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

β = 60 , fT = 650 MHz ,VBE = 6.0 V at collector current of IC = 1mA . The bias circuit are the resistors RC = 300 Ω and RE =10 Ω at the collector and emitter respectively and the feedback resistor RF = 47 KΩ between the collector and base. The emitter current is found from the formula:

V −V I = CC BE = 10.56mA E R F + R + R 1+ β C E

The collector current is thus IC = β ⋅ I E /(1+ β ) =10.38mA and its voltage is

VC = Rc ⋅ Ic = 8.835V . The small-signal parameters of the hybrid-∏ model at the quiescent point are:

IC gm = = 415 mS VT β rπ = = 145 Ω gm

gm Cπ ≅ = 102 pF 2π ⋅ fT

Substituting these parameters in the expression for the small-signal voltage gain results in:

    V g 1 R R A = out = − m −  ⋅ F C = -12 V V    R  R + R in 1 F 142F 43C 1+  gm +  ⋅ RE    R || R   Zπ   F C

The input impedance is calculating using:

−1 Vin  1 1− AV  RF Zin = =  +  = []Zπ + ()1+ gmZπ ⋅ RE || Iin Zπ + ()1+ gmZπ ⋅ RE RF  1− AV

470 560 + 100K

.1 µ 6V Tuning 100 µ 1n 1n 1K 220K 560 µ

100K 10p

24V CV L3 100K 10p 47 µ .1 µ .1 µ .47 µ 22K

NE602

+12V

300 30p

30p 10.7MHz 47K

MPSH10 100p

Antenna 10 .1 µ

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 C1 = C2 = 30 pF capacitors and a coil:

1 L = = 169 nH 2 C1C2 ()2πf0 ⋅ C1 + C2

At f0 = 100 MHz , the networks transforms the RC = 300 Ω impedane to:

2  C   1  Zout ()jf 0 ≅ 1+  ⋅ Rc = ,1 200 Ω  C2  as of the input impedance of the SA602.

C2

L Zout =1,200 Ω

C1 R c = 300 Ω

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 fIF = 10.7MHz . The tuning of the local oscillator frequency is voltage-controlled by a 100K Ω potentiometer. The VCO is built in a Colpitts configuration with components as summarized in Table 1. The oscillator frequency is calculated by:

1 fLO ()V =  C1C2  2π L3 CV ()V +   C1 + C2 

The frequency of the received radio station is given by fRF (V ) = fLO (V )m fIF and shown in Figure 3. The range of the tuning voltage should be set with the two 100K Ω trimmers to be between 5.5V to 19V.

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.

L+R Pilot RDS L-R L-R SCA

0 15 19 23 38 53 57 67 Frequency [ KHz ] 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.