ITCS Chapter 6 Notes: Receivers

Single Sideband (SSB) Receivers

  • Section 3 covers single sideband receivers.
  • Recap of SSB:
    • The Carrier is not needed to propagate the RF signal.
    • Transmitter's Purpose: To convert the modulating signal to the frequency range of the carrier.
    • Receiver's Purpose: To reverse the process and extract the intelligence.
  • SSB Receiver Block Diagram:
    • Antenna -> RF Amplifier -> First Mixer
    • Local Oscillator: Tuned to the original suppressed carrier frequency (e.g., 455 kHz).
    • Mixer Output -> IF Amplifier Stage -> Second Mixer (Detector)
    • Beat Frequency Oscillator (BFO): Tuned to the IF center frequency.
  • Important Clarification: The book refers to reinserting the carrier, but in SSB, only one sideband is transmitted. The BFO provides a reference for demodulation.
  • The second mixer is required to properly demodulate the SSB signal.
  • Example 6-9 (Page 169):
    • SSB receiver with outputs at 1 kHz and 3 kHz.
    • Carrier frequency (suppressed at transmitter): 2 MHz.
    • Upper sideband utilized.
    • IF frequency: 455 kHz.
  • Transmitter:
    • Carrier: 2 MHz.
    • Intelligence: 1 kHz and 3 kHz tones.
    • Only the upper sideband is used, resulting in frequencies of 2.001 MHz and 2.003 MHz.
  • Receiver:
    • RF Amplifier receives 2.001 MHz and 2.003 MHz.
    • Local Oscillator: 2.455 MHz (455 kHz above the original carrier).
    • Mixer:
      • Subtracts the local oscillator frequency from the received signals.
      • 2,455 kHz−2,001 kHz=454 kHz2,455 \text{ kHz} - 2,001 \text{ kHz} = 454 \text{ kHz}
      • 2,455 kHz−2,003 kHz=452 kHz2,455 \text{ kHz} - 2,003 \text{ kHz} = 452 \text{ kHz}
    • IF Amplifier:
      • Passes frequencies around 455 kHz.
      • Outputs 454 kHz and 452 kHz.
    • Second Mixer (Detector):
      • BFO set to 455 kHz (the intermediate frequency of the transmitter).
      • Extracts the intelligence by finding the difference between the IF amplifier output and the BFO frequency.
      • 455 kHz−454 kHz=1 kHz455 \text{ kHz} - 454 \text{ kHz} = 1 \text{ kHz}
      • 455 kHz−452 kHz=3 kHz455 \text{ kHz} - 452 \text{ kHz} = 3 \text{ kHz}
    • Audio Amplifier amplifies and outputs the 1 kHz and 3 kHz tones through the speaker.
  • Handout Example:
    • Determine frequencies at various stages of an SSB receiver.
    • IF frequency: 455 kHz (BFO setting).
    • Original carrier: 1940 kHz.
    • Intelligence: 1 kHz.
    • Upper sideband utilized, resulting in a transmitted frequency of 1941 kHz (1.941 MHz).
    • Local Oscillator: Set to 455 kHz above 1940 kHz:
      • 1940 kHz+455 kHz=2395 kHz1940 \text{ kHz} + 455 \text{ kHz} = 2395 \text{ kHz}
    • Mixer output: 454 kHz.
    • IF Amplifiers amplify the 454 kHz signal.
    • Second Mixer:
      • Subtracts 454 kHz from the BFO frequency (455 kHz).
      • 455 kHz−454 kHz=1 kHz455 \text{ kHz} - 454 \text{ kHz} = 1 \text{ kHz}

