1. CHAPTER 3:
AM RECEIVERS
1

2. Topics
• AM Demodulators
• Tuned Radio Frequency Receivers
• Superheterodyne Receivers
• RF Section and Characteristics
• Path and Frequency Changing
• Intermediate Frequency (IF) & IF Amplifier
• Detector and Automatic Gain Control (AGC)
2

3. AM Demodulator
3

4. Demodulator
• Definition:
– A demodulator is an electronic circuit used to recover the information
content from the carrier wave of a signal. The term is usually used in
connection with radio receivers, but there are many kinds of
demodulators used in many other systems. Another common one is in a
modem, which is a contraction of the terms modulator/demodulator.
– For AM, the most popular demodulator used are the Envelop Detector
and Product Detector.
RF IF AF
Section Demodulator Stage
Section
Figure 3.1 Receiver Block Diagram
4

5. AM DEMODULATOR
• Demodulation of DSBFC AM
– Simplest demodulator for DSBFC is envelop detector.
– The recovery of the baseband signal undergoes the process of rectifying
the incoming signal, remove half of the envelop, then use low pass filter
to remove the high frequency component of the signal.
– Major advantage of AM = ease of the demod process.
– No need for synchronous demodulator.
Figure 3.2
Envelope
detection of a
conventional
AM signal
5

6. AM Demodulation
• Demodulation of SSBSC AM
– For SSBSC, product detector is used to recover the signal.
– The product detector multiplies the incoming signal by the signal of a local
oscillator with the same frequency and phase as the carrier of the incoming
signal.
– After filtering, the original audio Product
signal will result. Detector
– This method will decode both AM AM or SSB
Audio
Out
and SSB, although if the phase Low Pass
cannot be determined a more Beat Filter
complex setup is required. Frequency (LPF)
Oscillator
(BFO)
Figure 3.3 Product Detector for AM and SSB
6

7. Demodulator Circuit
Diode Demodulator
• D1 rectifies the signal producing only positive result.
• The rectified signal will quickly charge C1.
• RC time constant of R1 and C1 is made long enough so
that C1 does not have to discharge before the next pulse is
received.
• Voltage across C1 follows the amplitude variation of carrier
signal, not the carrier signal itself.
Figure 3.4: Diode Demodulator
• Finally DC component is removed by C2.
Transistor Demodulator
• AM input is applied to Q1 base (common emitter).
• C1 is the coupling capacitor  block DC from the input
source.
• R1 and R2 establish base bias and R3 establish collector
bias.
• Transistor is biased-for-class B operation that allows
positive pulses on the output.
• C2 filter out carrier frequency. Figure 3.5: Transistor Demodulator
• C3 removes DC component. 7

8. AM RECEIVERS
8

9. Process in a Receiver
Antenna
Speaker
RF Audio
Detector
Amplification Amplification
Fig. 3.6 Simple block diagram of a receiver
1. Signal received at antenna is very low, need to amplify
(LNA) and tuned to desired freq. to avoid
interference.
2. Detector finds the info signal from the rf signal.
3. Further amplification needed to give it enough power
to drive a loudspeaker.
9

10. RECEIVER PARAMETERS
• Parameters used to evaluate the ability of a receiver to
successfully demodulate radio signal :-
– Selectivity
– Sensitivity
– Bandwidth Improvement Factor
– Dynamic Range
– Fidelity
– Insertion Loss
10

11. Selectivity
• Ability of a receiver to • Higher Q the narrower the
BW and the better the
accept a given band of selectivity.
frequency and reject all • i.e. using the bandwidth of
others. the receiver at the – 3dB
• Obtained using tuned points  not necessarily
show rejection characteristic
circuits. • Most common used two
• Selectivity Q, is given by: points; another at -60dB 
XL ratio of the two called shape
Q=
R factor: B( − 60 dB )
SF =
• The bandwidth curve B( − 3dB )
from the tuned circuit is: BW = fr
Q
11

12. Example 3.1
• High-Q tuned cct are used to keep the BW narrow to
ensure that only desired signal is passed. Assumed
that 10µH coil with resistance of 20Ω is connected in
parallel with 101.4pF variable capacitor. The circuit
resonates at what freq.?
• What is the inductive reactance?
• What is the selectivity of the cct?
• The bandwidth of the tuned cct?
• Find the upper and lower cutoff frequencies?
12

13. Answer Eg. 3.1
1. f = 1
= 5MHz
4. BW = f r = 318.47kHz
r
2π LC Q
2. X = 2π f L = 314Ω 5. One half on each side of
L r
center freq. of 5MHz is
318.47/2 = 0.159 MHz.
3. Q = X L = 15.7
R ∴
Upper, f 2 = 5 + 0.159 = 5.159 MHz
Lower, f1 = 5 − 0.159 = 4.841MHz
13

