MPX stereo and SCA FM tuner

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.