FM Receivers

  • Block diagram is similar to AM receivers but with key differences.
  • Superheterodyne FM Receiver
  • Differences from AM Receivers:
    • RF Amplifier: Absolutely necessary in FM receivers due to weaker received signals compared to AM.
    • Limiter: Added to remove noise from the FM signal.
    • Discriminator: Replaces the detector in AM receivers; extracts the intelligence.
    • De-emphasis Network: Used for filtering and amplitude boosting before the speaker output.
    • IF (Intermediate Frequency:)
      • AM: 455 kHz
      • FM: 10.7 MHz
  • RF Amplifier:
    • Necessary to overcome mixer noise.
    • Boosts the desired signal.
    • FM receivers can function with weaker signals than AM due to their inherent noise reduction.
      • Input signal as low as 1 microvolt.
    • RF amp boosts the signal to 10-20 microvolts to overcome mixer noise.
      • RF amplifier reduces image frequency problems.
      • RF amp reduces local oscillator reradiation effect.
      • Local oscillator signal can couple back into the receiving antenna causing interference.
  • Discrete Setup of RF Amplifier (MOSFET RF Amplifier):
    • Virtually all high-quality FM receivers use FETs (Field Effect Transistors).
    • FETs have a square law relationship:
      • The input signal's magnitude is squared at the output, causing an exponential increase.
      • Good for amplification.
    • Dual-gate MOSFET provides a convenient injection point for the local oscillator signal.
    • MOSFETs have increased dynamic range over JFETs.
    • A wider range of input signals can be tolerated while offering the desired square law input-output relationship
  • Circuit Analysis.
    • Input impedance of 50 ohms (antenna must match).
    • FM signal of 88 MHz to 108 MHz is fed in and amplified.
    • A tuned frequency selective circuit, probably the preselector is in the front end of the circuit.
    • Two resistors form a voltage divider for biasing needs.
    • Capacitors and a radio frequency choke are used for decoupling.
    • Variable capacitors and an inductor form frequency selective circuits for output coupling or tuning.
    • Gang tuning tunes the input and output circuits together.
    • Optional AGC (Automatic Gain Control) varies the gain of the MOSFET.
  • Mixer, IF, and LO:
    • Similar to AM receivers, but the intermediate frequency is 10.7 MHz.
  • Limiter:
    • Output of the limiter is constant for all inputs above a critical value
    • Constant amplitude for all passing signals, removes any noise that may be present.
    • Removes amplitude variations/noise.
    • Needs to receive a signal at a certain amplitude for it to do its job.
    • If threshold not met, the noise can not be removed.
    • Zener diode limiter.
    • Transistor limiter.
    • The signal may be expressed with noise spikes on the signal
    • There should be a threshold met for the output of the signal to not have random variances in amplitude.
  • Quieting:
    • When enough signal arrives, the limiter quiets.
    • When that quieting threshold is reached, the signal is limited
    • Level of quieting is generally established at a certain signal to noise level, (i.e. 30 dB).
  • Limiting and Sensitivity:
    • Sensitivity is the level of input signal to a receiver that causes a level of quieting.
    • It can be expressed as the minimum value to give a desired output, the antenna must meet a minimum level.
    • V<em>in=V</em>outVoltageGainV<em>{in} = \frac{V</em>{out}}{Voltage Gain}
    • Sensitivity = 1 microvolt in order for it to limit.
  • Extended Problem with sensitivity and local oscillator for an FM receiver
    • An FM receiver is tuned to a station, the intermediate frequency is known.
      • In order to find the frequency in which to change the local oscillator, sum the frequencies needed.
      • 103.7 MHz+10.7 MHz=114.4 MHz103.7 \text{ MHz} + 10.7 \text{ MHz} = 114.4 \text{ MHz}
    • The DB value must take into account all values, then must be transposed.
      # Discrete Component FM Receivers
      # Section 4: Direct Conversion Receivers
  • A single mixer local oscillator is used to down convert the received signal to the baseband or the intelligence signal.
  • Eliminates image frequency problems
  • Also known as zero IF receivers
  • Works with AM, double sideband, it single sideband.
  • Does not work well with FM or PM
  • Heavily uses of cell phones, will be discussed throughout the following chapter(s)