14. Sensitivity
• The minimum RF signal that can be detected at the input of a
receiver and still produce a usable demodulated info signal.
• Also called receiver threshold.
• Depends on the noise power present at the input of the receiver,
the receiver’s noise figure, sensitivity of the AM detector and the
bandwidth improvement factor of the receiver.
• The best way to improve sensitivity is by reducing the noise level
reduce temperature, reduce bandwidth of the receiver, or
improving receiving noise figure.
14

15. Bandwidth Improvement Factor
• One way of reducing the noise level is by reducing the bandwidth
of the signal
• There is limitation for reducing the bandwidth to make sure
information is not lost
• As RF bandwidth at the input of the receiver is higher than the IF
bandwidth at the output of the receiver, reducing the RF
bandwidth to IF bandwidth ratio effectively reducing the noise
figure of the receiver, thus reducing the noise
• Bandwidth improvement expressed mathematically as
BRF
BI =
BIF
• Noise figure improvement expressed as
NFimprovement = 10 log BI
15

16. Dynamic Range
• The minimum input level necessary to discern a signal
and the input that will overdrive the receiver and
produce distortion.
• Minimum receive level is a function of front-end noise,
noise figure and the desired signal quality.
• Input that produce distortion is a function of the net
gain of the receiver.
• 1 dB compression point is used for the upper limit for
usefulness.
16

17. FIGURE 3.7 Linear gain, 1-dB compression point, and third-order
intercept distortion for a typical amplifier
17

18. Fidelity
• A measure of the ability of the receiver to
produce, at the output of the receiver, an exact
replica of the original source information.
• Any amplitude, frequency or phase variations
present in the demodulated waveform that are
not included in the original signal are consider as
distortion.
18

19. Insertion Loss
• Loss occur when a signal enter the input of the
receiver.
• Parameters associated with the frequencies that
fall within the passband of a filter.
• Defined as the ratio of the power transferred to
the load with a filter in the circuit to the power
transferred to the load without a filter.
19

20. Tuned Radio Frequency Receiver
• Tuned RF Receiver (TRF) Antenna
– It is the earliest and simplest receiver design
(Fig. 3.8). D
E Recovered
Output
– TRF consist of RF amplifiers stages, detector M
O
and audio amplifier stages (Fig. 3.9) C L
D
U
L
– The received signal is tuned by LC circuit to a A
T
passband centered at carrier frequency. O
R
– Selectivity pass only the desired signal, others
are rejected.
Figure 3.8 Basic TRF receiver
– The tuned signal is boost up by an amplifier
block diagram, showing simple
for better info detection.
structure.
– Signal info detection is made at the
demodulator and further amplified for the
speaker output.
20

21. FIGURE 3.9 Noncoherent tuned radio frequency receiver block diagram
21

22. TRF cont…
– TRF has high sensitivity – ability to drive the speaker to an
acceptable level (to amplify).
– Disadvantages :-
• BW is inconsistent and varies with center frequency when
tuned over a wide range of input frequencies  selectivity
changes, (means the extent to which a rx can differentiate
between the desired signal and other signal).
• Instability due to the large number of RF amplifier all tuned to
the same center frequency  oscillation.
• Gain is not uniform over a wide range of frequency.
22

23. Superheterodyne Receiver
• Superhets was designed to overcome the problems in TRF.
• Complex circuitry compared to TRF but excellent
performance under many conditions (Fig. 3.10).
• Heterodyne mean:
– to mix 2 frequencies together in a nonlinear device or
– to translate one frequency to another using nonlinear device.
• Superhets concept:
– Rx tunes to desired signal and converts the signal to intermediate
frequency via a signal mixing circuit.
– Then IF signal is optimized to fully recovered the modulated info
signal.
23

24. FIGURE 3.10 AM superheterodyne receiver block diagram
24

25. Stages in Superhets
• RF Stage:
– Which takes the signal from the antenna and amplifies it to a level large
enough to be used in the following stages.
• Mixer and Local Oscillator:
– Converts the RF signal to IF signal.
• IF Stage:
– Further amplifies the signal and has bandwidth and passband shaping
appropriate for the received signal.
• Detector Stage:
– Recovers (demodulates) the info signal from the carrier.
• AF Stage:
– The received signal is amplified for loudspeaker or interconnections to
comm systems.
25

