ENGG1030 - Prac 2

LC AM Radio

Axon Blg. 47 - Room 506

(Proper shoes must be worn in the laboratory!)
Updated - 17 September, 2001
[Copyright] | [Modified] | [Duration] 

Goal

See the following topics: Back to Timetable Information

Copyright

This document was prepared by:

School of Computer Science and Electrical Engineering
The University of Queensland
St. Lucia Qld 4072 Australia

Tel: (07) 3365 4137
Fax: (07) 3365 4999
International: +61 7 3365 4999
email: pja@csee.uq.edu.au

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Modified

This document was last modified on 16 February 2001.

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Duration

2 Hours, no more.

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Introduction
There are a number of modulation techniques. All of these methods modulate a particular characteristic of the carrier waveform (usually at radio frequencies) with the signal information. Once this modulated waveform is received the signal is then extracted (or demodulated) from this waveform. In AM (or Amplitude Modulation) the amplitude of the carrier waveform is made to vary in proportion to the amplitude of the signal.

There are several ways to demodulate this waveform. In this experiment the simple method of Envelope Detection is used. In this method the waveform is rectified and passed through a low pass filter.  After removing the dc component with the coupling capacitor, (ideally) the original signal only, remains. (see Figure 1.below)

Figure 1. Envelope Detection.




 

AM broadcast radio operates around the 500 - 1700 kHz range. This means the receiver must be able to tune in to the carrier at these frequencies. Since the information we want is contained in only a narrow band of frequencies, the receiver needs to be able to reject other stations so that they do not interfere. Thus some sort of bandpass filter is required. In this experiment a LC Tank circuit is used for the frequency selection.
(See Figure 2. below)


Figure 2. LC Tank circuit
This introduces a selectivity metric defined as the "Q" of the circuit or it is fair to say the Q of the coil as for a RLC parallel circuit we can assume that the capacitor is lossless and the load R is very large and hence lossless.
The L is normally a coil with some series resistance which can not be considered lossless.
Q can be defined as 2*pi (Energy stored)/(Energy dissipated per cycle).
An alternative definition for Q is the ratio of reactance to resistance or woL/R.
Also Q can be defined as the ratio of resonant frequency to Band Width.





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Preparation

This should be discussed within your group as to who will do the following calculations.

Using the formula  f0 = 1/(2pi *[SQRT (LC)])
                Q   = wo L/Rs
                Q   =  wo/Band Width
where Q is the Quality factor and wo is the carrier frequency in radians. i.e wo =2pi*fo
With L= 0.5mH ; Rs =3ohms . The series resistance of the inductance will determine the Q of the tuned circuit and hence the bandwidth.
One would like a Rs   to be smaller than 3 ohms , but this may not be the case. In the experimental setup the "Tank circuit" has an adjustable L :which is adjusted by sliding a brass contact along the inductance coil . The contact resistance of this brass contact with the the coil may not be negligible. A larger R will give a larger bandwidth which is undesireable. A larger value of Q will give a narrower bandwidth and thus a more 'selective' tuned circuit which is desireable.

 (1) Calculate C for a f0 of 612 kHz 4QR and 1116kHz 4BC
 (2) Calculate the expected Q of our "tank circuit" for these two radio stations.
 (3) What is then the calculated Bandwidth of our "tank circuit"
 (4) Determine a necessary  Band Width of our tuned circuit.
   (So that other stations (see list in the Appendix) will not also be selected).
 (5) What value of Q results from this requirement.

The group discussion should decide
(a) if a "trimmer " capacitor of 20-60pF is of sufficient range to  tune in the available radio stations.
     See the list of Radio Stations for Brisbane in the Appendix.
 (b) if the selectivity of our radio ,the Q of our "tank circuit" is acceptable or not.

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Notes:to get you started.
Working as a group of two.   One person will do the  calculations as suggested by the preparation.
The other person will measure the inductance and capacitance available in the "Tank circuit", (which is the grey perspex box with a large obvious air cored inductor).To measure the L and C use the LCR meter in the lab.

