2 9 am receiver
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2.9 : AM Receiver. AM demodulation is the reverse process of AM modulation. A conventional double sideband AM receiver converts the amplitude-modulated waveform back to the original source by receiving, amplifying and demodulating the wave.

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2.9 : AM Receiver

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2 9 am receiver

2.9 : AM Receiver

  • AM demodulation is the reverse process of AM modulation.

  • A conventional double sideband AM receiver converts the amplitude-modulated waveform back to the original source by receiving, amplifying and demodulating the wave.

  • The receiver also functioning to bandlimit the total RF spectrum to a specific desired band of frequency – tuning the receiver

  • Simplified block diagram of typical AM receiver

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2 9 am receiver1

2.9 : AM Receiver

  • RF section (Receiver front end)

    • used to detect, bandlimit and amplifying the received RF signal.

  • Mixer/converter

    • Down-converts the received RF frequencies to intermediate frequencies (IF).

    • Intermediate frequencies are the frequencies that fall somewhere between the RF and the information frequencies.

  • IF section

    • Used for amplification and selectivity.

  • AM detector

    • Demodulates the AM wave and converts it to the original information signal.

  • Audio section

    • Used to amplify the recovered signal

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2 9 1 receiver parameters 2 9 1 1 selectivity

2.9.1 : Receiver Parameters2.9.1.1 : Selectivity

  • Selectivity – parameter used to measure the ability of the receiver to accept a given band of frequencies and reject all others.

    • Ex : for the commercial AM broadcast band, each stations transmitter is allocated a 10 kHz bandwidth. For a receiver to select only those frequencies assigned in a single channel, the receiver must limit its bandwidth to 10 kHz.

  • A method to describe the selectivity of the receiver is to give the receiver a bandwidth at 2 levels of attenuation (e.g. -3 dB and -60 dB).

  • The ratio of these 2 bandwidths is called as shape factor (SF),

    (31)

  • In ideal, both bandwidth would be equal and the value of the shape factor would be 1. But this is impossible to be achieve in practical circuit.

    • Ex :AM broadcast-band radio receiver : SF = 2

      satellite, microwave & 2-way radio receivers: SF = closer to 1

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2 9 1 1 selectivity

2.9.1.1 : Selectivity

  • A radio receiver must be capable of separating the desired channel’s signal without allowing interference from an adjacent channel to spill over into the desired channel’s passband.

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2 9 1 2 bandwidth improvement

2.9.1.2 : Bandwidth Improvement

  • Thermal noise is one form of noise occurs in communication system that is proportional to a bandwidth.

  • As signal propagates from the antenna through the RF section, mixer/converter section and IF section, the bandwidth of signal is reduced thus reducing the noise.

  • Noise reduction ratio achieved by reducing the bandwidth is called bandwidth improvement (BI) expressed as follow,

    (32)

    where BI = bandwidth improvement

    BRF = RF bandwidth

    BIF = IF bandwidth

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2 9 1 3 bandwidth improvement

2.9.1.3 : Bandwidth Improvement

  • The corresponding reduction in noise due to reduction in bandwidth is called as noise figure improvement

    (33)

  • Ex 5-1

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2 9 1 4 sensitivity

2.9.1.4 : Sensitivity

  • Sensitivity of the receiver is defined as - the minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal.

  • Signal-to-noise ratio (SNR) and the power of signal at the output of the audio section are used to determine the quality of the received signal and whether it is usable.

    • Typical AM broadcast-band receivers, a 10 dB or more SNR with approximately 0.5W of signal power at audio section is considered usable.

  • Sensitivity of a receiver is expressed in microvolts of the received signal.

    • Typical sensitivity for commercial broadcast-band AM receiver is 50 μV.

  • Sensitivity of the receiver depends on :

    • Noise power present at the input to the receiver

    • Receiver noise figure

    • Sensitivity of the AM detector

    • Bandwidth improvement factor of the receiver

  • The best way to improve the sensitivity is to reduce the noise level

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2 9 1 5 dynamic range

2.9.1.5 : Dynamic range

  • Dynamic range of a receiver is defined as - the difference in decibels between the minimum input level necessary to recognize a signal and the input level that will overdrive the receiver and produce distortion.

    • The minimum received levelis a function of the desired signal quality, front-end noise and the noise figure of the receiver : X

    • The level that will produce overload distortion is a function of the net gain of the receiver (total gain of all stages in the receiver) : Y

    • A dynamic range of 100 dB (between X and Y) is considered about the highest possible.

    • A low dynamic range can cause severe intermodulation distortion.

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2 9 1 6 fidelity

2.9.1.6 : Fidelity

  • Fidelity is defined as – a measure of the ability of a communication system to produce an exact replica of the original source information at the output of the receiver.

  • Any variations in the demodulated signal that are not in the original information signal is considered as distortion.

  • 3 forms of distortions :

    • Phase distortion

    • Amplitude distortion

    • Frequency distortion

  • Phase distortion

    • Filtering is the predominant cause of phase distortion

    • Frequencies at or near the break frequency of a filter undergo varying the values of the phase shift (i.e. the phase is shifted/delayed).

