo to the incoming frequency (coherent) • Consists of Five sections: Superheterodyne Receivers • RF: Presector and amplifier; consists of a broad-tuned bandpass filter to prevent unwanted signal (image frequency); the amplifier needs to have high gain and SNR with good sensitivity • Mixer: consists of a local oscillator and a non-linear mixer; converts the incoming RF to IF Intermediate frequency) – this called heterodyning • IF Section: Uses multiple amplifiers (IF strip) – amplification as IF is easier and less expensive! • Detector: Diode or PLL Operation of Super-heterodyne Receivers • Assume we are using a SHR for AM radio • AM Frequency 535-1605 KHz • Separation between each channel is 10KHz (BW = 10 KHz) • Note that the IF range is much smaller • Tuning occurs at LO & Pre-selector We are using high-side injection LO >RF Sideband inversion Information content Is preserved! PLL Application in SHR • PLL-based frequency Synthesizers can used for SHR Example: PLL Application in SHR • Find N and M for the Frequency Synthesizer assuming we want to tune to 530KHz station • In this case what will be the frequency of the local oscillator? IF: 455KHz AM: 530-1710 KHz) @ 10KHz steps High-side LO Injection F input to mixer Rf. Osc = 1MHz 10KHz/2 = 5KHz References • Leon W. Couch II, Digital and Analog Communication Systems, 8th edition, Pearson / Prentice, Chapter 4 • Contemporary Communication Systems, First Edition by M F Mesiya– Chapter 5 • (http://highered.mcgraw-hill.com/sites/0073380369/information_center_view0/) See Notes .