lab2 am fm radio autumn2009
TRANSCRIPT
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 1/18
Autumn 2009: Radio Electronics (TSEK-02) 1/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Date: ________ Student Name: _________________________ Lab Supervisor: ___________
Personal Number:- Signature: _______________
LAB-2
Measurements on AM / FM Radio
Receiver Building Blocks
Prepared by
Rashad.M.Ramzan
Note: The gain values in above block diagram are typical values.
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 2/18
Autumn 2009: Radio Electronics (TSEK-02) 2/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
1. Objectives
• To understand how the radio broadcast AM/FM receivers work.
• To understand the RF receiver building blocks; their functionality and tradeoffs.
• To familiarize with basic RF measurement techniques and instrumentation likeoscilloscope, spectrum analyzer and RF signal generator.
2. Instructions
2.1. You should read the LAB manual carefully and answer all questions labeled as PE
(Preparatory Exercise) before starting the lab. If you fail to do this, it might not be possible for you to complete the lab assignments within the scheduled hours.
2.2. Spectrum Analyzer in the LAB should be used with extreme care; it must not be
used without DC blocker (already connected to RF out) and must not be exposed to
power levels higher than 30dBm. Failing to do so might permanently damage theequipment!
3. Equipment Used
The ElencoTM
Superhet 108 AM/FM radio receiver board
Power Supply: SN16A
Oscilloscope: HP 54600B
Spectrum analyzer: R&S FSL or R&S FS315 or HP 8562E
Signal generator: R&S SM300 or R&S SMB100A or HP E4432B
DVM (Digital Voltmeter), cables and connectors.
4. Introduction:
In this lab you will investigate characteristics and the behaviour of functional blocks used
in traditional AM and FM receivers. You will also learn how to use the RF measurementequipment and how to carry out the measurements on radio circuits.
The function of the broadcast radio receiver is to recover the audio signal that was
modulated onto the RF carrier at the radio station, and apply it to the speaker, reproducing
the sounds of the announcer. There are various ways to combine the carrier frequency andthe audio signal together. This process is called modulation. The most commonly used
modulation methods are amplitude modulation (AM), frequency modulation (FM), and
phase modulation (PM). Also digital signals can be modulated onto radio frequency carrier.When the signal is transmitted, there is a number of impairments that degrade the signal on
the way until it gets to the receiver. We can broadly categorize those impairments as
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 3/18
Autumn 2009: Radio Electronics (TSEK-02) 3/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
channel impairments and impairments in transmitter and receiver. The typical impairments
are: The absorption of radio signal power, air and surrounding environment absorb signal.
Signal reflections caused by the ground and obstacles, signal detected in the receiver is
a sum of direct and reflected waves which can cause an effect known as fading. Co-channel interference, distant radio transmitters on the same or close to frequency of
interest will disturb the reception of a neighbouring station. Image-channel interference which deteriorates the receiver sensitivity. Non-linearities and saturation effects in different transmitter and receiver blocks give
rise to intermodulation distortion.
Noise, background noise in receiver, thermal noise generated by the receiver electronics
itself, atmospheric noise (burst noises from thunder storms and similar) and industrialnoise (RF noise from electronics, sparks)
The radio receiver ElencoTM Superhet 108 AM/FM to be investigated in this lab is shownin Fig-1. To facilitate measurements it is mostly built of discrete components loosely
spaced on the board.
Fig-1: AM/FM radio receiver
The receiver is a “superheterodyne” of the standard AM and FM broadcast frequencies. Its
block diagram with depicted 9 sections is shown in Fig-2. The receiver sections can be
characterized as follows. Section-1, the Audio Amplifier Stage, is used to increase the power of the audio signal
received from either AM or FM detector to a power level capable of driving the 8Ω
speaker.
Section-2 includes the AM detector circuit and the AGC (automatic gain control) stage.
The AM detector converts the amplitude modulated IF (intermediate frequency) signal
to audio signal. The AGC stage feeds back a DC voltage to the first AM IF amplifier in
order to maintain a near constant level of audio at the detector.
