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FM – TUNER
Stereo MPX decoder
SCA demodulator
Prof. Yosef PINHASI
The tuner is aimed at receiving commercial FM radio stations broadcasting at
the VHF band 88-108MHz. It enables detection of the frequency modulated
signals including decoding of stereophonic transmissions and also Subsidiary
Communications Authorization (SCA) signals.
The RF stage shown in Figure 1, consists of a broad-band low-noise amplifier
(LNA), based on the MPSH10 high frequency bipolar transistor, and followed
by the balanced mixer NE602 (or today – SA602). The fundamental
parameters of the front-end transistor MPSH10 of the LNA are
VVMHzf BET 6.0,650,60 ===β at collector current of mAIC 1= . The bias circuit
are the resistors Ω= 300CR and Ω=10ER at the collector and emitter
respectively and the feedback resistor Ω= KRF 47 between the collector and
base. The emitter current is found from the formula:
10.56mA
1
=++
+
−=
ECF
BECCE
RRR
VVI
β
The collector current is thus ( ) 10.38mA1/ =+⋅= ββ EC II and its voltage is
8.835V=⋅= ccC IRV . The small-signal parameters of the hybrid-∏ model at the
quiescent point are:
pFf
gC
gr
mSV
Ig
T
m
m
T
Cm
1022
145
415
=⋅
≅
Ω==
==
π
β
π
π
Substituting these parameters in the expression for the small-signal voltage
gain results in:
-121
11
||
=+
⋅
−
⋅
++
−==43421CF RR
CF
CF
F
Em
m
in
outV
RR
RR
RR
Zg
g
V
VA
π
The input impedance is calculating using:
( )( )[ ]
V
FEm
F
V
Emin
inin
A
RRZgZ
R
A
RZgZI
VZ
−⋅++=
−+
⋅++==
−
1||1
1
1
11
ππππ
+470
100K
1K 220K 560µ
.1µ .47µ
1n 1n
22K
10p
10p
47µ .1µ
24V
.1µ 100µ 6V
560
NE602
10
300
47K
100p
MPSH10
+12V
30p
30p
.1µ
10.7MHz
Antenna
L3 CV
100K
100K
Tuning
Figure 1: The RF front-end and mixer of the tuner.
The LNA is coupled to the following mixer IC SA602 via impedance
matching network composed of two pFCC 3021 == capacitors and a coil:
( )nH
CC
CCf
L 169
2
1
21
212
0
=
+⋅
=π
At MHzf 1000 = , the networks transforms the Ω= 300CR impedane to:
( ) Ω=⋅
+≅ 200,11
2
2
10 cout R
C
CjfZ
as of the input impedance of the SA602.
Figure 2: The matching network between the LNA and mixer.
The mixer circuit in the NE602 down converts the RF signal to an
intermediate frequency (IF) of 10.7MHz=IFf . The tuning of the local oscillator
frequency is voltage-controlled by a 100KΩ potentiometer. The VCO is built
in a Colpitts configuration with components as summarized in Table 1. The
oscillator frequency is calculated by:
( )( )
++
=
21
2132
1
CC
CCVCL
Vf
V
LO
π
The frequency of the received radio station is given by ( ) ( ) IFLORF fVfVf m= and
shown in Figure 3. The range of the tuning voltage should be set with the two
100KΩ trimmers to be between 5.5V to 19V.
C 1
C 2
L
R c = 300 Ω
Z out =1,200 Ω
Table 1: Components of the voltage-controlled oscillator .
Figure 3: Received signal frequency as a function of the tuning voltage.
The IF stage and detector is based on the CA3089 (or its equivalent
TDA1200) FM radio IF system. The electronic scheme of the IF circuit is
shown in Figure 4. Note that the differential 741 amplifier is provided for
an Automatic Frequency Control circuit is also shown but not used in this
application. A photo of the board of the RF front-end and the IF
demodulator is shown in Figure 5.
Figure 4: IF and FM quadrature detector.
Figure 5: The board of the RF front-end and IF stages.
The spectrum of the demodulated multiplexed (MPX) signal is shown in
Figure 5. It is composed of the monophonic L+R signal at base-band, the
19KHz pilot tone, the double-side band modulated L-R signal at 38KHz,
the Radio Data System (RDS) 1187.5BPS data at 57KHz and the
Subsidiary Communications Authorization (SCA) FM signal at 67KHz.
The decoding of the MPX stereophonic broadcast is carried out by the
MC1310 (or NTE801) FM stereo demodulator, and the SCA broadcast is
demodulated with the NE565 PLL, as illustrated in Figures 6 and 7.
The layout of the components is shown Figure 8.
19 15 38 23 53
L+R
L-R L-R
Pilot
Frequency [ KHz ]
0 57 67
SCA
RDS
Figure 5: The spectrum of the MPX signal of
the FM demodulated radio broadcast.
Figure 6: Stereo MPX decoder and SCA demodulator.
(a)
(b)
Figure 7: The MC1310 stereo MPX decoder system.
Figure 8: Inner look at the tuner layout.