    • If all the frequencies are not delayed by the same amount of time, the frequency-versus-phase relationship of the received signal is not consistent with the original signal and the recovered signal is distorted.

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2 9 1 6 fidelity1

2.9.1.6 : Fidelity

  • Amplitude distortion

    • Occurs when the amplitude-versus-frequency characteristics of the output signal of a receiver differs from those of the original signal.

    • It is the result of nonuniform gain in amplifiers and filters

  • Frequency distortion

    • Occurs when frequencies that are present in a received signal are not present in the original source information.

    • It is a result of harmonic and intermodulation distortion and caused by nonlinear amplification

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2 9 1 7 insertion loss

2.9.1.7 : Insertion Loss

  • Insertion loss – ratio of the power transferred to a load with a filter in the circuit to the power transferred to a load without a filter in the circuit

  • Filters are generally constructed from lossy components such as resistorand imperfect capacitor that tend to attenuate (reduce the magnitude) the signal

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2 9 1 8 noise temperature equivalent noise temperature

2.9.1.8 : Noise Temperature/Equivalent Noise Temperature

  • Thermal noise is directly proportional to temperature and can be expressed in degress as well as watts and volts.

    where T = environmental temperature (kelvin)

    N = Noise power (watts)

    K = Boltsmann’s constant (1.38 x 10-23 J/K)

    B = bandwidth (Hz)

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2 9 2 types of receiver

2.9.2 : Types of receiver

  • 2 basic types of receiver

    • Coherent receiver – the frequencies generated in the receiver and used for demodulation are synchronized to oscillator frequencies generated in the transmitter.

    • Noncoherent receiver – frequencies that are generated in the receiver or the frequencies that are used for demodulation are completely independent from the transmitter’s carrier frequency

  • For AM DSBFC scheme, the noncoherent receivers are typically used.

    • Tuned Radio Frequency receiver (TRF)

    • Superheterodyne Receiver

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2 9 2 1 tuned radio frequency receiver trf

2.9.2.1 : Tuned Radio Frequency Receiver (TRF)

  • Block diagram of 3-stages TRF receiver that includes an RF stage, a detector stage and an audio stage :

    • Two or three RF amplifiers are required to filter and amplify the received signal to a level sufficient to drive the detector stage.

    • The detector converts RF signals directly to information.

    • An audio stage amplifies the information signals to a usable level

    • Simple and have a relatively high sensitivity

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2 9 2 1 tuned radio frequency receiver trf1

2.9.2.1 : Tuned Radio Frequency Receiver (TRF)

  • 3 distinct disadvantages :

    1. The bandwidth is inconsistent and varies with the center frequency when tuned over a wide range of input frequencies.

    • As frequency increases, the bandwidth = f/Q increases. Thus, the selectivity of the input filter changes over any appreciable range of input frequencies.

    • Ex 5-2

      2. Instability due to large number of RF amplifiers all tuned to the same center frequency

    • High frequency, multi stage amplifiers are susceptible to breaking into oscillation.

      3. The gains are not uniform over a very wide frequency range.

    • The nonuniform L/C ratios of the transformer-coupled tank circuits in the RF amplifiers.

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2 9 2 2 superheterodyne receiver

2.9.2.2 : Superheterodyne Receiver

  • Heterodyne – to mix two frequencies together in a nonlinear device or to transmit one frequency to another using nonlinear mixing.

  • Block diagram of superheterodyne receiver :

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2 9 2 2 superheterodyne receiver1

2.9.2.2 : Superheterodyne Receiver

  • 1. RF section

    • Consists of a pre-selector and an amplifier

    • Pre-selector is a broad-tuned bandpass filter with an adjustable center frequency used to reject unwanted radio frequency and to reduce the noise bandwidth.

    • RF amplifier determines the sensitivity of the receiver and a predominant factor in determining the noise figure for the receiver.

  • 2. Mixer/converter section

    • Consists of a radio-frequency oscillator and a mixer.

    • Choice of oscillator depends on the stability and accuracy desired.

    • Mixer is a nonlinear device to convert radio frequency to intermediate frequencies (i.e. heterodyning process).

    • The shape of the envelope, the bandwidth and the original information contained in the envelope remains unchanged although the carrier and sideband frequencies are translated from RF to IF.

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2 9 2 2 superheterodyne receiver2

2.9.2.2 : Superheterodyne Receiver

  • 3. IF section

    • Consists of a series of IF amplifiers and bandpass filters to achieve most of the receiver gain and selectivity.

    • The IF is always lower than the RF because it is easier and less expensive to construct high-gain, stable amplifiers for low frequency signals.

    • IF amplifiers are also less likely to oscillate than their RF counterparts.

  • 4. Detector section

  • Gambar

    • To convert the IF signals back to the original source information (demodulation).

    • Can be as simple as a single diode or as complex as a PLL or balanced demodulator.

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2 9 2 2 superheterodyne receiver3

2.9.2.2 : Superheterodyne Receiver

  • 5. Audio amplifier section

    • Comprises several cascaded audio amplifiers and one or more speakers

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2 9 3 receiver operation 2 9 3 1 frequency conversion

2.9.3 : Receiver Operation2.9.3.1 : Frequency Conversion

  • Frequency conversion in the mixer stage is identical to the frequency conversion in the modulator except that in the receiver, the frequencies are down-converted rather that up-converted.

