2.9 : AM Receiver

1
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 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 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

4
2.9.1.1 Selectivity

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

5
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

• The corresponding reduction in noise due to
  reduction in bandwidth is called as noise figure
  improvement
• (33)
• Ex 5-1

7
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

• 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 level is 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

• 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 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

• 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

12
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 Boltsmanns constant (1.38 x 10-23 J/K)
• B bandwidth (Hz)

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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 transmitters carrier
  frequency
• For AM DSBFC scheme, the noncoherent receivers
  are typically used.
• Tuned Radio Frequency receiver (TRF)
• Superheterodyne Receiver

14
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 (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

• 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 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 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 Receiver

• 5. Audio amplifier section
• Comprises several cascaded audio amplifiers and
  one or more speakers

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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

• Mathematically expressed
• High side injection (33)
• Low side injection (34)

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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 Conversion

• Ex 5-3

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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

• 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 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 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

• 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 ? (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 ratio

• Ex 5-5

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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.

31
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

32
2.9.5 Net Receiver Gain

• Ex 5-8