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Tutorial 4
EKT313 Electronic Communication Systems
EKT 313
PPKKP
TUTORIAL 4
Radio transmitters
1. Discuss the four basic requirements of a transmitter.
Every transmitter has four basic requirements:
1. It must generate a carrier signal of the correct frequency at a desired point in
the spectrum.
2. It must provide some form of modulation that causes the information signal to
modify the carrier signal.
3. It must provide sufficient power amplification to ensure that the signal level is
high enough to carry over the desired distance.
4. It must provide circuits that match the impedance of the power amplifier to that
of the antenna for maximum transfer of power.
2. What circuits are typically part of every radio transmitter?
Crystal oscillator or frequency synthesizer, buffer amplifier, frequency multiplier, power
amplifiers, modulator. Some transmitters may include a mixer.
3. What is the name given to the bias for a class C amplifier produced by an input RC
network?
Signal bias.
4. Why are crystal oscillators used instead of LC oscillators to set a transmitter frequency?
Crystal oscillators may be more precisely set to a specific frequency, but more important, the
frequency remains more constant and stable over time with variations in temperature, supply
voltage, and so on.
5. How is the output frequency of a frequency of a PLL synthesizer changed?
By changing the frequency division ratio.
6. What is the purpose of the loop filter in a PLL?
The loop filter smoothes the output of the phase detector into a varying direct current to
control the VCO frequency.
7. What is the most efficient class of RF power amplifier?
Class D or E.
8. What are parasitic and how are they eliminated in a power amplifier?
Parasitics are very-high-frequency oscillations unrelated to the operating frequency that can
occur in RF amplifiers because of stray inductances and capacitances. They are normally
eliminated by connecting a small value of resistor or parallel RL circuit in series with the base
or collector leads in the problem amplifier. A ferrite bead on the appropriate lead can also
solve the problem.
9. Compare the differences between class D and Class E amplifier.
Both are switching-type amplifiers, but class D uses two transistors and dual power supplies,
whereas the class E amplifier uses a single supply and transistor.
10. Explain how a feed forward power amplifier reduces distortion.
A feedforword power amplifier generates the amplified signal with and without distortion and
subtracts out the distortion(harmonics) before being sent to the output.
11. How is impedance matching handled in a broadband linear RF amplifier?
Transformers.
Sahadah ahmad
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Tutorial 4
EKT313 Electronic Communication Systems
PPKKP
12. Why are π and T networks are preferred over L networks?
Higher Q or Q selected for a specific bandwidth. Improved selectivity and minimized
harmonics.
13. An antenna will be connected to class C amplifier that has a supply voltage of 36
V and a collector current of 2.5 A. Calculate the RF output power if the efficiency of the
amplifier is 80% and find the turns ratio of the transformer to match 6 Ω amplifier
impedance with the 72 Ω antenna load. (72 W, 3.464)
14. An FM transmitter has an 8.6 MHz crystal carrier oscillator and frequency multipliers of
2,3 and 4. What is the output frequency? (206.4 MHz.)
15. A PLL frequency synthesizer has an ouput frequency of 162.7 MHz. The reference is a 1
MHz crstal oscillator followed by a divider of 10. What is the main frequency divider
ratio?( 1627)
16. Using the principles of PLL frequency synthesizers, create a synthesizer that will
generate all the AM radio broadcast frequencies from 1MHz to 10 MHz in 100 KHz
increments. This unit will be used as the carrier source in an AM broadcast transmitter.
(Refer to exercise given in the class)
17. Design an LCC T network that will match 5Ω internal resistance to a 52 Ω load at 54
MHz. Assume Q of 12.
L = 176.92 nH, C1 = 15.76 pF, C2 = 34.18 pF.
Sahadah ahmad
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Tutorial 4
EKT313 Electronic Communication Systems
PPKKP
Communication Receivers
1. Define the terms sensitivity and selectivity .
Sensitivity is the ability to provide amplification to recover the modulating signal
Selectivity is the ability to pick out the desired signal from others in the frequency spectrum
2. With the aid of the block diagram of a simple receiver, explain the basic superheterodyne
principle.
