FM Radio Essentials
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How the FM Radio Works
To aid the description of the circuit, we have split it into two parts. The first deals with the decoding of the FM radio
signal and the second deals with the audio amplification.
Radio decoding
For the radio circuit to work, it needs a stable power supply (not one that falls as the battery becomes flat). To
provide the stable power supply, a voltage regulator is used. This provides a 3V output for the circuit to run from.
Capacitor C17 is to remove any unwanted noise from this 3V supply.
The key component of the radio circuit is the TDA7010T chip. This chip is used to receive radio signals via an aerial
and then decodes this signal to recover the audio signal that it carries. To recover the radio signal the chip needs to
generate another radio signal by using a circuit called VCO (voltage controlled oscillator) which it mixes with the
received radio signal to recover the desired radio station. The signal that the VCO generates can be altered, allowing
radio stations on different frequencies to be recovered. The VCO uses a resonant circuit that is made up of inductor
(L1), capacitor (C16) and the varicap diode (D1). A varicap diode is a diode that also has a capacitance. This
capacitance varies as the voltage across the diode varies. By adjusting the potentiometer (R5), which is used to tune
the circuit, the voltage that is fed to the varicap diode can be adjusted. This, in turn, changes the capacitance of the
varicap diode and, therefore, the frequency of the resonant circuit (and thus the frequency to which the radio is
tuned).
The remaining capacitors are used by the chip to filter the recovered radio signal.
Resistor R2 is used to disable the mute feature of the chip. If this is not fitted when the radio is not tuned into a radio
station, the audio output is muted (silent). When it is fitted you get a hissy sound between stations. We have fitted
it as it makes it easier to manually tune into stations.
Potentiometer R1 is used to control the amount of the audio output signal that is fed to the amplifier circuit. By
doing this, it will therefore control the overall volume that comes out of the radio.
Battery
(15V max)
C17
470uF
Audio
out
C20
10nF
R6
4.7K
TDA7010T
Mute
Audio out
LF
V+
Voltage
regulator
In
Out
Gnd
VCO
Mix1
Mix2
Mix Out
CR
DM
Gnd
IF
Mix 3
RF in
IF
IF
R7
15K
R5
100K
Tune
R4
100K
D1
L1
56nH
C16
180pF
R1
10K
Volume
C12
10nF
C19
10nF
Aerial
R2
10K
C1
C2
C4
C3
C11
C10
C9
C8
C7
C6
C5