Person One. Calculations.
You should supply the group with the following:
(i)  C(4QR) =             pF , where a pF =1E-12 Farads
(ii) C(4BC) =             pF
(iii)Q(4QR) =
(iv) Q(4BC) =
(v) BW(4QR) =           kHz
(vi) BW(4BC) =          kHz
(viii) Calc BW of our tank circuit =              kHz
(ix) Necessary BW of our tuned circuit =          kHz
(x) Q of tank circuit to meet this necessary condition =

Once you have finished the calculations consider what effect the Q will have on your attempt to "select" a radio station carrier frequency out of the spectrum of possible radio noise intercepted by the antenna of your radio receiver.
And also consider what the calculated bandwidth will mean in your attempt to tune in a single carrier frequency.
How could the Q be improved? Does it need to be increased or decreased?

Person Two. Measurements.
The LCR meter has two grey test leads with a red or black clip which should be connected to the component to be measured.  Ensure the RANGE is set to AUTO, the LEVEL set to 1, the FREQ set to 1kHz and the TRIG set to INT
To measure L
ensure the LCR meter FUNCTION is set to L and the CIRCUIT MODE is set to SER
Measure total L =            mH  . i.e. between L1 and L3
Then measure L1 to L2 , and or L2 to L3 to check that the Brass "dolly" contact with the wire turns on the former is OK. If there is a reading of 1999 this indicates an open circuit either at the 'dog clip' connections or the slider connection to the top of the coil. You may need to be aware of the possibility of a bad connection with the coil when you attempt to adjust the coil's inductance."Wiggle" the slider  to investigate the nature of the contact and the possibility of adjusting the inductance to say 0.25mH.
To measure C
ensure the LCR FUNCTION is set to C and the CIRCUIT MODE is set to PARA
then measure between C1 and C2 and or C2 and C3
Find minimum  C =                 pF
         maximum C  =                pF

Once the calculations and measurements have been completed, the group needs to answer Questions (a) and (b) from the preparation.
Firstly can our tank ciruit "tune" in the two radio sations? :4QR, at the low end of possible radio stations, and 4BC, at the high end of possible radio stations.
Both the L and C can be varied within limits. L from uH to our maximum of 0.55mH. For our C, extra capacitance can be added in parallel to the trimmer capacitance in the tank circuit. The stray capacitance of our circuit can only be minimised and may already be too large. You may need to give it consideration.

Keep in mind that the tank circuit has to be in "Resonance" at the carrier frequency of the selected radio station.
If the fo for our resonant circuit is too high then an increase in C and or L will decrease the resonant frequency.
If the fo for our resonant circuit is too low then a decrease in C and or l will increase the resonant frequency.

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Procedure

Note that there is a demonstration setup of this circuit at the 'front' of the lab. It works . So you should emulate it as closely as possible. Stray capacitances have to be minimised for our tuned circuit to be resonant at the required frequency;i.e., the carrier frequency of our selected radio station. If our our tuned circuit isn't 'tuned' then our radio signal along with all the other unwanted carrier signals will pass through our tank circuit to ground and our radio will  be receiving only noise.
0) Please examine the demonstartion circuit carefully.

1)

Construct the Envelope Detector circuit (as shown in Figure 3.) on the prototype board provided.
The audio amplifier is a separate module, pre-assembled in a box, that can be connected to your prototype
board. The schematic of the amplifier is only shown here for your interest.





The Tank Circuit Box  (tuned circuit) has terminals L1, L2, L3 and C1, C2, C3. Note that L2 is connected to the slider on top of the inductance coil, while C2 is the wiper of the capacitance. These terminals allow the inductance coil and the capacitor to be connected using short 2mm -4mm patch cords. Notice that on the prototype board the green 4mm terminal connector is connected to the Aluminium chassis and this should be used as the ground connection for your circuit. Connect a short piece of green hookup wire from the ground terminal to a horizontal bus line which becomes a ground node for your circuit. Connect one side of the Tuned circuit to the ground terminal using a green patch cord. The other side of the Tuned circuit should be connected along with the Antenna, ( a long piece of hookup wire), to a 4mm terminal on the Prototype board.
Try to keep the leads from the tuning coil to the board as short as is practical. Connect the tuned circuit to the anode of the Ge diode and the top node of the RC filter to the cathode of the diode. The coupling capacitor should be added now but the Audio Amplifier can be connected after you have observed the radio's respone to a AM signal , on the oscilloscope. That is after part 4. Note that the audio amplifier is already constructed and mounted in a plastic box.

