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

RL

DetectorHead

phones

RL

DetectorHead

phonesRL

Detector Speaker

Un-tuned

Tuned Amplified

3

Tuned radio frequency(TRF) receiver

RL

Detector Speaker

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

LO

BPF DetectorBPF BPF Speaker

AGC

The local oscillator is tunable, so the intermediate frequency (IF) is fixed regardless of the signal frequency

5

Super-heterodyne receiver components

LO

Converter

BPF The BW of the filter can be fixed

Example:

Signal frequency: 540kHz, LO: 540 + 455 = 995 kHz

Signal frequency: 1700kHz, LO: 1700 + 455 = 2155 kHz

6

Optical-heterodyne receiver

See the PTL paper

7

Receiver block diagram

Detector Speaker

Autodyne converter

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. 6.11 envelope detector

15

SSBSC AM

LO

LPFSSB or DSBSC

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