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TDA7338D
STEREO DECODER
TDA7338STEREO DECODER
INTEGRATED 19KHz SC NOTCH FILTER
FOR PILOT CANCELLATION
ON CHIP FILTER FOR PILOT DETECTOR
AND PLL
ADJUSTMENT FREE VOLTAGE CONTROL-
LED OSCILLATOR
AUTOMATIC PILOT DEPENDENT MONO/
STEREO SWITCHING
NOISE BLANKING WITH PROGRAMMABLE
THRESHOLD
HIGH CUT CONTROL AND STEREO BLEND
INTEGRATED HIGH PASS FILTER FOR IN-
TERFERENCE DETECTOR
LEVEL INPUT FOR ADDITIONAL SPIKE DE-
TECTION ON FIELDSTRENGHT SIGNAL
VERY HIGH SUPPRESSION OF HARMONIC
AND INTERFERENCE SIGNALS
DESCRIPTIONThe TDA7338 is a new concept of monolithic inte-
grated stereo decoder with noise blanking for FM
car radio applications.
With the used BICMOS technique, the 19KHz
Notch Filter, the PLL Filter and Phase Filter is re-
alized on the chip with a Switched Capacitor con-
cept.
Avoiding the use of multipliers and non linear cir-
cuits a very high performance in terms of noise
suppression and total harmonic distortion is
reached.
PIN CONNECTION (Top view)
ABSOLUTE MAXIMUM RATINGS
THERMAL DATA
ESDAll pins are protected against ESD according to the MIL883 standard.
BLOCK DIAGRAM AND TEST CIRCUIT
TDA73382/11
ELECTRICAL CHARACTERISTICS (VCC = 9V; modulation frequency: 1KHz; de-emphasis time:T = 50μs; nominal MPX input voltage: VMPX = 1.5VPP; m 100% (75KHz deviation, fmod = 1KHz);
RIN = 40.2kΩ, ROUT = 40.2kΩ; Tamb = 27°C; CREF = 4.7μF; unless otherwise specified)
CARRIER AND HARMONIC SUPPRESSION AT THE OUTPUT
INTERMODULATION (note 1)
TRAFFIC RADIO (note 2)
SCA - SUBSIDIARY COMMUNICATIONS AUTHORIZATION (note 3)
ACI - ADJACENT CHANNEL INTERFERENCE (note4)
MONO/ STEREO SWITCH
STEREO BLEND
TDA7338
ELECTRICAL CHARACTERISTICS (continued)HIGH CUT CONTROL
VCO
NOISE INTERFACE DETECTOR (test condition: VSB > VR + 50mV)
NOTES TO THE CHARACTERISTICS1 INTERMODULATION SUPPRESSION
α2 = VO (signal) (at1KHz)
VO (spurious) (at1KHZ) ; fs = (2 x 10KHz) - 19KHz
α3 = VO (signal) (at1KHz)
VO (spurious) (at1KHZ) ; fs = (3 x 13KHz) - 38KHz
measured with : 91% mono signal; 9% pilot signal; fm=10KHz or 13KHz
2. TRAFFIC RADIO (V.F.) suppression
α57 (V.W.F.) = VO(signal) (at1KHz)
VO (spurious) (at1KHZ ±23Hz)
measured with : 91% stereo signal; 9% pilot signal; fm=1KHz; 5% subcarrier
(f=57KHz, fm = 23Hz AM, m = 60%)
3. SCA (SUBSIDIARY COMMUNICATIONS AUTHORIZATION)
α67 = VO(signal) (at1KHz)
VO (spurious) (at9KHZ) ; fs = (2 x 38KHz) - 67KHz
measured with : 81% mono signal; 9% pilot signal; fm=1KHz;
10% SCA - subcarrier (fs = 67KHz, unmodulated)
TDA73384/11
FUNCTIONAL DESCRIPTION
Signal PathThe TDA7338 Stereodecoder contains all neces-
sary functions for processing the MPX signal. Due
to the external input resistance (Pin 20) the circuit
can be adapted to different MPX input levels. Be-
hind a 80kHz lowpass filter the adjustment free
PLL for the pilot Tone is placed. The only external
component needed for the PLL is the ceramic
resonator for the oscillator which runs at 456kHz.
The pilot detector output is designed as an open
collector output, therefore an external pullup re-
sistor is needed. To force the decoder to "MONO"
Pin 19 has to be clamped to a voltage below
0.8V.
The voltage level (signal strength from the IF part)
applied to Pin 15 (VHCC) allows to control the
time constant of the deemphasis (nom. = 50μs,
see fig. 1). If the RF-signal is weak, the corner
NOTES TO THE CHZARACTERISTICS (continued)4. ACI (ADJACENT CHANNEL INTERFERENCE)
α114 = VO (signal) (at1KHz)
VO (spurious) (at4KHZ) ; fs = 110KHz - (3 x 38KHz)
α190 = VO (signal) (at1KHz)
VO (spurious) (at4KHZ) ; fs = 186KHz - (5 x 38KHz)
measured with : 90% mono signal; 9% pilot signal; fm=1KHz; 1% spurious signal
(fs = 110KHz or 186KHz, unmodulated)
5. Control range typ 11% of VR (see figure 1 and figure2)
6. MEASUREMENT OF TRIGGER THRESHOLDS
All thresholds are measured by using a pulse
with TR = 2μs, THIGH = 2μs, and TF = 10μs.