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 4/18
Autumn 2009: Radio Electronics (TSEK-02) 4/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Fig-2: Block diagram of AM/FM radio receiver
Section-3 is the second AM IF amplifier. The second AM IF amplifier is tuned to 455
kHz and has a fixed gain of 50 at this centre frequency.
Section-4 is the first AM IF amplifier which has a variable gain that depends on theAGC voltage received from the AGC stage. The first AM IF amplifier is also tuned to
455 kHz.
Section-5 includes the AM mixer, AM oscillator and AM antenna stages. When the
radio wave passes through the antenna, it induces a small voltage across the antennacoil. This voltage is coupled to the mixer and is down-converted to the IF frequency of
455 kHz. This change is accomplished by mixing the radio frequency signal with the
oscillator signal.
Section-6 is the FM ratio detector. In the AM detector section we observed that the
audio was detected from changes in the amplitude of the incoming signal. In FM
detection, the audio is detected from changes in frequency of the incoming signal. The
so called ratio detector is used here, which has a built-in limiting circuit to limit thesignal amplitude so that noise at the carrier will be minimized. The FM ratio detector
has a fixed gain of about 20.
Section-7 is the 2nd FM IF amplifier. This amplifier is tuned to 10.7 MHz and has afixed voltage gain of approximately 20. The 3dB bandwidth of this stage is
approximately 350 kHz.
Section-8 is the 1st FM IF amplifier. This amplifier is also tuned to 10.7 MHz with 3dB
bandwidth of 350 kHz and has a voltage gain of approximately 10.
Section-9 includes the FM mixer, FM oscillator, FM RF amplifier, AFC (Automatic
Frequency Control) stage and FM antenna. The incoming radio signals are amplified by
the FM RF amplifier, which is tuned to a desired radio station in the FM frequency
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 5/18
Autumn 2009: Radio Electronics (TSEK-02) 5/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
bandwidth of 88 MHz to 108 MHz. These amplified signals are then coupled to the FM
mixer stage to be down-converted to an IF of 10.7 MHz. This change, as in AM, isaccomplished by mixing radio frequency signal with the oscillator signal. The AFC
stage feeds back a DC voltage to the FM oscillator to minimize the oscillator frequency
drift.
Preparatory Exercise
• PE: What portions of the frequency spectrum are allocated for typical AM and FM broadcasts?
___________________________________________________________________
___________________________________________________________________
• PE: Why is the IF fixed the superhet receiver, when we tune to the certain radio
station by tuning its RF stage?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
• PE: Draw a simplified diagram of the ratio detector to illustrate its working
principle by a phasor diagram.
Hint : Refer to R. Blake pp. 247-248 or http://tpub.com/neets/book12/51d.htm
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 6/18
Autumn 2009: Radio Electronics (TSEK-02) 6/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
• PE: Draw a simplified diagram of the AM envelope detector and illustrate its
working principle in the time domain.
PE: Harmonic distortion is defined as distortion components which are integer
multiples of the fundamental signal frequency (i.e. they are harmonically related to
the fundamental frequency). Symmetric distortion is usually represented by odd
harmonics while asymmetric distortion instead corresponds to both even- and oddharmonics. Draw a tone signal with typical symmetrical distortion (caused by
clipping) in the time domain and the corresponding spectrum. Similarly consider
asymmetrical distortion (e.g. one side clipping).
Hint : You can refer to http://hyperphysics.phy-astr.gsu.edu/hbase/audio/amp.html)and click Harmonic Distortion inside the page. For more confidence on the
asymmetric case you can also support your answer by Matlab simulations.
