Amplitude Modulation 2.2 AM RECEIVERS CHAPTER 2 Amplitude Modulation 2.2 AM RECEIVERS

OBJECTIVES 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

LECTURE OVERVIEW Demodulation Receiver parameters Tuned radio frequency (TRF) receiver Superheterodyne receiver

Introduction 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

Simplified block diagram of an AM receiver

Receiver Parameters Selectivity Bandwidth improvement Sensitivity Dynamic range Fidelity Insertion Loss Noise temperature & Equivalent noise temperature

Selectivity 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.

Cont’d… Give the receiver bandwidth at two levels of attenuation. Eg: -3dB, -60dB The ratio of two BW ~ 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

Cont’d… 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

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

Cont’d… Bandwidth Improvement, BI BI = BRF /BIF Where BRF = RF Bandwidth (Hz) BIF = IF Bandwidth (Hz) Noise figure improvement, NF = 10 log BI

Sensitivity 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.

Cont’d… Depends on: To improve ~ reduce the noise level The noise power present at the input to the Rx. Rx noise figure. AM detector sensitivity. BI factor of the Rx To improve ~ reduce the noise level Reducing the temperature or Rx BW or RX noise figure

Dynamic range 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.

Cont’d… 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.

Fidelity 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.

Cont’d… Forms of distortion that can deteriorate the fidelity of a communication system:- Amplitude Frequency Phase

Noise Temperature & Equivalent noise Temperature Thermal noise directly proportional to temperature ~ can be expressed in degrees, watts or volts. Environmental temperature, T (kelvin) T = N/KB Where N = noise power (watts) K = Boltzman’s Constant (1.38 X 10-23 J/K) B = Bandwidth (Hz)

Cont’d… Equivalent noise temperature, (Te) Te = T(F-1) Where T = environmental temperature (kelvin) F = Noise factor Te often used in low noise, sophisticated radio receivers rather than noise figure.

Insertion loss 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 (Pout /Pin)

AM RECEIVERS Two basic types of radio receivers. Coherent Non-coherent Synchronous receivers The frequencies generated in the Rx & used for demodulation are synchronized to oscillator frequencies generated in Tx. Non-coherent Asynchronous receivers Either no frequencies are generated in the Rx or the frequencies used for demodulation completely independent from the Tx’s carrier frequency. Non-coherent detection = envelope detection.

Coherent demodulation 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 wct Coherent demodulation

Non-Coherent Rx Tuned Radio Frequency Rx Superheterodyne Rx

Non-coherent tuned radio frequency receiver (TRF Rx)

Cont’D 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

DisAdvantages of trf 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.

Cont’d… 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

AM superheterodyne receiver

Cont’d… 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

CONt’d… Mixer/ converter section: IF 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.

Cont’d… Detector section: Audio amplifier 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

Frequency conversion High side injection, flo = fRF + fIF Low side injection flo = fRF - fIf

INTERMEDIATE FREQUENCY (IF) Mixers generate signals that are the sum and difference of the incoming signal frequency (fS) and the frequency of the local oscillator (fLO). The difference frequency is more commonly chosen as the IF. Some receivers use the sum frequency for the IF.

IMAGES An image (fIM) is an undesired signal that is separated from the desired signal frequency (frf) by two times the IF (fIF). fI = frf + 2fIF or frf - 2fIF 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.

con’t’d… Image frequency fim = fRF + 2fIF Image Frequency rejection ratio IFRR = √ (1 + Q²ρ²) Where ρ = (fim/fRF) –(fRF/fim)

Example 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 APPLICATION AM Radio broadcasting Commercial AM radio broadcasting utilizes the frequency band 535 – 1605 kHz for transmission voice and music. Carrier frequency allocation range, 540-1600 kHz with 10 kHz spacing.

Cont’d… 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 fIF = 455 kHz.

AT THE END OF THIS CHAPTER, YOU SHOULD BE ABLE 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 2.2 : AM RECEIVER