    • In the mixer, RF signals are combined with the local oscillator frequency

    • The local oscillator is designed such that its frequency of oscillation is always above or below the desired RF carrier by an amount equal to the IF center frequency.

    • Therefore the difference of RF and oscillator frequency is always equal to the IF frequency

    • The adjustment for the center frequency of the pre-selector and the local oscillator frequency are gang-tune (the two adjustments are tied together so that single adjustment will change the center frequency of the pre-selector and at the same time change the local oscillator)

    • when local oscillator frequency is tuned above the RF – high side injection

      when local oscillator frequency is tuned below the RF – low side injection

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2 9 3 2 frequency conversion

2.9.3.2 : Frequency Conversion

  • Mathematically expressed :

    High side injection(33)

    Low side injection(34)

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2 9 3 2 frequency conversion1

2.9.3.2 : Frequency Conversion

  • Illustration of the frequency conversion process for an AM broadcast-band superheterodyne receiver using high side injection :

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2 9 3 2 frequency conversion2

2.9.3.2 : Frequency Conversion

  • Ex 5-3

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2 9 3 3 local oscillator tracking

2.9.3.3 : Local oscillator tracking

  • Local oscillator tracking – the ability of the local oscillator in a receiver to oscillate either above or below the selected radio frequency carrier by an amount equal to the intermediate frequency throughout the entire radio frequency band.

    • With high side injection- local oscillator should track above the incoming RF carrier by a fixed frequency equal to fRF + fIF

    • With low side injection- local oscillator should track below the incoming RF carrier by a fixed frequency equal to fRF - fIF

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2 9 3 4 image frequency

2.9.3.4 : Image frequency

  • Image frequency – any frequency other than the selected radio frequency carrier that will produce a cross-product frequency that is equal to the intermediate frequency if allowed to enter a receiver and mix with the local oscillator.

  • It is equivalent to a second radio frequency that will produce an IF that will interfere with the IF from the desired radio frequency.

    • if the selected RF carrier and its image frequency enter a receiver at a same time, they both mix with the local oscillator frequency and produce different frequencies that are equal to the IF.

    • Consequently, 2 different stations are received and demodulated simultaneously

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2 9 3 4 image frequency1

2.9.3.4 : Image frequency

  • The following figure shows the relative frequency spectrum for the RF, IF, local oscillator and image frequencies for a superheterodyn receiver using high side injection.

    • For a radio frequency to produce a cross product equal to IF, it must be displaced from local oscillator frequency by a value equal to the IF.

    • With high side injection, the selected RF is below the local oscillator by amount equal to the IF.

    • Therefore, the image frequency is the radio frequency that is located in the IF frequency above the local oscillator as shown above, i.e.

      (35)

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2 9 3 4 image frequency2

2.9.3.4 : Image frequency

  • The higher the IF, the farther away the image frequency is from the desired radio frequency. Therefore, for better image frequency rejection, a high IF is preferred.

  • However, the higher the IF, it is more difficult to build a stable amplifier with high gain. I.e. there is a trade-off when selecting the IF for a radio receiver (image frequency rejection vs IF gain and stability)

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2 9 3 5 image frequency rejection ratio

2.9.3.5 : Image frequency rejection ratio

  • Image frequency rejection ratio (IFRR) – a numerical measure of the ability of a pre-selector to reject the image frequency

  • Mathematically expressed as,

    (36)

    where ρ= (fim/fRF) – (fRF/fim)

    Q = quality factor of a pre-selector

  • Once an image frequency has down-converted to IF, it cannot be removed. In order to reject the image frequency, it has to be blocked prior to the mixer stage. I.e. the bandwidth of the pre-selector must be sufficiently narrow to prevent image frequency from entering the receiver.

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2 9 3 5 image frequency rejection ratio1

2.9.3.5 : Image frequency rejection ratio

  • Ex 5-5

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2 9 4 double conversion receivers

2.9.4 : Double Conversion Receivers

  • For good image rejection, relatively high IF is desired. However, for a high gain selective amplifiers that are stable, a low IF is necessary.

  • The solution fro above constrain is to use 2 intermediate frequencies, i.e. by using double conversion AM receiver.

    • The 1st IF is a relatively high frequency for good image rejection.

    • The 2nd IF is a relatively low frequency for good selectivity and easy amplification.

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2 9 5 net receiver gain

2.9.5 : Net Receiver Gain

  • Net receiver gain is simply the ratio of the demodulator signal level at the output of the receiver to the RF signal level at the input to the receiver.

  • In essence, net receiver gain is the dB sum of all gains to the receiver minus the dB sum of all losses.

  • Gains and losses found in a typical radio receiver :

    Net Receiver Gain GdB = gainsdB – lossesdB

    where gains = RF amplifier gain + IF amplifier gain + audio amplifier gain

    losses = pre-selector loss + mixer loss + detector loss

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2 9 5 net receiver gain1

2.9.5 : Net Receiver Gain

  • Ex 5-8

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