MC13027 Datasheet, PDF (18/28 Pages) Motorola, Inc – AMAX STEREO CHIPSET

MC13027 MC13122
CAR RADIO APPLICATION
Figure 6 shows a car radio circuit using a TOKO pre–tuned
RF module. The RF module includes a 4 diode tracking circuit
to eliminate mistracking between the oscillator and RF circuits
over the 530 to 1700 kHz AM band. This is important for stereo
performance because mistracking will cause mono distortion
and will significantly reduce the stereo separation. The
THB122 module contains the variable 10 kHz notch filter. This
module can be replaced with discrete components as shown
in Figure 8, using 1% resistors and 5% capacitors.
Some manufacturers add a PIN diode attenuator at the
antenna input. An example is shown in Figure 7.
The WB AGC sensitivity can be adjusted by changing R4
in series with the WB AGC input, Pin 1. The internal input
resistance is 15 k.
R15, R17 and R19 are the blanker timing resistors. They
were setup for this circuit and can be changed if desired.
FL1 is a linear phase IF filter . We recommend a Gaussian
(rounded) filter, such as SFG or SFH for lower distortion and
better separation than one with a flatter amplitude response.
The SFG types of filters have poorer selectivity than the ones
with flat GDT (group delay time) so some compromise has
been made on adjacent channel selectivity.
The blanker can be disabled for testing by grounding the
blanker AGC on Pin 2 in the MC13027.
The blanker and mixer inputs must be biased from the
4.0 V regulator through a low dc resistance like the
secondary winding of the RF coil.
The receiver VCO operates at 4 times the local oscillator
frequency and is divided internally in the MC13027 so that
both the mixer input and the LO out is the same as in other
receivers. This receiver can be connected to an existing
synthesizer. For AM stereo, the synthesizer must have low
phase noise. The Motorola MC145173 is recommended. For
bench testing of this receiver, the Motorola MC145151
parallel input synthesizer may be useful. It will operate on
9.0 V and the phase detector can provide tuning voltage
without a buffer amplifier.
The SS (stop–sense) output can be used for station
searching and scanning. The best way to use it is to connect
the SS signal to a comparator or A–D converter in the control
microprocessor. If Pin 23 is grounded during searching by
turning on Q3, the SS voltage changes from less than 0.5 V
to around 2.2 V when an RF threshold is exceeded, as is
shown in the graph in Figure 15. This system results in very
reliable stopping on usable signals and fast detection of AM
stereo signals. After a station is detected, Q3 should be
turned off.
This receiver is very easy to set up because the TOKO
module is pre–aligned. The only adjustments are to tune T1
and T2 for maximum voltage of the SS out line or maximum
audio with a weak signal. If desired, they can be changed
slightly to maximize stereo separation.
If different components are used, the blanker resistors can
be setup as follows:
Ground Pin 2 of the MC13027. Apply a 1.0 µs pulse or 50
Hz square wave of about 10 mV through a dummy antenna
and synchronize an oscilloscope to the pulse generator.
Observe the signal at the mixer collector (Pin 11). It should be
a sine wave burst. Remove the ground on Pin 2 and adjust
R15 so the burst is just suppressed. Check the performance
at the ends and middle of the band because the width might
change due to RF circuit bandwidth.
Mix the pulse signal with a CW signal of about 300 µV with
a power combiner and connect the oscilloscope to Pin 7 or
Pin 14 of the MC13122. Adjust R17 so the blanking starts at
the beginning of the audio pulse and R19 so the audio
blanking is just long enough to suppress the audio pulse. The
audio blanking time should not be made longer than
necessary because it will be more noticeable in the normal
program. The effectiveness of the blanker can be determined
in field testing by connecting a switch from Pin 2 of the
MC13027 to ground and bringing it outside the radio.
Figures 10 to 19 refer to the performance of the
Application Circuit of Figure 6.
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MOTOROLA ANALOG IC DEVICE DATA