Receivers Notes

Single Sideband Receivers

• Single sideband receivers and FM receivers will be covered.
• Single sideband receivers: Block diagram towards the bottom of page 169.
• Carrier is not needed to propagate the RF signal.
  • The carrier's purpose in the transmitter is to convert the modulating signal to the frequency range of the carrier.
  • To extract intelligence, the process is reversed at the receiving side.
• Block Diagram:
  • Antenna feeds into an RF amplifier.
  • Goes into the first mixer.
  • The local oscillator is tuned to the original suppressed carrier frequency of $455$ kHz.
  • Output of the mixer is feeding into the IF amplifier stage.
  • Goes into another mixer, also being fed by reinserted carrier.
  • BFO (beat frequency oscillator) is tuned to the IF center frequency.
• The carrier is not necessarily reinserted.
  • Beat frequency oscillator is tuned to the intermediate frequency, center frequency.
  • The second mixer is needed to properly demodulate the single sideband signal.
• Example 6-9 on page 169 reviews the single sideband receiver.
  • Single sideband receiver has outputs at $1$ kHz and $3$ kHz.
  • The carrier used and suppressed at the transmitter was $2$ MHz.
  • The upper sideband was utilized to determine the exact frequencies at all stages for a $455$ kHz IF frequency.
  • At the transmitter:
    • Carrier of $2$ MHz.
    • Intelligence is sending a $1$ kHz tone and a $3$ kHz tone.
    • Only the upper sideband is utilized.
    • Upper sideband is fluctuating between $2.001$ MHz and $2.003$ MHz.
    • Taking this original carrier and adding $1$ kHz or $3$ kHz.
    • Sending a two-toned signal.
  • Being transmitted and received at the receiver.
  • At the RF amplifier:
    • Receiving what is being transmitted: $2.001$ MHz, $2.003$ MHz.
  • The local oscillator is set to $455$ kHz above the original carrier.
    • The local oscillator is $2,455$ kHz.
    • When these two frequencies are mixed, we end up with a sum and difference within the mixer.

FM Receivers

• Additional block portions are necessary to demodulate FM.
• Many of these blocks look identical to AM receivers.
• Block configuration makes it a superheterodyne FM receiver.
• Differences from AM receivers:
  • RF amp is definitely needed.
    • With FM, you're actually receiving a weaker signal in comparison to amplitude.
    • RF amplifier is needed within an FM receiver.
  • There is a limiter.
  • Detector has been renamed the discriminator.
    • Still extracts the intelligence.
  • A de-emphasis network also filters and boosts amplitudes.
  • The intermediate frequency is changed from $455$ kHz within AM receivers to $10.7$ MHz in FM receivers.

RF Amplifier

• Definitely needed to overcome mixer noise.
• Boost the desired signal because the radio receiver is acting noisy from internal components.
• FM receivers can function with weaker received signals than AM or single sideband due to their inherent noise reduction capability.
  • The input signal that FM receivers receive is as low as $1$ microvolt.
• An RF amp boosts the received signal to $10$ to $20$ microvolts to overcome mixer noise.
• At frequencies above $1$ GHz, diode mixers are used due to increased transistor noise and decreased gain at those frequencies.
  • Diodes are passive devices and less noisy.
• The radio frequency amplifier reduces image frequency problems.
  • There is a preselector being deployed.
• RF amp reduces local oscillator re-radiation effect.
  • Local oscillator signal can be coupled back into the receiving antenna and results in transmitted interference.

Discrete Setup of an RF Amplifier

• The circuit itself of an RF amplifier is a MOSFET RF amplifier.
• Almost all RF amplifiers used in quality FM receivers use FETs (field effect transistors) as the active device.
• FETs have a square law relationship:
  • An input signal being fed into a FET has an exponential increase, a squared increase. If you're receiving a signal, the FET can amplify by a squared value, increasing the signal exponentially.
  • Gives an output signal at the input frequency and a small distortion component at twice the input frequency, which can result in received noise known as cross modulation (similar to intermodulation distortion).
• The use of FETs in radio frequency amplifiers greatly minimizes the possibility of intermodulation distortion.
• Input: * $50$ ohm impedance. * The antenna that would attach has to match to $50$ ohms.
  • Output:
    • $50$ ohm impedance.
    • Connections made beyond need to match with $50$ ohms.
  • FM signal ranges from $88$ MHz to $108$ MHz.
    • That's the FM broadcasting spectrum.
    • The RF Amplifier boosts the amplitude.

Analysis of the Circuit

• FETs are used in virtually all high-quality receivers, not for their high input impedance but because they are a square law device.
• A dual gate input provides an extra gate for a convenient injection point for the local oscillator signal.
• MOSFET devices have increased dynamic range over JFETs.
  • A wider range of input signals can be tolerated by the MOSFET while still offering the desired square law input-output relationship.
• There is a tuned frequency selective circuit.
  • A tank circuit exists.
  • At the front end of an RF amplifier.
  • Connected to the antenna.
  • Most likely the preselector doing some wide filtering of the signal passing through initially.
• Two resistors like this are most likely for biasing. - Voltage divider.
• Many capacitors are seen and a radio frequency choke.
  • The radio frequency choke is hindering signals from interfering with the DC biasing voltage. - Most likely used for decoupling.
• Variable capacitors are seen.
• Output: some frequency selective circuits happen here. - Most likely output coupling or tuning.
• There is some gang tuning.
  • The output circuit and the input circuit are tuned together.
• An optional automatic gain control is used to vary the gain of this MOSFET.

Mixer, IF, and LO

• Basically the same as AM.
• The only difference is that the IF amplifier frequency has changed.
• It is no longer $455$ kHz as it was for AM receivers, but is now $10.7$ MHz.

Limiter

• Output of the limiter is constant for all inputs above a critical value.
• It will remove amplitude variations, also known as noise
• It needs to receive the signal at a certain amplitude.
• Needs the RF amplifier to do its job
• Needs the IF amplifier to also do its job.
• Amplitude abnormalities in FM signal can have noise spikes. Zener Diode Limiter:
  • Tuned frequency selective circuit.
  • This circuit is functional as long as a threshold has been met and a signal is received at a certain amplitude, such that the Limiter can remove variants in amplitude or noise spikes.