Amplitude Modulation Part 2 - AM RECEPTION

 To define AM demodulation  To define and describe the receiver parameters  To describe the operation of a tuned radio frequency (TRF) receiver  To describe the operation of a superheterodyne receiver

 Demodulation  Receiver parameters  Tuned radio frequency (TRF) receiver  Superheterodyne receiver

 AM demodulation – reverse process of AM modulation.  Demodulator: converts a received modulated-wave back to the original source information.  Basic understanding of the terminology commonly used to describe radio receivers & their characteristics is needed to understand demodulation process

Used to evaluate the ability of a receiver to successfully demodulate a radio signal:  Selectivity  Bandwidth improvement  Sensitivity  Dynamic range  Fidelity  Insertion Loss  Noise temperature & Equivalent noise temperature

 Used to measure the ability of the receiver to accept a given band of frequencies and reject all others.  Way to describe selectivity is to simply give the bandwidth of the receiver at the -3dB points.  Not necessarily a good means of determining how well the receiver will reject unwanted frequencies.

 Given the receiver bandwidth at two levels of attenuation. Eg: -3dB, -60dB  The ratio of two BW is called the Shape factor SF = B (-60 dB) / B (- 3dB) Where SF – Shape factor B (-60dB) – BW 60dB below max signal level B (-3dB) – BW 3dB below max signal level

 If both BW equal, the shape factor would be 1.  Impossible to achieve in practical circuit  Example application for SF nearly 1  Satellite  Microwave  Two way radio Rx

 Thermal noise directly proportional to bandwidth.  Reduce BW ~ reduce noise, improving system performance.  Reducing BW = improving the noise figure of the RX

Bandwidth Improvement, BI BI = B RF /B IF Where B RF = RF Bandwidth (Hz) B IF = IF Bandwidth (Hz) Noise figure improvement, NF = 10 log BI

 The minimum RF signal level that can be detected at the input to the Rx and still produce a usable demodulated information signal.  Usually stated in micro volts of received signal.  Rx sensitivity also called Rx threshold.

 Depends on:  The noise power present at the input to the Rx.  Rx noise figure.  AM detector sensitivity.  BI factor of the Rx  The best way to improve the sensitivity is to reduce the noise level Reducing either the temperature or receiver’s bandwidth or improving receiver’s noise figure

 The difference (in dB) between the minimum input level necessary to discern a signal and the input level that will overdrive the Rx and produce distortion.  Input power range over which the Rx is useful.

 A dynamic range of 100dB is considered about the highest possible.  A low dynamic range can cause a desensitizing of the RF amplifiers and result in severe intermodulation distortion of the weaker input signal.

 A measure of the ability of a communication system to produce (at the output of the Rx) an exact replica of the original source information.

 Forms of distortion that can deteriorate the fidelity of a communication system:-  Amplitude  Frequency  Phase

 Thermal noise is directly proportional to temperature ~ can be expressed in degrees, watts or volts.  Environmental temperature, T (kelvin) T = P n /KB Where P n = noise power (watts) K = Boltzman’s Constant (1.38 X J/K) B = Bandwidth (Hz)

 Equivalent noise temperature, (T e ) T e = T(F-1) Where T = environmental temperature (kelvin) F = Noise factor  T e often used in low noise, sophisticated radio receivers rather than noise figure.

 IL is a parameter associated with the frequencies that fall within the passband of a filter.  The ratio of the power transferred to a load with a filter in the circuit to the power transferred to a load without the filter. IL (dB) = 10 log (P out /P in )

 Two basic types of radio receivers. 1. Coherent  Synchronous receivers  The frequencies generated in the receiver & used for demodulation are synchronized to oscillator frequencies generated in transmitter. 2. Non-coherent  Asynchronous receivers  Either no frequencies are generated in the receiver or the frequencies used for demodulation completely independent from the transmitter’s carrier frequency.  Non-coherent detection = envelope detection.

