1. Improving Tuned Circuits

2. Limitations of Simple crystal set receivers
Before being able to improve any device we must first be able to understand it’s limitations.

3. Limitations of Simple crystal set receivers
Poor sensitivity
The tuning and demodulation sections of this type of receiver rely on the signal to provide power. Weaker stations do not provide this and thus only local stations with a strong signal can be received.
This can be overcome slightly by using an amplifier that works with Radio Frequencies. This is called an RF amplifier.
RF Amp
Tuned Circuit
Demodulation

4. Limitations of Simple crystal set receivers
Poor selectivity
If there are two strong stations with very close frequencies, the simple crystal set will not be able to separate them out and they will interfere with each other
Good Q Factor – is to have a narrow but tall curve
Unloaded
Voltage across tuned circuit
Loaded
Frequency

5. TRF System
We have now seen that by using multiple RF Amplifiers it is possible to create a stronger signal that can take the strain of being placed in a loaded circuit. By using 3 RF Amplifiers we have seen that the effect is to narrow the bandwidth of the stations and thus improve the selectivity.
Main Disadvantage
It can be very difficult to set several capacitors to the correct value at the same time.
TRF Amp 1
TRF Amp 2
TRF Amp 3
Mechanical Link

6. Superheterodyne Receivers
We can already improve the strength of our RF and AF signals by using the relevant amplifiers but we need to find a better way to improve the selectivity of our radios. The most popular way in modern radios is to use the superheterodyne method. fo
( fo + fc )
( fo – fc ) fc
Tuned RF Amplifier
Mixer
RF Filter Amplifiers
Local Oscillator
fo = frequency of oscillator
fc = frequency of carrier
Mechanical Link

7. How it works
The frequency of the RF carrier signal can obviously change so we need to make something a bit more constant in our circuit.
The Oscillator produces a frequency that is always 470 kHz above that of the carrier via a mechanical link between the two. The carrier signal and the newly produced oscillator signal are then fed into the Mixer. The mixer now outputs many signals due to the arrangement being fed in. fo
( fo + fc )
( fo – fc ) fc
Mixer

8. How it works ( fo – fc ) ( fo + fc ) Carrier Frequency
Oscillator Frequency
( fo – fc )
( fo + fc )
1 MHz
1.47 MHz
470 kHz
2.47 kHz
1.5 MHz
1.97 MHz
3.47 kHz