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Integrated Circuits and Systems http://www.ics.isy.liu.se/en/
TSEK02 – Radio Electronics LAB 2: RADIO SYSTEM MEASUREMENTS By Fahad Qazi and Morteza Abbasi, 2013 2014-10 2015-12
Updated by Duong Quoc Tai (
[email protected]) Updated by Ted Johansson (
[email protected])
Electronic Devices Laboratory (EK)
TSEK02 Radio Electronics, HT2 2015
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Introduction In this lab you will be introduced to basic RF measurement techniques and instrumentation like oscilloscope, spectrum analyzer and RF signal generator and characterize some building blocks of an AM receiver. Please note that the objective of this lab is not to understand how the circuits operate or how they are designed but rather to be able to measure their performance. You can gain more in depth knowledge about circuits in other courses. Performing measurements with delicate instruments requires a certain level of responsibility and courtesy. We are certain that you will follow the instructions of this lab carefully and ask for assistance in case something is not clear to you.
Equipment Used
The ElencoTM Superhet 108 AM/FM radio receiver board
Power Supply: SN16A
Oscilloscope: HP 54600B
Spectrum analyzer: R&S FS300
Signal generator: HP E4432B
DVM (Digital Voltmeter), cables and connectors.
The Spectrum Analyzer 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 30 dBm. Failing to do so might permanently damage the equipment!
Please do not bring any food or beverages with you into the lab!
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Background 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. The radio receiver ElencoTM Superhet 108 AM/FM to be investigated in this lab is shown in Fig. 1. To facilitate measurements it is mostly built of discrete components loosely spaced on the board. The board consists of one FM and one AM receiver. Its block diagram with depicted nine sections is shown in Fig. 2.
Fig. 1. AM/FM radio receiver.
Fig. 2. Block Diagram of the FM and AM radio receivers on the board.
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In this lab we will only characterize the AM receiver which is highlighted in Fig. 2.
Section 1, the Audio Amplifier Stage, is used to increase the power of the audio signal received from either the AM or FM detector to a power level capable of driving the 8 Ω speaker.
Section 2 includes the AM detector circuit and the AGC (automatic gain control) stage. The AM de tector 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 con stant level of audio at the detector.
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 center frequency.
Section 4 is the first AM IF amplifier, which has a variable gain that depends on the AGC 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 sensed by the antenna, it induces a small voltage across the antenna coil. This voltage is coupled to the mixer and is down-converted to the IF frequency of 455 kHz by mixing the radio frequency signal with the oscillator signal.
Section 6, 7, 8, and 9 are FM radio blocks. We will not measure those blocks in this lab.
On the last page of this lab manual, you find the detailed schematics of the full PCB for reference. The AM part is the left (lower) part of the schematics.
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1. Measurement Tasks 1. DC Power Consumption Power and current measurement of the AM receiver: use a 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
calculated power
= _____________________ mW
2. Audio Amplifier Gain and BW (Section 1) The purpose of the audio amplifier is to increase the signal power to the level sufficient to drive the 8 speakers.
Fig. 3. Test setup for audio amplifier measurements
2.1 Gain Measurement Power up the AM/FM radio receiver board with 9 V, 0.1 A current limited supply. Set the AM/FM switch to AM position and use the LF output of the signal generator for gain 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 20 mV (RMS). (VRMS = Vpp/{2√2}) TSEK02 Radio Electronics, HT2 2015
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Connect the signal generator output to TP2, and the oscilloscope CH1 to jumper J3 (input of the amplifier). Connect oscilloscope CH2 to the audio amplifier output point TP1 (output of the amplifier). Record the input and output voltage levels (please note that gain varies with volume control). Voltage Gain = _______- ________ 2.2 Bandwidth Measurement
Use the LF output of the 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 the oscilloscope to monitor the input and output signals at TP2 and TP1, respec tively.
Change the volume control so that the output voltage is 2 V pp.