Asymmetric Distortion Symmetric Distortion
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 7/18
Autumn 2009: Radio Electronics (TSEK-02) 7/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
• PE: Define selectivity of a radio receiver. What does it depend on ?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
• PE: Define sensitivity of a radio receiver. What does it depend on ?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
• Describe the tradeoff between selectivity and sensitivity in a typical superhetreceiver.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
• PE: A strong and a weak radio transmitter are transmitting very close on frequencyscale. If you want to listen to the to weak radio station with good fidelity, whatenhancement will you recommend for present radio receiver design?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 8/18
Autumn 2009: Radio Electronics (TSEK-02) 8/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Part-I: AM Radio Receiver
Power and current measurement of AM receiver (use DVM)
Fix the supply voltage to 9V and measure the current and calculate the power.
Supply current (receiver off) ___________ mA
Supply current (with moderate volume, FM/AM switch on AM position)
__________ mA Calculate Power = _____________________ mW
4.1. Audio Ampli fier Gain and BW (Section-1)
The purpose of the audio amplifier is to increase the signal power to the level sufficient to
drive the 8Ω speakers. LM386 is a general-purpose audio amplifier with two pins provided for external gain control.
Fig-3: Test setup for audio amplifier measurements
Gain Measurement
Power up the AM/FM radio receiver board with 9V, 0.1A current limited supply.
Set AM/FM switch to FM position and use the LF output of signal generator forgain measurement.
Setup the circuit as shown in the Fig-3. Keep the radio volume knob at
approximately middle position.
Set the signal generator frequency at 1 kHz and voltage at 20mV (RMS).
(VRMS = V pp/2√2)
Connect the signal generator output to TP2 and oscilloscope CH1 to jumper J3
(Input of amplifier).
Connect oscilloscope CH2 to audio amplifier output point TP1 (output of
amplifier).
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 9/18
Autumn 2009: Radio Electronics (TSEK-02) 9/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Record the input and output voltage levels (Please note that gain varies with
volume control).
Voltage Gain = _______= ________
Bandwidth Measurement
Use LF output of signal generator for BW measurement.
Set the signal generator frequency at 10 kHz and voltage at 20mV (RMS).
Connect the signal generator output to TP2.
Connect oscilloscope to monitor input and output signals at TP2 and TP1
respectively.
Change the volume control so that output voltage is 2 V pp.
Now, slowly decrease the frequency to lower 3dB corner frequency so that output
is approx. 0.7 x 2 V pp = 1.4 V pp , note down:
f low,3dB = ________
Now slowly increase the frequency to higher 3dB corner frequency so that output
is 0.7 x 2 V pp = 1.4 V pp , note down:
f high,3dB = ________
Bandwidth = f high,3dB - f low,3dB = _________
4.2. AM Detector and AGC (Section-2)
The purpose of the detector is to change the amplitude modulated IF signal to an audio
signal. First, the amplitude modulated IF signal is applied to a diode in such a way as toleave only the negative portion of that signal (see Fig-6). When the diode is in
conduction, it will force the capacitors C33 and C38 to charge to approximately the same
voltage as the negative peak of the IF signal.
Fig-6: AM detector and AGC stage
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 10/18
Autumn 2009: Radio Electronics (TSEK-02) 10/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
After conduction stops in the diode, the capacitors will discharge through resistors R36
and R42. The discharge time constant must be small enough to follow the audio signalotherwise high frequency audio distortion will occur. The discharge time constant must be
large enough, however, to remove the intermediate frequency (455 kHz) and leave only
the audio as shown in Fig-6.
The purpose of the automatic gain control (AGC) circuit is to maintain a constant level atthe detector, regardless of the strength of the incoming signal. Without AGC, the volume
control would have to be adjusted for each station and even moderately strong stations
would saturate the 2nd
IF amplifier causing audio distortion. AGC is accomplished by
adjusting the DC bias of the first IF amplifier to lower its gain as the signal strengthincreases. There is negative DC bias present with audio signal. This negative bias
corresponds to the strength of the incoming signal. The large signal results in the more
negative the DC component. At test point five (TP5), the audio is removed by a low passfilter, R36 and C32, leaving only the DC component. Resistor R35 is used to shift the
voltage at TP5 high enough to bias the base of transistor Q8 to the full gain position when
no signal is present. Resistors R35 and R36 also forward bias diode D4 just enough to
minimize “On Condition” threshold voltage.
