1. 1
2.9.2.1 : Tuned Radio Frequency Receiver (TRF)
 Block diagram of 3-stages TRF receiver that includes an RF stage, a detector
stage and an audio stage :
 Two or three RF amplifiers are required to filter and amplify the received signal to a
level sufficient to drive the detector stage.
 The detector converts RF signals directly to information.
 An audio stage amplifies the information signals to a usable level
 Simple and have a relatively high sensitivity

2. 2
2.9.2.1 : Tuned Radio Frequency Receiver (TRF)
 3 distinct disadvantages :
1. The bandwidth is inconsistent and varies with the center frequency when tuned over a
wide range of input frequencies.
2. Instability due to large number of RF amplifiers all tuned to the same center
frequency
3. The gains are not uniform over a very wide frequency range.

3. 3
2.9.2.2 : Superheterodyne Receiver
 Heterodyne – to mix two frequencies together in a nonlinear device or to
translate one frequency to another using nonlinear mixing.
 Block diagram of superheterodyne receiver :

4. 4
2.9.2.2 : Superheterodyne Receiver
 1. RF section
 Consists of a pre-selector and an amplifier
 RF amplifier determines the sensitivity of the receiver and a
predominant factor in determining the noise figure for the
receiver.
 2. Mixer/converter section
 Consists of a radio-frequency oscillator and a mixer.
 Choice of oscillator depends on the stability and accuracy desired.
 The shape of the envelope, the bandwidth and the original
information contained in the envelope remains unchanged
although the carrier and sideband frequencies are translated from
RF to IF.

5. 5
2.9.2.2 : Superheterodyne Receiver
 3. IF section
 Consists of a series of IF amplifiers and bandpass filters to
achieve most of the receiver gain and selectivity.
 4. Detector section
 To convert the IF signals back to the original source information
(demodulation).

6. 6
2.9.2.2 : Superheterodyne Receiver
 5. Audio amplifier section

7. 7
2.9.3 : Receiver Operation
2.9.3.1 : Frequency Conversion
 Frequency conversion in the mixer stage is identical to the frequency
conversion in the modulator except that in the receiver, the frequencies are
down-converted rather that up-converted.
 In the mixer, RF signals are combined with the local oscillator frequency
 Therefore the difference of RF and oscillator frequency is always equal to the IF
frequency
 The adjustment for the center frequency of the pre-selector and the local oscillator
frequency are gang-tune (the two adjustments are tied together so that single
adjustment will change the center frequency of the pre-selector and at the same time
change the local oscillator)

8. 8
2.9.3.2 : Frequency Conversion
 Illustration of the frequency conversion process for an AM broadcast-band
superheterodyne receiver using high side injection :

9. 9
2.9.3.2 : Frequency Conversion
 Ex 5-3

10. 10
2.9.3.3 : Local oscillator tracking
 Local oscillator tracking – the ability of the local oscillator in a receiver to
oscillate either above or below the selected radio frequency carrier by an
amount equal to the intermediate frequency throughout the entire radio
frequency band.

11. 11
2.9.3.4 : Image frequency
 Image frequency – any frequency other than the selected radio frequency
carrier that will produce a cross-product frequency that is equal to the
intermediate frequency if allowed to enter a receiver and mix with the local
oscillator.
 It is equivalent to a second radio frequency that will produce an IF that will
interfere with the IF from the desired radio frequency.

12. 12
2.9.3.4 : Image frequency
 The following figure shows the relative frequency spectrum for the RF, IF,
local oscillator and image frequencies for a superheterodyne receiver using
high side injection.
 For a radio frequency to produce a cross product equal to IF, it must be displaced
from local oscillator frequency by a value equal to the IF.
 With high side injection, the selected RF is below the local oscillator by amount
equal to the IF.
 Therefore, the image frequency is the radio frequency that is located in the IF
frequency above the local oscillator as shown above.
(35)
IFRFIFloim fffff 2+=+=

