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MC13028A Datasheet PDF : 20 Pages
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MC13028A
The LA1832 tuner IC (U2) is set for AM operation by
switch S2 connecting Pin 12 to ground. An AM Stereo signal
source is applied to Pin 2 of the RF coil contained within the
BL–70 tuning block. That coil applies the signal to Pin 21 of
U2. The L.O. coil is connected from Pin 23 to VCC. The
secondary is tuned by a varactor which is controlled by a dc
voltage output from the synthesizer circuit. The reactance of
this oscillator tank is coupled back to Pin 23. It is through this
reactance that the frequency of the L.O. is determined. A
buffered output from the L.O. emerges at Pin 24. This signal
is routed to Pin 1 of the synthesizer (U3), thus completing the
frequency control loop.
The mixer output at Pin 2 is applied to the IF coil T1. Coil
T1 provides the correct impedance to drive the ceramic
bandpass filter F1. The IF signal returns to U2 through
Pin 4, and also to the input, Pin 4 of the AM Stereo decoder
(U1). The ceramic filter F1 is designed to operate into a
load resistance of 2.0 kΩ. This load is provided at Pin 4 of
U2.
The stereo outputs exit from Pins 15 and 16 of U1. The
design amplitudes of the audio outputs will vary according to
the values used for the resistors to ground at Pins 15 and 16
of the decoder, (labeled RO in the Electrical Characteristics
Table and the Test Circuit on page 2 and 3, and in Figure 1,
and called R2 and R3 in Figure 18). While the values chosen
for RO are left to the discretion of the designer, the numbers
chosen in this data sheet are reflective of those required to
set the general industry standard levels of audio outputs in
receiver designs.
Pins 15 and 16 are also good locations for the insertion of
simple RC filters that are used to comply with the United
States NRSC requirement for the shape of the overall
receiver audio response. The following curve, Figure 20,
shows the response of this U.S. standard.
Figure 20. NRSC De–Emphasis Curve
for the United States
0
– 2.0
– 4.0
– 6.0
– 8.0
–10
100 200
500 1.0 k 2.0 k
f, FREQUENCY (Hz)
5.0 k 10 k
There are many design factors that affect the shape of the
receiver response, and they must all be considered when
trying to approximate the NRSC de–emphasis response. The
mixer output transformer (IF coil, T1), and ceramic filter
probably have the greatest contribution to the frequency
response. The ceramic filter can be tailored from its rated
response by the choice of transformer impedance and
bandwidth. When designing an overall audio response
shape, the response of the speakers or earphones should
also be considered.
Component Values.
The Pin Function Description table gives specific
information on the choice of components to be used at
each pin of U1. A similar section in the Sanyo LA1832 data
sheet should be consulted as to the components to be used
with U2.
Tuning
The frequency to which the test circuit will tune is set by
the eight binary switches contained in the S5 assembly,
numbered from 1 to 8. Number 1 connects to Pin 11 of U3
and number 8 connects to Pin 18. The other switches
connect to the pins in between and in order. Each individual
switch is a SPST type.
To tune to a specific RF frequency, a computation must be
made in order to ascertain the divide ratio to input to the
synthesizer via the switch array. The divide ratio is simply the
eight digit binary equivalent number for the local oscillator
frequency divided by 10 kHz. The local oscillator frequency is
the desired RF frequency plus 450 kHz, the IF frequency. Any
local oscillator value within the AM band can be represented
by a binary number. Each binary bit represents a switch
setting where a “1” is an open switch and a “0” is a closed
switch. The most significant bit represents switch 8 which is
connected to Pin 18.
To illustrate, consider the setting for an input frequency of
1070 kHz. (This frequency was used to test the circuit board
as described further on.) The local oscillator frequency is
1070 kHz plus 450 kHz which equals 1520 kHz. Dividing by
10 kHz yields the number 152. The binary number for 152 is
10011000. Thus the switches are set to:
Switch
Position
Number
8
Open
1
7
Closed
0
6
Closed
0
5
Open
1
4
Open
1
3
Closed
0
2
Closed
0
1
Closed
0
Circuit Adjustments
The FM circuit requires no adjustment. The AM L.O. must
be able to tune from 980 to 2150 kHz to cover the broadcast
range. Adjust the core of the L.O. coil if needed in order to be
able to cover this range. The AM RF coil and trimmer can be
adjusted for best signal after connection to the loop antenna.
The coil is adjusted near the low end of the band, and the
trimmer is adjusted at the top of the band. The IF coil, T1, is
first adjusted for maximum signal out of the filter, F1. This is
a “coarse” adjustment. The final “fine tune” adjustment
occurs after the following conditions are met. From an AM
Stereo generator with the pilot tone off, feed the decoder an
input signal of approximately 70 dBµV that is modulated with
an 80% L–R audio signal at 3.0 kHz. While monitoring either
the left or the right output from the decoder on an
oscilloscope, precisely fine tune the IF coil for a minimum
residual signal, see the following diagram. If there is no
sideband tilt in the system, this adjustment should hold for
both channels. Otherwise, the best compromise adjustment
for both channels should be used.
16
MOTOROLA ANALOG IC DEVICE DATA
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