2)
A HP Function Generator 33120A is to be used to tune the radio. Connect the output of the generator to a 4mm terminal on the prototype board close to the aerial terminal. Use a 4mm patch cord with one end connected to the function generator terminal and the lead wrapped around the aerial wire. This will capacitively couple a signal to the antenna without disturbing the tuned circuit.

3)
Determine which station you intend to receive. A list of available stations has been provided in Appendix B. However in the first instance either 4QR or 4BC should be selected.
You will be using a Tektronix TDS320 Digitising Oscilloscope and a HP 33120A Function Generator and copies of the instruction/user manuals are provided. However the tutor is available to help you use these instruments though the aim of the experiment is to start to become familiar with using these instruments.

   If the output of the demodulator does not pass through a maximum then the tank circuit does not reach resonance at the appropriate frequency. You must then determine the frequency at which your tank circuit is resonant by changing the frequency of the carrier until the demodulated output is a maximum. If the resonant frequency is lower then the radio station frequency, ( the desired frequency) , then reduce the inductance . Change the generator frequency back to the selected radio station frequency and adjust the capacitance from it's minimum to it's maximum and see if the output of the demodulator now passes through a maximum. Continue this process of adjusting the inductance/capacitance until your Tank circuit is tuneable.

4)

Reduce the volume so that others are not disturbed.


5)
Determine which station you have and try to tune in to others. Keep in mind that 4QR and 4BC should provide the best reception for our locality. After tuning these two stations try a station with an intermediate frequency. Finally choose the station with the best reception (the loudest) ie., 4BH 882 for nice musik.

Can you now explain why sometimes more than one station can be heard? How can this be improved?
Demonstrate the receivers operation to the Tutor.
This is were you generate the "mark" for your group/individual effort by discussing with the tutor your calculations and how the experiment supports your effort to select a radio frequency from what is generally considered as noise.
These next two parts, (6 and 7) , should be accepted as a challenge, to be completed if time permits and if attempted discuss your results with the tutor. (The tutor may readjust your mark).

6)
Again using the function generator at the appropriate carrier frequency but now with no modulation,
display the input waveform and the output of the tuned circuit on the CRO. By adjusting the frequency of
the carrier, determine the two frequencies that the received tone is 3dB below its peak amplitude and hence
determine the 3dB bandwidth of the receiver.

NOTE: 3dB relates to  Power and this is equivalent to 0.707 of the Voltage Gain.

Determine the Q of the tank circuit.  How does it agree with your calculated Q.
 

7)
Now that the radio is receiving your preferred radio station;  without moving the adjustment, remove the tuning coil from the circuit and use the LCR meter to determine its inductance. Replace the coil and add a 100 pF capacitor to the tuning circuit. Tune in to the same station. As before measure the inductance of the coil.
      From these measurements you should be able to estimate the stray capacitance of your circuit. Using this value calculate if it is possible to receive any stations. Confirm this by removing all the capacitors from the tuning circuit and attempt to tune in to a station.

      NOTE: the CRO lead has approx. 50 - 100 pF of capacitance thus you may need to reduce the amount of capacitance in the tuning circuit. (It may work with only the lead capacitance.)

Using the function generator at the appropriate carrier frequency and 50% modulated with a 1kHz tone tune the receiver so that the tone can be heard.

Display both waveforms on the CRO. Explain.
 
 

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Equipment 
located in the Electronics Laboratory Axon Bldg rm 506 Workstations ME1-ME10

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References

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