 EXAMPLE OF COHERENT DEMODULATION: SSB  The received signal is heterodyned /mixed with a local carrier signal which is synchronous (coherent) with the carrier used at the transmitting end. LPF X SSB cos w c t Coherent demodulation

 Tuned Radio Frequency (TRF) Receiver  Superheterodyne Receiver

 RF amplifier - to filter and amplify the received signal to a level sufficient to drive the detector  Audio detector - converts RF signals directly to information  Audio stage – amplifies the information signals to a usable level  Advantages – simple and have relatively high sensitivity

 Bandwidth is inconsistent and varies with center frequency when tuned over a wide range of input frequencies  This is caused by a phenomenon called the skin effect  Skin effect phenomenon: B = f/Q Where Q is quality factor.

 Instability due to large number of RF amplifiers all tuned to the same center frequency.  Can be reduced by tuning each amplifier to a slightly different frequency, slightly above or below the desired center frequency.  Their gains are not uniform over a very wide frequency range because of the non-uniform L/C ratios of the transformer-coupled tank circuits in the RF amplifiers

Heterodyne means to mix two frequencies together in a nonlinear device or to translate one frequency to another using nonlinear mixing.

 RF section:  Preselector is use to provide enough initial bandlimiting to prevent a specific unwanted radio frequency called the image frequency from entering the receiver  Preselector also reduces the noise bandwidth of the receiver  RF amplifier determines the sensitivity of the receiver

 Mixer/ converter section:  Is a nonlinear device and its purpose is to convert radio frequencies to intermediate frequencies (RF-to-IF translation)  IF section:  Most of the receiver gain and selectivity is achieved in the IF section  IF is always lower in frequency than the RF because it is easier and less expensive to construct high-gain, stable amplifiers for low- frequency signals.

 Detector section:  To convert the IF signals back to the original source information  Audio amplifier section:  Comprises several cascaded audio amplifiers and one or more speakers

 Mixers generate signals that are the sum and difference of the incoming signal frequency (f S ) and the frequency of the local oscillator (f LO ).  The difference frequency is more commonly chosen as the IF.  Some receivers use the sum frequency for the IF.

 High side injection, f LO = f RF + f IF  Low side injection f LO = f RF – f IF f lo = local oscillator frequency (Hz) f RF = radio frequency (Hz) f IF = intermediate frequency (Hz)

 An image frequency (f IM ) is any frequency other than the selected radio frequency carrier, f rf,that, if allowed to enter a receiver and mix with the local oscillator will produce a cross- product frequency that is equal to the intermediate frequency, f IF  f IM = f RF + 2f IF or f RF + 2f IF  Images interfere with the desired signal.  Images can be eliminated or minimized by:  Proper selection of the IF in design.  Use of highly selective filters before the mixer.  Use of a dual conversion receiver.

 Image-frequency rejection ratio (IFRR) is a numerical measure of the ability of a preselector to reject the image frequency.  Image Frequency rejection ratio IFRR = √ (1 + Q²ρ²) Where ρ = (f IM /f RF ) –(f RF /f IM )

For a citizens band receiver using high-side injection with an RF carrier of 27 MHz and an IF center frequency of 455 kHz, determine a. Local oscillator frequency b. Image frequency c. IFRR for a preselector Q of 100

 AM Radio broadcasting  Commercial AM radio broadcasting utilizes the frequency band 535 – 1605 kHz for transmission voice and music.  Carrier frequency allocation range, kHz with 10 kHz spacing.

 Radio stations employ conventional AM for signal transmission – to reduce the cost of implementing the Rx.  Used superheterodyne Rx.  Every AM radio signal is converted to a common IF frequency of f IF = 455 kHz.

 To define AM demodulation  To define and describe the receiver parameters  To describe the operation of a tuned radio frequency (TRF) receiver  To describe the operation of a superheterodyne receiver

END OF CHAPTER 2 PART 2