Simple block diagram of receivers
4f.1 Understand
the block diagrams of the crystal diode receiver, tuned radio frequency
(TRF) or straight receiver and superhet receiver.
4f.2 Understand
the functions of the RF amplifier, mixer, local oscillator, IF
amplifier, demodulator (detector), and audio amplifier. Intermediate frequency
4g.1 Recall that the intermediate frequency is the sum of or difference between the RF and local oscillator frequencies.
Frequency selection
4h.1 Understand that tuned circuits in RF and IF amplifiers select the wanted signal.
Detectors
4i.1 Understand how a diode detector will recover the audio from
amplitude modulated signals. Understand that to generate the audio from
CW signals a beat frequency oscillator (BFO) is used; for the recovery
of single side band audio, a carrier insertion oscillator (CIO) and
product detector is used; for the recovery of FM audio, a discriminator
is used.
4i Identify the waveforms produced in a diode AM detector.
This is a crystal diode receiver: The tuning stage consists of a parallel inductor and variable capacitor between the antenna and earth. The diode rectifies strong AM signals
The earphone converts the rectified electrical signal to sound.
Key features
No power supply required
Not very selective – receives several AM stations at the same time
Receives AM signals only
No amplification, so not very sensitive.
Only receives strong stations
This is a tuned radio frequency receiver
The first stage filters out the required band and amplifies the signals The demodulator is still a diode which separates out the AF from the RF The audio amplifier increases the gain The BFO can be used to receive Morse and SSB
Key features over a crystal set:
More selectivity only receives one station at a time
More sensitivity
Can be used to drive a loud speaker
Needs a power supply
This is a superhet receiver There
may be an RF amplifier between the antenna and the mixer. The RF
signal from the antenna is amplified. It also has tuned circuits to select signals on the received band.
The mixer combines the RF signal with a signal from the local oscillator to produce an intermediate frequency (see previous slide on sum and difference). The local oscillator generates the tuning frequency
After the mixer there is a filter, usually a crystal filter that selects IF signals only. The IF is fixed at one frequency – for example 10.7 MHz. It further amplifies and filters the signal. As it works on one frequency there is no need for tuning each stage with variable capacitors. This is where most gain is achieved.
For CW and SSB receivers.The demodulatoris
usually a transistor or an IC mixer that mixes the IF frequency with a
signal close to the IF frequency produced by the BFO to produce an
audio signal.
For FM receivers a discriminator is used.
For
AM reception a diode is used. This rectifies the AC signal and then
filters out the RF leaving the envelope which is an audio signal. The BFO (Beat Frequency Oscillator) or CIO (Carrier Insertion Oscillator) is mixed with the IF to demodulate CW and SSB The audio amplifierincreases the audio power to drive aspeaker or headphones
4j.1 Understand that the automatic gain control (AGC) of a receiver
operates by sensing the strength of the received signals at the
detector and adjusting the gain of the IF and sometimes the RF
amplifiers to keep the audio output level fairly constant.
AGC (Automatic Gain Control)
Signals from a superhet
receiver can vary from S1 to S9+60. This change can take place over
several seconds. One way to cope with this is to constantly have your
hand on the RF gain, turning it up and down to keep the audio signal
constant. Fortunately an AGC circuit will do this automatically. This
monitors the output from the detector, and turns it to a voltage which
automatically controls the gain of the IF and sometimes the RF stages.
If the signal strength increases, the IF gain is turned down.
If the signal strength decreases, the IF gain is turned up.