26. RF Stage and
Characteristics
26

27. RF Stage and Characteristics
• The RF section is a tunable circuit connected to the antenna.
• It is where the wanted signal is selected and the unwanted signal
is rejected.
• Some basic receiver does not have amplifier but for rx that has
one is much more superior in performance.
• The main advantage having RF amplifiers are:
– Greater gain – better sensitivity
– Improved image frequency
– Improve SNR
• Two main characteristic of RF stage are:
– Sensitivity – ability to amplify weak signals
– Selectivity – ability to reject unwanted signals
27

28. Path and Frequency
Changing
28

29. Path & Frequency Changing
• Converter / Mixer (Fig. 3.11)
– RF is down converted to IF, but shape of the envelope remains the same
 info is conserved, bandwidth is unchanged.
– Output of the mixer : infinite no. of harmonic and cross product
including fRF, fLO, fRF + fLO , fRF – fLO.
– LO is designed so that its frequency is always above or below the desired
RF carrier by an amount equal to IF center frequency.
– fLO is usually higher than fRF because up conversion leads to a smaller
tuning range (smaller ratio of the maximum to minimum tuning
frequency)  much easier to design an oscillator that is tunable over a
smaller frequency ratio.
– If mixer and LO are in a single stage, it is called converter.
– Common IF : 455 kHz.
– Adequate selectivity because it is difficult to design sharp band bass filter
if the center frequency is very high.
– Center frequency is fixed and factory-tuned  effectively suppressed
because of its high selectivity.
29

30. fi,fo
fi
fo + f i
fo fo – fi or fi - fo
Mixer
Tuned circuit or
filter
Figure 3.11: Mixer input - output
30

31. Intermediate Frequency
&
IF Amplifiers
31

32. IF & IF Amplifiers
• Intermediate Frequency
– Sum or difference in the output of a mixer that enters the IF
stage.
– A down-converted frequency that carries the information.
• IF amplifiers
– One or more stage(s).
– Provide most gain and selectivity.
– IF is much lower than RF  easier to design and good
sensitivity is easier to obtain with tuned circuit.
32

33. • Image Frequency & Rejection
– It is formed after the mixer circuitry.
– It is an image of the input frequency IF IF
that enters the mixer.
– Represented in two form: high side
injection and low side injection. fi f LO f image
– The image is an equal distance from
the LO frequency on the other side Fig. 3.12 High-side Injection
of it from the signal.
– An image must be rejected prior to
mixing, because it’s indistinguishable
and impossible to filter out.
IF IF
For high side :
f image f LO fi
f image = f i + 2 f IF
For low side : Fig. 3.13 Low-side Injection
f image = f i − 2 f IF 33

34. • Image Frequency Rejection Ratio
– Is defined as the ratio of voltage gain at the input
frequency to which the receiver is tuned to gain the
image frequency.
– Numerical measure of the preselector ability to reject
the image frequency.
The Image Rejection, IR, α = 1 + Q 2 ρ 2
f im f RF fsi fs
where The rejection ratio ρ = − = −
f RF f im fs fi
Q = Quality factor of tuned circuit
X
= L = f where B = bandwidth
R B
IR(dB) = 20 log α
34

35. Example 3.2
Determine the image frequency for a standard broadcast band receiver
using 455-kHz IF and tuned to station at 620 kHz.
The first is determine the frequency of the LO
The LO frequency minus the desired station’s frequency of 620 kHz should equal the
IF of 455 KHz
Hence,
fLO – 620 kHz = 455 kHz
fLO = 620 KHz + 455 kHz
fLO = 1075 kHz
Now determine what other frequency, when mixed with 1075 kHz, yields an output
component at 455 kHz
X – 1075 kHz = 455 kHz
X = 1075 kHz + 455 kHz
X = 1530 kHz
Thus, 1530 is the image frequency in this situation. To solve the problem associated
with image frequency, sometimes a technique known as double conversion is
employed.
35

36. Detector And Automatic
Gain Controller
36

37. • Detector/Demodulator
– to recover the original signal
– eg : diode detector
• Audio amplifier
– to amplify the detected audio signal to be passed to the user
• Automatic Gain Control
– A dc level proportional to the received signal’s strength is
extracted from the detector stage and fed back to the IF and
sometimes to the mixer and/or the RF amplifier.
– This is the automatic gain control (AGC) level, which allows
relatively constant receiver output for widely variable received
signals.
37

38. Cont..
– The AGC help to maintain a constant output voltage level over a
wide range of RF input signal levels
– Without AGC, to not miss a weak station, you would probably
blow out your speaker while a weak station may not be audible.
– The received signal from the tuned station is constantly changing
as a result of changing weather and atmospheric conditions.
– The AGC allows you to listen to a station without constantly
monitoring the volume control.
38

39. Figure 3.14 Automatic Gain Control
39