3848
DUAL-CONVERSION AM RECEIVER
3848
Providing the AM signal processing functions for an electronically
tuned AM receiver (ETR), the A3848EEQ includes two balanced
mixers, a crystal local oscillator, an L/C-tuned local oscillator, oscillator
buffer, IF amplifier, AM detector, scan control detectors, and a switch-
able voltage regulator. This dual-conversion device typically mixes the
incoming RF up to a first IF of 10.7 MHz, then down to 450 kHz, and
then detects the audio. The addition of a JFET matched to a whip
antenna, RF low-pass filter, IF selectivity, and audio stages gives a
complete AM radio which can be used in automotive receivers. The
frequency-detecting stop circuit is also capable of recovering narrow-
band FM, making it useful for scanners or weather band radio applica-
tions. Two AGC and field-strength indicator modes provide special
features for scanning.
The A3848EEQ uses the dual criteria of frequency and amplitude
for establishing a valid stop. Tuning accuracy (frequency criterion) is
established by evaluating phase shift across the detector coil. The
circuitry is similar to that used in FM discriminators. Because this
detection system is phase operated, it remains effective even in the
presence of strong signals, which can cause false stops in systems
using narrow-band filters. The amplitude criterion for stop is deter-
mined by evaluating the IF level. It includes a unique circuit that
removes the effect of the AGC action. This allows the AGC tuning
components to be selected for low-frequency audio performance
without compromising scanning speed.
Data Sheet
27121.15
DUAL-CONVERSION AM RECEIVER
HIGH-LEVEL
GROUND
AUDIO OUT
REGULATOR
AFC OUT
STOP
MIX 1 IN
MIX 1 OUT
A
MIX 1 OUT
B
WB AGC 1 IN
12
RESET
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, V
CC
. . . . . . . . . . . . . .
12 V
Package Power Dissipation, P
D
. . . .
1.2 W
Operating Temperature Range,
T
A
. . . . . . . . . . . . . . .
-40
°
C to +105
°
C
Storage Temperature Range,
T
S
. . . . . . . . . . . . . . .
-65
°
C to +150
°
C
T
C
U Y
D L
O N
R O
P E
D C
E N
U E
N R
I E
T F
N E
O R
C R
S
I O
D F
—
SUPPLY
31
IF BIAS
IF OUT
V
CC
SUB
32
30
4
2
3
1
DECOUPLE
DETECTOR
FSI
5
6
7
8
9
29
28
27
MIX 2 OUT
B
IF
V
REG
DET
AGC (MUTE)
26
25
24
CRYSTAL
AGC RATE 2
MIX 2 OUT
A
MIX 2 E
B
10
11
23
WB
AGC
22
MIX 2 E
B
13
21
MIX 2
BYPASS
V
REF
14
15
17
18
19
16
REFERENCE
LOW-LEVEL
GROUND
OSC BUFFER
WB AGC OUT
WB AGC 2 IN
OSC COIL
MIX 2 IN
20
Dwg. PS-012-1
Dwg. No. PS-012-1
In the normal AGC mode (AGC RESET low), a slow, narrow-band
field-strength indicator (FSI) is provided for controlling signal-depen-
dent functions such as stereo blending. A fast AGC mode (AGC
RESET high) resets the AGC holding capacitors to maximum gain.
This mode allows cataloging station strengths quickly during a band
sweep.
This AM signal processor is packaged in a rectangular, 32-lead,
plastic, leaded chip carrier (PLCC) for surface-mount applications and
is rated for operation over the temperature range of -40°C to +105°C
are available on special order.
FEATURES
s
Low Noise Figure
s
Field-Strength Indicator
s
Balanced Mixers
s
Buffered Oscillator
s
High Dynamic Range First Mixer
s
Fast Scan Mode
Always order by complete part number:
A3848EEQ
.
3848
DUAL-CONVERSION AM RECEIVER
ELECTRICAL CHARACTERISTICS at T
A
= +25°C, V
CC
= 10 V, f
o
= 1 MHz, f
if1
= 10.7 MHz,
f
if2
= 450 kHz, V
in
= 10 mV; f
m
= 1 kHz , Mod = 30% (except as noted).