Now, slowly decrease the frequency to the lower 3 dB corner frequency so that the output is approx. 0.7 x 2 Vpp = 1.4 Vpp , note down: flow,3dB = ________
Use the RF output of the signal generator (replace the LF output).
Set the signal generator frequency (RF) to 100 kHz and voltage at 20 mV.
Connect the oscilloscope to monitor the input and output signals at TP2 and TP1, respectively.
Change the volume control so that the output voltage is 2 V pp.
Now slowly increase the frequency to the higher 3dB corner frequency so that the output is approx. 0.7 x 2 Vpp = 1.4 Vpp , note down: fhigh,3dB = ________ Bandwidth = fhigh,3dB - flow,3dB = _________
3. AM Detector and AGC (Section 2) 3.1 AM Detector Bandwidth Measurement
Connect the circuit as shown in Fig. 4 and set the switch to the AM position.
Set the signal generator for AM at 455 kHz, 80 % modulation, 1 kHz, and 300 mV (RMS)
Connect the RF output of the signal generator to TP3 via a 0.001 uF capacitor.
Connect the CH1 of oscilloscope to TP3.
Connect the CH2 of oscilloscope to TP2.
Set the volume of radio receiver at comfortably audible level. TSEK02 Radio Electronics, HT2 2015
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Increase/decrease the signal generator amplitude until the output on the oscilloscope CH2 (de modulated signal) is approximately 200 mV peak, make sure that it is free of distortion.
Leave the frequency of the signal generator at 455 kHz.
Increase the modulation frequency of 1 kHz until the output drops to approx. 200 mV x 0.7 (3dB) = 140 mV 3 dB BW of AM detector = fhigh,3dB = _________
Connect the spectrum analyzer to TP3 and observe the 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.
Fig. 4. AM detector bandwidth measurement setup
4. 1st IF Amplifier (Section 3 and 4) Gain and Bandwidth Measurement
Set the volume of the radio receiver at minimum level.
Connect the circuit as shown in Fig. 5 and short TP3 to R38 as shown.
Set the signal generator at 455 kHz, no modulation, 60 mV (RMS) approx. and connect the RF output to TP6 via a 0.001uF capacitor.
Connect the 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 V PP.
Record the input voltage (base of Q8) and output voltage (TP4). Voltage Gain = _______ - ________
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Change the Signal Generator frequency and record the 3 dB bandwidth (4VPP x 0.7 = 2.82VPP) Bandwidth = fhigh,3dB - flow,3dB = ________-________ = __________
Remove the short circuit and observe what happens to the output voltage at TP4 (AGC effect)?
Fig. 5. Test setup for 1st IF amplifier stage
5. AM Mixer and Oscillator (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 amplifies the RF signal, and simultaneously oscillates at a frequency 455 kHz (AM IF frequency) above the desired radio station frequency. So the RF amplifier, local oscillator, and mixer are in one circuit here. During the mixing process the following four frequencies 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. 5.1 Measurement of Tracking Range of Local Oscillator Connect the circuit as shown in Fig. 6. Connect the Oscilloscope to the collector of Q7. Turn the Tuning Knob from one extreme to another extreme and note the oscillator frequency. fmin ___________ ,
fmax ____________.
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Tracking Range: _________________________ Calculate the AM frequency band reception possible for an IF of 455 kHz. fmin - 455 kHz = ___________ ,
fmax - 455 kHz = ____________
Connect the spectrum analyzer at R33 and observe the spectrum of the local oscillator (LO) signal and its harmonics.
Fig. 6: Test setup for measurement of Tracking Range
5.2 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.
Turn the Tuning Knob to adjust the frequency of oscillator to 1.455 MHz which after mix ing with the AM signal (from signal generator) produces an IF of 455 kHz.
Connect the spectrum analyzer to R33 and observe and identify the signal components be tween: 900 kHz to 1.1 MHz: _________________________ 1.4 MHz to 1.5 MHz: _________________________ 400 kHz to 500 kHz: _________________________
You have successfully passed the lab! Date: Name of the lab assistant:
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TSEK02 Radio Electronics, HT2 2015