The principle of operation of the superheterodyne receiver depends on the use
of heterodyning or frequency mixing. The signal from the antenna is filtered sufficiently at
least to reject theimage frequency (see below) and possibly amplified. A local oscillator in the
receiver produces a sine wave which mixes with that signal, shifting it to a
specific intermediate frequency(IF), usually a lower frequency. The IF signal is itself filtered
and amplified and possibly processed in additional ways. The demodulator uses the IF signal
rather than the original radio frequency to recreate a copy of the original modulation (such as
audio).
Block diagram of a typical superheterodyne receiver.
The diagram shows the minimum requirements for a single-conversion superheterodyne
receiver design. The following essential elements are common to all superhet circuits: [6] a
receiving antenna, a tuned stage which may optionally contain amplification (RF amplifier), a
variable frequency local oscillator, a frequency mixer, a band pass filter and intermediate
frequency (IF) amplifer, and a demodulator plus additional circuitry to amplify or process the
original audio signal (or other transmitted information)
3. What are the advantages that the superheterodyne receiver has over the TRF receiver? Are there
any disadvantage?
The superheterodyne receiver offers superior sensitivity, frequency stability and selectivity.
Compared with the tuned radio frequency receiver (TRF) design, superhets offer better stability
because a tuneable oscillator is more easily realized than a tuneable amplifier. Operating at a
lower frequency, IF filters can give narrower passbands at the same Q factor than an equivalent
RF filter. A fixed IF also allows the use of a crystal filter[6] or similar technologies which cannot be
tuned.Regenerative and super-regenerative receivers offered a high sensitivity, but often suffer
from stability problems making them difficult to operate.
Although the advantages of the superhet design are overwhelming, we note a few drawbacks
which need to be tackled in practice.
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Tutorial 4
EKT313 Electronic Communication Systems
PPKKP
Image frequency (fimage)
One major disadvantage to the superheterodyne receiver is the problem of image frequency. In
heterodyne receivers, an image frequency is an undesired input frequency equal to the station
frequency plus twice the intermediate frequency. The image frequency results in two stations
being received at the same time, thus producing interference. Image frequencies can be
eliminated by sufficient attenuation on the incoming signal by the RF amplifier filter of the
superheterodyne receiver.
4. Most of the gain and selectivity in superheterodyne receiver is obtained in the IF amplifier.
Discuss the effect on receiver selectivity if a resistor is connected in parallel with the tuned
transformer
The bandwidth increases and selectivity decreases
5. A superheterodyne receiver must cover the frequency range from 220 to 224 MHz. The
first IF is 10.7 MHz and the second IF is 1.5 MHz. If frequency of a local oscillator is
higher than the input by the IF, calculate:
i)
Local oscillator tuning range.
ii)
The frequency of the second local oscillator
iii)
The first IF image frequency range.
i)
220+10.7 =230.7MHz
224+10.7 =234.7MHz
Tuning range=> 230.7 to 234.7 MHz
ii)
2nd LO = 1.5 MHz higher than the !st IF
Therefore freqof 2nd LO = 10.7 +1.5 =12.2 MHz
iii)
230.7 +10.7 = 241.4 MHz
234.7 +10.7 = 245.4 MHz :
The first IF image frequency range. => 241.4 – 245.4 MHZ
6. What is the difference between forward AGC and reverse AGC?
In forward AGC, increasing the collector current of an IF amplifier transistor decreases the
gain. In reverse AGC, increasing the collector current increases the gain.
7. Name the three main sources of selectivity for receivers implemented with ICs.
Crystal, ceramic, and SAW filters.
8. Design a receiver with no separate IF filter and using only single detector. Besides that
propose a low cost filter at the mixer output for selectivity in your design.
Refer to class’s exercise
Sahadah ahmad
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