1. Receivers Receivers perform the inverse operations to transmitter
Two important specifications are fundamental to all receivers:
Sensitivity: the signal strength required to achieve a given signal-to-noise ratio
Selectivity: the ability to reject unwanted signals at frequencies different from that of the desired signal

2. Receiver topologies Three receiver configurations Head phones DetectorRL
Un-tuned
Head phones
Detector
Detector
Speaker RL RL
Tuned
Amplified

3. Tuned radio frequency(TRF) receiver
Detector
Speaker RL
B = f / Q
Q = XL/ R
Thus the bandwidth varies with the square root of frequency
A 10kHz bandwidth would become 17kHz when tuning from 540kHz to 1700kHz

4. Super-heterodyne receiver
To solve the problem of varied bandwidth, the super-heterodyne receiver is proposed
BPF
BPF
BPF
Detector
Speaker
AGC LO
The local oscillator is tunable, so the intermediate frequency (IF) is fixed regardless of the signal frequency

5. Super-heterodyne receiver components
Converter LO
The BW of the filter can be fixed
BPF
Example:
Signal frequency: 540kHz, LO: = 995 kHz
Signal frequency: 1700kHz, LO: = 2155 kHz

6. Optical-heterodyne receiver
See the PTL paper

7. Receiver block diagram
Autodyne converter
Detector
Speaker
AGC
The mixer and LO use a single transistor in a configuration called an autodyne converter

8. High side and low side injection
High side injection: the LO is tuned so that it generates a frequency that is always 455kHz higher than the incoming signal frequency
Low side injection: the LO is always 455kHz lower than the signal frequency. Percentage wise, the LO has to have a wider tuning range

9. Receiver characteristics
Sensitivity: The ability to receive weak signals with an acceptable signal-to-noise ratio (S/N) is called sensitivity. It is expressed in terms of the voltage or power the the antenna necessary to achieve a specified SNR
One common specification for AM receivers is the signal strength required for a 10dB (S+N)/N ratio

10. Selectivity
The ability to discriminate against interfering signals is known as selectivity
The ratio between the B-60dB and B-6dB is called the shape factor
Adjacent channel rejection
Alternate channel rejection
B-6dB
B-60dB

11. Distortion
Harmonic distortion – when the frequencies generated are multiples of those in the original modulating signal
Inter-modulation distortion – when frequency components mix in a nonlinear device
Un-even frequency response
Phase distortion – irregular phase shift rather than linear phase shift that increases with frequency

12. Dynamic range The response to weak signal is usually limited by noise
Signals that are too strong will overload one more more stages
The ratio between these two signals levels is the dynamic range of the receiver
In many receivers the dynamic range is actually the AGC range

13. Spurious responses Image frequencies
Signal LO
Image
Image LO
Signal
High side injection
Low side injection
An image must be rejected prior to mixing

14. Demodulators Full-carrier AM: envelope detector
Out
Fig envelope detector

15. SSBSC AM V0 = sin(ωct) x cos(ωc + ωm)t
SSB or DSBSC
LPF LO
V0 = sin(ωct) x cos(ωc + ωm)t
= 0.5 [sin(- ωm t) + sin(2 ωc + ωm)t]

16. Impact of frequency offset
For SSB, the impact is relatively small
For DSBSC, the offset may cause significant degradation of the the signal
Would be a good study to try out

17. FM demodulation kd = Vo / δ kd is the detector sensitivity
Vo is the output voltage
δ is the frequency deviation
S-curve of the FM detector

18. Slope detector
The simplest FM demodulator is a slope detector f