AM Detector Bandwidth Measurement
Connect the circuit as shown in Fig-7 and set the switch to AM position.
Set the signal generator for AM at 455 kHz, 80% modulation, 1 kHz and 300 mV
(RMS) EMF.
Connect the RF output of the signal generator to TP3 via 0.001uF.
Connect the CH1 of oscilloscope to TP3
Connect the CH2 of oscilloscope to TP2. Set the volume of radio receiver at comfortably audible level.
Increase/decrease the signal generator amplitude until output on oscilloscope CH2
(demodulated signal) is approximately 200 mV peak, make sure that it’s free ofdistortion.
Leave the frequency of signal generator at 455 kHz.
Increase the modulation frequency of 1 kHz until output drops to approx.
200mV x 0.7 (3dB) = 140 mV
f high,3dB = ________
3 dB BW of AM detector = f high,3dB = _________
Connect the spectrum analyzer to TP3 and observe AM spectrum for 455 kHz,80% modulation, 1 kHz and 300 mV (RMS) EMF.
Identify the carrier and sidebands.
Change the depth of modulation between 10% and 100% and observe the effect
both on the oscilloscope and spectrum analyzer.
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 11/18
Autumn 2009: Radio Electronics (TSEK-02) 11/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Fig-7: AM detector bandwidth measurement setup
4.3. 1st IF Amplifier Gain Measurement (Section-3 and 4)
The operation of the 1st IF amplifier is the same as the second IF amplifier with one
important difference. The gain of the first IF amplifier decreases after the AGC threshold iscrossed to keep the audio output constant at the detector and prevent overload of the second
IF amplifier. This is accomplished by making the voltage on the base of transistor Q8 lower
as the signal strength increases. Since the voltage from base to emitter is fairly constant, the
drop in voltage at the base produces a similar drop in voltage at the emitter of Q8. Thisdrop lowers the voltage across R37 and thus, reduces the DC current through R37. Since all
of the DC current from the emitter of Q8 must go through R37, the DC current in Q8 is
therefore lowered. When the DC current in a transistor is lowered, its effective emitterresistance increases. The AC gain of transistor Q8 is equal to the AC collector load of Q8
divided by its effective emitter resistance. Raising the value of the effective emitter
resistance, thus, lowers the AC gain of Q8.
Gain and Bandwidth Measurement
Set the volume of radio receiver at minimum level.
Connect the circuit as shown in Fig-8 and short TP3 to R38 as shown.
Set the signal generator at 455 kHz, no modulation, 60 mV (RMS) EMF approx.and connect RF output to TP6 via 0.001uF capacitor.
Connect oscilloscope to TP4 for output monitoring. The oscilloscope must have
probe capacitance of 12 pF or less to avoid loading and detuning of IF AMP.
Change the signal generator amplitude so that the output is 4 VPP.
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 12/18
Autumn 2009: Radio Electronics (TSEK-02) 12/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Fig-8: Test setup for 1st IF amplifier stage
Record the input voltage (base of Q8) and output voltage (TP4).
Voltage Gain = _______ = ________
Change the Signal Generator frequency and record the 3 dB bandwidth
(4VPP x 0.7 = 2.82VPP)
Bandwidth = f high,3dB - f low,3dB = ________-________ = __________
Remove the short circuit and observe what happens to the output voltage at TP4(AGC effect)?
4.4. AM Mixer and Oscil lator (Section-5)
In a superheterodyne receiver, the radio waves at the antenna are amplified and then mixed
with the local oscillator to produce the intermediate frequency (IF). Transistor Q7 amplifiesthe RF signal, and simultaneously oscillates at a frequency 455 kHz above the desired radio
station frequency. So the RF amplifier, local oscillator, and mixer are in one circuit here.
