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PRINCETON UNIVERSITY
Physics Department
PHYSICS 104 LAB
Week #12
EXPERIMENT XI
BUILD AND SIGNAL-TRACE AN AM RADIO CIRCUIT
Introduction. Like the transistor amplifier lab, this one is designed to go beyond the material
that you learn in lecture and class and to let you use your knowledge of electronic circuits to
construct an AM radio and understand how it works.
AM means amplitude modulation. This was not the first way that people communicated
by “wireless” (the first radio waves were called CW (continuous wave), an interrupted Morse
Code, high frequency sinusoidal electromagnetic wave), but the principles that you will see in
this lab underlie all radio, TV and optical communications. In AM, the audio frequency signal
(approximately 50 Hz to 20,000 Hz) which carries the sound information modulates the radio
frequency (RF) signal (the commercial AM band: 550 kHz to 1600 kHz) That is, the amplitude
of the RF wave is varied by the audio signal from the microphone in the transmitting station. So
the broadcast tower radiates an electromagnetic wave of the form:
A(t)cos(2fRFt)
where A(t) is an amplified version of the voltage out of the microphone. The audio signal itself
is too low in frequency to be radiated by the transmitter; the LC tuned circuit and the antenna
would have to be enormous. The low frequency (audio) information is superimposed on the RF
carrier wave whose frequency (1350 kHz in this lab) is much easier to radiate and receive. The
job of the AM receiver is to
1. Select the desired frequency (radio station).
2. Detect the RF signal. That is, use a diode to make the signal's average value non-zero.
3. Amplify the RF signal. (It is possible to amplify then detect, or to filter before amplifying, or
even to amplify more than once. We choose this order for clarity, performance, and to give
the most possible “test points” which you can probe on the scope.)
4. Filter out the RF part of the signal with a (guess what?) low-pass filter for clearer sound.
In previous labs this semester you have seen circuits which perform each of these functions. In
this lab we want you to put them together to get a working radio, then use the oscilloscope to
examine the signal at each stage. As usual you should make all voltage measurements with the
10x probe and all of them with respect to ground.
A Basic AM Radio Circuit. On the next page you will see a schematic drawing of the basic
radio circuit that you will breadboard and study. The four subsections are separated by dashed
lines. You should understand the function of each independently; that will make it easier to see
how the whole thing works. The tuning circuit in this radio consists of a variable capacitor (the
capacitor in the schematic with an arrow through it) and a variable inductor whose inductance
will change as you move a ferrite core in and out of it.
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Schematic for an AM Radio
Antenna and
tuner circuit
Diode
detector
Amplifier
Low-pass
Filter
Tuned circuit
+15 V
100k


F
A
B
W
L
Y
C
G
10k

1k

C
B
B
0.1F
C
D
2N2222
E
100

1k

E
0.1F
+
F
Amplifier
/ Speaker
Ground
NOTES:
1. Some signal levels are very low. You may need to use the extra
10X voltage gain available at the oscilloscope by pulling out the
center knob on the voltage scale control.
2. There are three physical time scales involved in this circuit: (a)
radio frequency -- ~ 1MHz, (b) audio frequencies -- ~ 1 kHz,
and (c) syllable and spoken word rates -- ~ 10 Hz. Be sure to
change the oscilloscope time scale widely enough to see all three.
of these as you look at the various test points, and record what you
see in your notebook.
The antenna and tuned circuit select the radio station, in this case by varying the inductance or
the capacitance of the parallel LC circuit. At resonance the LC circuit has a large impedance to
ground and a large signal thus appears at the transistor base; off resonance the impedance to
ground is small, and so is the signal to the base. This is what you want in order to exclude
signals from stations at nearby frequencies. The strongest AM station around Princeton is
WHWH at 1350 kHz.
What is the wavelength of its RF carrier signal?
For AM stations a long wire serves as an antenna. There should be one hooked to your LC
circuit. Use the scope to look for the signal at point A.
At maximum sensitivity of the voltage scale you might just be able to see the radio station signal.
The audio modulation appears on both the positive and negative peaks of the RF signal. At this
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point the radio signal has an average value of zero Volts; when the positive peaks get bigger, so
do the negative ones. In order to get the average value of the voltage to change synchronously
with the audio signal, we chop off the + (or the  ) half of the signal with a diode, it's called a
detector. The signal should look something like this before detection (shown on two very
different time bases):
RF
Audio
1 s
2 ms
You will need to hook up the RF amplifier and use its amplification in order to see the rectified
radio signal very well. The amplifier is quite similar to the one you studied last week. The main
difference is that the 100 “feedback” resistor has a high-frequency bypass capacitor across it.
This increases the gain of the amplifier at high frequencies. Since the audio signal is riding on
the RF, it also gets amplified.
The signal at point B should have a DC component of about 1 Voltthe transistor bias.
Now look at the signal at C. This should have a large DC component ( 10 V). To examine the
small AC part more carefully, switch to AC coupling on the scope and increase the scope’s
vertical sensitivity. Now you should see the amplitude modulated RF signal; on a good day with
a strong signal it should be about 1 Volt peak-to-peak.
At this point you should “peak up” the signal with the variable inductor and capacitor in your LC
tuning circuit, this is how you tune your radio.
What is the AC voltage gain of your amplifier; this is the voltage reading at D divided by the
voltage reading at A?
The signal at D should look like the one at C, but without a DC component. The coupling
capacitor takes care of that; remember that capacitors are open circuit to DC signals.
Finally, you see a low-pass filter. Can you explain why these values were chosen for the resistor
and the capacitor to make the filter? Look at the signal at point E. (You may have to increase
the scope sensitivity.)
What happened to the RF signal? If you haven't already done so, hook up the earphone and listen
to the station. Singing in the lab is allowed this week!
Draw the schematic diagram of the circuit in your notebook and draw the signals you measure at
each test point. Be sure to indicate the DC and AC voltage levels and the time base scale.
In your own words, describe how a radio works.