Elenco AM Radio Kit - Manual (Page 20)

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DYNAMIC MEASUREMENTS
Connect the VOM and audio generator as shown in
Figure 17.Turn the power ON. Normally the AC gain is
measured at a frequency of 1 kilohertz (kHz). Your VOM,
however, may not be able to accurately read AC voltages
at this frequency. It is recommended, therefore, that this
test be performed at 400Hz. Set the audio generator at
400Hz and minimum voltage output. Set your VOM to
read an AC voltage of 1 volt at the output of your Audio
Amplifier (TP8). Slowly increase the output of the audio generator until the VOM reads 1 volt AC. Leave the audio
at this setting and move the positive lead of your VOM
to TP9. Record the AC voltage input to the amplifier
here: Vin=___________ volts. You may have to change
scales on your VOM for the most accurate reading. Turn
the power OFF. The AC voltage gain of your Audio
Amplifier is equal to the AC output voltage divided by the
AC input voltage, or 1/Vin. Your calculated AC Gain
should be approximately 30 - 50.
If an oscilloscope is not available, skip the following test and go directly to Section 2.
AC GAIN
Figure 17
Battery
Output Adjust
GND
TP10
10 μF
Generator
GND
TP10
Connect the oscilloscope and audio generator to your
circuit as shown in Figure 18.
Set the audio generator for a frequency of 1kHz and
minimum voltage output. Set the oscilloscope to read 0.5
volts per division. Turn the power ON and slowly
increase the generator output until the oscilloscope
displays 2 volts peak to peak (Vpp) at TP8. Move the
oscilloscope probe to TP9 and record the input voltage
here: Vin=___________ Vpp, (at this point you may
want to verify the AC Gain). Move the oscilloscope probe
back to TP8 and slowly increase the frequency from the
audio generator until the waveform on the oscilloscope
drops to 0.7 of its original reading, 1.4 Vpp or 2.8 divisions Use the oscilloscope probe to check TP9 to
make sure the input voltage did not change. The
frequency of the generator when the output drops to 0.7
of its original value is called the high frequency 3 decibel
(dB) corner.
Repeat this procedure by lowering the frequency from
the generator to obtain the low frequency 3dB corner.
Leave the oscilloscope connected to TP8 and turn the
power OFF. By subtracting the frequency of the low
corner from the frequency of the high corner, you
calculate the bandwidth of the Audio Amplifier. Your
bandwidth should be greater than 100kHz.
AC BANDWIDTH