To make the oscillator a positive feedback from the collector to the emitter of Q7 is
provided by coil L5 and capacitor C31. During the mixing process the following fourfrequencies are present at the collector of Q7.
1. The local oscillator frequency, LO.
2. The RF carrier or radio station frequency, RF.
3. The sum of these two frequencies, LO + RF.
4. The difference of these two frequencies, LO – RF.
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 13/18
Autumn 2009: Radio Electronics (TSEK-02) 13/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
The “difference frequency” is used as the intermediate frequency in AM radios. The
collector of Q7 also contains an IF transformer (T6) tuned to the difference frequency. This
transformer rejects all frequencies except those near 455 kHz. T6 also couples the 455 kHzsignal to the base of Q8 to be processed by the IF amplifiers. The antenna and the oscillator
coils are the only two resonant circuits that change when the radio is tuned for different
stations. Since a radio station may exist 455 kHz above the oscillator frequency, it isimportant that the antenna filter rejects this station and selects only the station 455 kHz
below the oscillator frequency that we refer to as the image problem. If the selectivity ofthe antenna (Q factor) is high, the image will be reduced sufficiently. The oscillator circuitmust also change when the radio is tuned in order to remain 455 kHz above the tuning of
the desired radio station. The degree of accuracy in keeping the oscillator frequency exactly
455 kHz above the tuning of the antenna is called tracking accuracy.
Measurement of Tracking Range of Local Oscillator
Connect the circuit as shown in Fig-9.
Connect the Oscilloscope to the collector of Q7.
Fig-9: Test setup for measurement of Phase noise and Tracking Range
Turn the Tuning Knob from one extreme to another extreme and note the oscillator
frequency.
f min ___________ , f max ____________.
Tracking Range: _________________________
Calculate the AM frequency band reception possible for an IF of 455 kHz.
f min - 455 kHz = ___________ , f max - 455 kHz = ____________
Connect the spectrum analyzer at R33 and observe the spectrum of the local
oscillator (LO) signal and its harmonics.
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 14/18
Autumn 2009: Radio Electronics (TSEK-02) 14/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Down-Conversion
Set the signal generator for AM at 1 MHz, 80% modulation, 5 kHz and 60 mV(RMS) EMF and connect RF output to TP7.
Connect the oscilloscope to the collector of Q7 to observe the oscillator output.
Turn the Tuning Knob to adjust the frequency of oscillator to 1.455 MHz whichafter mixing with the AM signal (from signal generator) produces an IF of 455kHz.
Connect the spectrum analyzer to R33 and observe and identify the signal
components between:
900 kHz to 1.1 MHz: _________________________
1.4 MHz to 1.5 MHz: _________________________
400 kHz to 500 kHz: _________________________
5. Part-II: FM Radio Receiver
The architecture of FM radio receiver is same as AM in Part-I. The fundamental difference
is the type of the demodulator (detector) used to detect the base-band signal. In AM
receiver the AGC was used to keep the signal power constant for a certain station at theinput of the detector.
In FM receiver AFC (Automatic Frequency Control) is used to keep the FM oscillator
frequency stable and to avoid drift with time. The signal of interest in engraved infrequency of modulated signal instead of amplitude. So amplitude variations which are in
certain bound will have no bad effect on quality of FM reception.We will describe the FM ratio detector which is used here. The remaining FM receive
stages are very similar to those of the AM receiver.
Fig-10: FM Radio Receiver
Power and current measurement of FM receiver (use DVM)
Fix the supply voltage to 9V and measure the current and calculate the power.