The repetition rate must not increase the PEAK
voltage.
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 VHCC-VR(V)
(=50μs)
(KHz)
D95AU366
Figure 1: High Cut Control-0.40 -0.32 -0.24 -0.16 -0.08 VSB-VR(V)
SEP
(dB)
D95AU367
Figure 2: Stereo Blend
TDA7338
frequency is reduced down to 1kHz to improve
the signal to noise ratio. Furthermore the condi-
tions of the stereo separation (see fig.2) can be
controlled through the signal applied to Pin 16
(VSB). Both signal levels (VSB and VHCC) are
referred to Pin 17 (VR), with the characteristic
that the control range is 11% of VR.
By modifying the feedback resistor value of the
output stages (Pin 9 - 10, Pin 11 - 12) the total
gain of the stereodecoder can be modified.
Pin 7 and Pin 8 have an additional function. By
pulling them to ground the VCO-OFF (Pin 7) and
the MUTE (Pin 8) function are activated. The
MUTE signal disconnects the MPX-signal from
the circuit, while in combination with VCO-OFF
also the output buffers are disconnected from the
circuit. In this mode the output buffers can be
used for AM-stereo, cassette play back and other
purposes.
AM Mono ModeBy selecting VCO-OFF (Pin 7 to GND) the VCO
is switched off and the SB and HCC are disabled.
The deemphasis time constant is changed to
40μs (fc = 4KHz).
DESCRIPTION OF THE NOISE BLANKER
In the normal automotive environment the MPX
signal is disturbed by ignition spikes, motors and
high frequency switches etc.
The aim of the noise blanker part is to cancel the
influence of the spikes produced by these compo-
nents. Therefore the output of the stereodecoder
is switched off for a time of 40μs (average spike
duration).
In a first stage the spikes must be detected but to
avoid a wrong triggering on high frequency noise
a complex trigger control is implemented.
Behind the trigger stage a pulse former generates
the 40μs "blanking" pulse.
This duration of 40μs can be varied by changing
the capacitor at pin 7.
1.1 Trigger Path The incoming MPX signal is highpass-filtered,
amplified and rectified (block RECT-PEAK).
The second order highpass-filter has a corner-fre-
quency of 140KHz.
The rectifier signal, RECT, is used to generate by
peak-rectification a signal called PEAK, which is
available at the PEAK pin 8.
Also noise with a frequency >100KHz increases
the PEAK voltage. The value of the PEAK voltage
influences the trigger threshold voltage Vth (block
ATC). The higher the noise level the higher the
threshold.
Both signals, RECT and PEAK+Vth are fed to a
comparator (block PEAK-COMP) which outputs a
sawtooth-shaped waveform at the TBLANK pin 7. second comparator (block BLANK-COMP)
forms the internal blanking duration of 40μs.
The noise blanker is supplied by his own biasing
circuit (block BIAS-MONO) to avoid any cross talk
to the signal path (block BIAS-MONO).
1.2 Noise Controlled Threshold Adjustment
(ATC)The behaviour of the noise controlled threshold
adjustment is shown in fig. 4. It can be influenced
slightly by adding a resistor in parallel to the
PEAK capacitor at Pin 8 either to GND or VDD. A
resistor to GND will decrease the threshold
whereas a resistor to VDD will increase it. But it is
recommended to choose one of the internal
thresholds by use of the PROG pin (see table 1)
1.3 Automatic Threshold Control by the
Stereoblend voltage (ATC-SB)Besides the noise controlled threshold adjustment
there is an additional possibility for influencing the
trigger. It is controlled by the difference between
Vsb and Vr, similar to the Stereoblend. The rea-
son for implementing such a second control will
be explained in the following:
The point where the MPX signal starts to become
noisy is fixed by the RF part. Therefore also the
starting point of the normal noise controlled trig-
ger adjustment is fixed (fig.5). But in some cases
the behaviour of the noiseblanker can be im-
proved by increasing the threshold even in a re-
gion of higher fieldstrength, for the MPX signal
often shows distortion in this range, which leads
to an undesired triggering.
Because of the overlap of this range and the
range of the stereo/mono transition it can be con-
trolled by Vsb and Vr. This threshold increase is
programmable (see fig. 5).
1.4 Blend ModeAnother possibility to avoid a disturbing triggering
on modulation is to use the spikes on the field-
strength signal (LEVEL pin). But in the range of
higher fieldstrength the signal saturates and no
more spike detection is possible. For this reason
the TDA7338 offers the "BLEND MODE". When
"BLEND MODE" is activated a smooth transition
between the LEVEL- and the MPX-signal is used
to detect the spikes either on LEVEL or on MPX.
In the lower fieldstrength range mainly the
LEVEL-signal is used whereas in the higher
range mainly the MPX is used. This switching is
controlled also by the normal Stereoblend signal
to avoid additional pins.
"BLEND MODE OFF" is activated by connecting
the LEVEL pin to GND (LEVEL must be also con-
nected to GND if not used).
TDA73386/11