13. 13
2.9.3.5 : Image frequency rejection ratio
 Image frequency rejection ratio (IFRR) – a numerical measure of the ability of
a pre-selector to reject the image frequency
 Mathematically expressed as,
(36)
where ρ= (fim/fRF) – (fRF/fim)
Q = quality factor of a pre-selector
22
1 ρQIFRR +=

14. 14
 Occurs when a receiver picks up the same station at
two nearby points on the receiver tuning dial. One
point is the desired location, and the other point is
called the spurious point.
Double spotting

15. 15
2.9.3.5 : Image frequency rejection ratio
 For an AM broadcast-band superheterodyne
receiver with IF, RF, and local oscillator frequencies
of 455 kHz, 600 kHz, and 1055 kHz, respectively.
Determine:
a. Image Frequency
b. IFRR for a preselector Q of 100

16. 16
2.9.4 : Double Conversion Receivers
 For good image rejection, relatively high IF is desired. However, for a high
gain selective amplifiers that are stable, a low IF is necessary.
 The 1st IF is a relatively high frequency for good image rejection.
 The 2nd IF is a relatively low frequency for good selectivity and easy amplification.

17. Inductive Coupling
 Inductive or transformer coupling is the most
common techniques used for coupling IF
amplifiers.
17

18. Automatic Gain Control Circuits
 AGC circuits compensates for minor
variations in the received RF signal level.
18

19. Squelch Circuits
 The purpose of a squelch circuit is to quiet a
receiver in the absence of a received signal.
19

20. 20
2.9.5 : Net Receiver Gain
 Net receiver gain is simply the ratio of the demodulator signal level at the
output of the receiver to the RF signal level at the input to the receiver.
 In essence, net receiver gain is the dB sum of all gains to the receiver minus the
dB sum of all losses.
 Gains and losses found in a typical radio receiver :

21. RADIO-WAVE PROPAGATION

22. Electromagnetic Waves
An electromagnetic wave propagating through space
consists of electric and magnetic fields, perpendicular
both to each other and to the direction of travel of the
wave.
The fields vary together, both in time and in space,
and there is a definite ratio between the electric field
intensity and the magnetic field intensity.

23. Power Density
The amount of power that flows through each square
meter of a surface perpendicular to the direction of
travel.

24. Plane and Spherical Waves
Conceptually, the simplest source of electromagnetic
waves would be a point in space. Waves would radiate
equally from this source in all directions.
A wavefront is a surface on which all the waves
have the same phase, would be the surface of the
sphere. Such a source, called an isotropic radiator.
Polarization
The polarization of a plane wave is simply the
direction of its electric field vector.

25. Free-Space Propagation
Free-space Path Loss
It is often defined as the loss incurred by an
electromagnetic wave as it propagates in a straight line
through a vacuum with no absorption or reflection of
energy from nearby objects.

26. Transmitting Antenna Gain
Until now, we have been assuming an isotropic
antenna, that is, one that radiates equally in all
directions. Many practical antennas are designed to
radiate more power in some directions than others.
They are said to have gain in those directions in which
the most power is radiated.

27. Effective Isotropic Radiated Power
(EIRP)
is the equivalent power that an isotropic antenna
would have to radiate to achieve the same power
density in the chosen direction at a given point as
another antenna.

28. Receiving Antenna Gain
The power extracted from a wave by a receiving
antenna ought to depend both on its physical size and
on its gain.

29. Reflection
Electromagnetic reflection occurs when an incident
wave strikes a boundary of two media and some or all
of the incident power does not enter the second
material.

31. Refraction
is sometimes referred to as the bending of the radio-
wave path. However, the ray does not actually bend.
Electromagnetic refraction is actually the changing of
direction of an electromagnetic ray as it passes
obliquely from one medium to another with different
velocities of propagation.

34. Diffraction
is defined as the modulation or redistribution of
energy within a wavefront when it passes near the edge
of an opaque object.
Huygens’s Principle states that every point on a given
spherical wavefront can be considered as a secondary
point source of electromagnetic waves from which other
secondary wave (wavelets) are radiated outward.