Supply current (receiver off) ___________ mA
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 15/18
Autumn 2009: Radio Electronics (TSEK-02) 15/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Supply current (with moderate volume, FM/AM switch on FM position)
__________ mA Calculate Power = _____________________ mW
FM Ratio Detector
In FM detection, the audio is detected from changes in frequency of the incoming
signal. The ratio detector has built-in limiting action which limits the signal so that
any noise riding on the FM carrier will be minimized. When an incoming signal is
present at T4 and T5, a current flows through D2, D3, R26, R27, and a largecapacitor C25 which is a bypass for R28. For the IF signal R26 and R27 are
practically connected to ground (C24 is like a short circuit). At no modulation the
circuit is in resonance, and the currents through the diodes D2 and D3 are equal because T5 is center tapped and the voltages on the secondary of T5 are
perpendicular to the IF voltage across the secondary of T4 (also referred to ground
by C23 acting as a short). Thus, no current is drawn through C23 resulting in zeroaudio output voltage.
Fig-11: FM Ratio Detector
When the incoming signal is modulated, its frequency is different from theresonant frequency and the current through one diode will be larger than the other
because the voltages across the secondary of T5 are not any more perpendicular to
the voltage across the secondary of T4. Then, like in the Foster-Seeley
discriminator the driving voltage for one diode branch is different from the voltage
applied to the other. This causes a current to flow in C23 which will produce anaudio voltage across C23 (the same current flows through C24 as well). If themodulation is of opposite direction than before, more current will flow in the other
diode, which will again cause current to flow in C23 but in opposite direction
resulting in an audio voltage being produced across C23. The ratio detector isdecoupled by the resistor R23 and capacitor C21.
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 16/18
Autumn 2009: Radio Electronics (TSEK-02) 16/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Bandwidth Measurement of Ratio Detector
Connect the circuit as shown in Fig-12.
Set the AM/FM switch to FM position.
Set the volume of radio receiver at comfortable audio level.
Set the generator for FM at 10.7 MHz modulated at 1 kHz, 22.5 kHz deviationwith minimum voltage out (EMF ∼ 20mV (RMS)).
Connect the signal generator RF output to base of Q6 (Input of the ratio detector).
Connect the oscilloscope to TP2 to observe the output of detector.
Slowly increase the amplitude of the generator until a 1 kHz sine wave is
observable on the scope.
Connect the spectrum analyzer to base of Q6 to observe the FM spectrum as well.
Fig-12: Test Setup for FM Ratio Detector (Scope to TP2)
Increase/Decrease the deviation frequency, what happens at the output and why?
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
For 22.5 kHz deviation change the amplitude of FM signal so that the detector
output is 400 mVPP.
Increase the modulation frequency and record the 3 dB bandwidth
(400 mVPP x 0.7 = 282.8 mVPP)
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 17/18
Autumn 2009: Radio Electronics (TSEK-02) 17/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden
Bandwidth = f high,3dB = __________
Measurement of Tracking Range of Local Oscillator
Connect the spectrum analyzer to the emitter of Q3.
Set the start and stop frequency for spectrum analyzer to 70 MHz and 130 MHz,respectively.
Set the marker setting for spectrum analyzer to auto maximum peak .
Rotate the Tuning Knob from one extreme to another extreme and note theminimum and maximum oscillator frequency.
f min ___________ , f max ____________.
Tracking Range: _________________________
Down-Convertion
Set the signal generator for FM at 100 MHz modulated at 1 kHz, 22.5 kHzdeviation and 60 mV (RMS) EMF and connect RF output to TP13.
Connect the spectrum analyzer to the emitter of Q3 to observe the oscillator
output.
Turn the Tuning Knob to adjust the frequency of oscillator to 110.7 MHz which
after mixing with the FM signal (from signal generator) produces an IF of 10.7MHz.
Observe the spectrum and identify different signals between 1.0 MHz and 130
MHz at:
100 MHz: _________________________
110.7 MHz: _________________________
10.7 MHz: _________________________
Congratulations! You have completed the LAB.
8/13/2019 LAB2 AM FM Radio Autumn2009
http://slidepdf.com/reader/full/lab2-am-fm-radio-autumn2009 18/18
Autumn 2009: Radio Electronics (TSEK-02) 18/18
Electrical Engineering Department (ISY) Linköping University, Linköping Sweden