Characteristic
Supply Current
Symbol
I
CC
V
in
V
in
V
out
THD
V
o
V
STP
V
stp
BW
STP
V
AGC
Mod = 90%
V
17
V
8
, V
in
= 0
V
8
, Mod = 0%
Stop Sensitivity
Stop Bandwidth
Wide-Band AGC
V
12
= 2.5 V, Mod = 0%
V
8
= 2.5 V, = 0%
V
in
= 0
V
in
= 11 mV, Mod = 0%
V
in
= 26 mV, Mod = 0%
Field-Strength
Indicator
Output Voltage
(unmodulated,
AGC Reset Low)
V
FSI
V
in
= 0
V
in
= 30
µV,
Mod = 0%
V
in
= 100
µV,
Mod = 0%
V
in
= 1 mV, Mod = 0%
V
in
= 10 mV, Mod = 0%
Field-Strength
Indicator
Output Voltage
(unmodulated,
AGC Reset High)
V
FSI
V
in
= 0
V
in
= 30
µV,
Mod = 0%
V
in
= 100
µV,
Mod 0%
V
in
= 1 mV, Mod = 0%
V
in
= 10 mV, Mod = 0%
Overload
V
in
V
in
V
out
V
out
S+N/N
V
out
= 3% THD, Mod = 90%
First Mixer (Note 2)
-3dB Limiting
IF Output Voltage
FM Recovered Audio
Signal to Noise Ratio
Mod = 3 kHz peak deviation
V
in
= 1 mV
V
7
, Mod = 3 kHz peak deviation
V
in
= 1 mV
V
in
= 10 mV
AGC Figure of Merit
FOM
Ref. at V
in
= 10 mV, V
in
for V
out
= -10 dB
Test Conditions
I
2
, V
in
= 0
I
2
, V
in
= 0, V
27
= 0 (Muted)
Sensitivity
Usable Sensitivity
Recovered Audio
Total Harmonic Dist.
Oscillator Output
Stop Output Voltage
V
out
= 50 mV
S + N/N = 20 dB
Min
—
—
—
—
165
—
80
4.7
—
30
5.0
5.0
3.7
—
—
0.25
1.1
1.8
3.1
—
0.9
1.3
2.3
2.5
60
350
—
200
—
50
53
7.0
Limits
Typ
Max
50
1.0
6.0
14
215
0.4
120
5.0
0.10
45
6.5
6.7
5.0
—
0.1
0.60
1.6
2.5
3.7
—
1.1
1.6
2.9
3.7
100
450
12
250
380
53
56
10
65
6.0
10
21
265
2.0
—
—
0.16
60
8.0
8.0
—
1.0
0.4
1.05
2.2
3.2
4.4
0.5
1.5
2.0
3.5
4.0
—
—
—
320
—
—
—
14
Units
mA
mA
µV
µV
mV
%
mV
V
V
µV
kHz
V
V
V
V
V
V
V
V
V
V
V
V
V
mV
mV
µV
mV
mV
dB
dB
µV
Continued next page...
115 Northeast Cutoff, Box 15036
W
Worcester, Massachusetts 01615-0036 (508) 853-5000
Copyright © 1993, Allegro MicroSystems, Inc.
3848
DUAL-CONVERSION AM RECEIVER
ELECTRICAL CHARACTERISTICS at T
A
= +25°C, V
CC
= 10 V, f
o
= 1 MHz, f
if1
= 10.7 MHz,
f
if2
= 450 kHz, V
in
= 10mV; f
m
= 1 kHz, Mod = 30% (except as noted) (concluded)
Characteristic
Regulator Voltage
Symbol
V
REG
V
REF
Test Conditions
V
6
V
6
, V
27
= 0 (Muted)
Reference Voltage
V
15
Min
4.7
—
3.2
Typ
5.0
0
3.4
Limits
Max
5.3
0.3
3.6
Units
V
V
V
NOTES: 1. Typical data is for design information only.
2. Attenuate MIXER 1 output with 50
Ω
load on mixer coil secondary, V
out
=3% THD, Mod = 90%
Allegro MicroSystems, Inc. reserves the right to make, from time to
time, such departures from the detail specifications as may be required
to permit improvements in the design of its products.
The information included herein is believed to be accurate and
reliable. However, Allegro MicroSystems, Inc. assumes no responsibil-
ity for its use; nor for any infringements of patents or other rights of third
parties which may result from its use.
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