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PDF LM1863 Data sheet ( Hoja de datos )
| Número de pieza | LM1863 | |
| Descripción | AM Radio System for Electronically Tuned Radios | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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May 1989
LM1863 AM Radio System for
Electronically Tuned Radios
General Description
The LM1863 is a high performance AM radio system intend-
ed primarily for electronically tuned radios Important to this
application is an on-chip stop detector circuit which allows
for a user adjustable signal level threshold and center fre-
quency stop window The IC uses a low phase noise level-
controlled local oscillator
Low phase noise is important for AM stereo which detects
phase noise as noise in the L-R channel A buffered output
for the local oscillator allows the IC to directly drive a phase
locked loop synthesizer The IC uses a RF AGC detector to
gain reduce an external RF stage thereby preventing over-
load by strong signals An improved noise floor and lower
THD are achieved through gain reduction of the IF stage
Fast AGC settling time which is important for accurate stop
detection and excellent THD performance are achieved
with the use of a two pole AGC system Low tweet radiation
and sufficient gain are provided to allow the IC to also be
used in conjunction with a loopstick antenna
Features
Y Low supply current
Y Level-controlled low phase noise local oscillator
Y Buffered local oscillator output
Y Stop circuitry with adjustable stop threshold and adjust-
able stop window
Y Open collector stop output
Y Excellent THD and stop time performance
Y Large amount of recovered audio
Y RF AGC with open collector output
Y Meter output
Y Compatible with AM stereo
Block Diagram
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C1995 National Semiconductor Corporation TL H 5185
Order Number LM1863M
See NS Package Number M20B
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TL H 5185 – 1
RRD-B30M115 Printed in U S A
1 page  
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Performance Characteristics of Applications Circuit
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TL H 5185–12
TL H 5185 – 13
TL H 5185 – 14
Cross modulation is
measured using the
following dummy an-
tenna
The following procedure was used to measure cross modulation
1 Tune the radio to the center frequency of interest and tune VGEN to this same frequency
2 Set at 0 dB audio reference with VGEN e10 mV RMS and 30% AM mod fMOD e 1 kHz
3 Remove the modulation from VGEN1 and set the level of VGEN1
4 Set the modulation level of VGEN2 e 80% at fMOD e 1 kHz and tune VGEN2 g 40 kHz away from
center frequency
5 Increase the level of VGEN2 until b40 dB of audio is recovered The level of VGEN2 is the cross
modulation measurement
TL H 5185–15
Additional Performance Information
THD for 80D%amtoaduSlahtioen feortf4MOUDe.c1okHmZ at
VGENe1V is 0 5%
VGENe10 mV is 0 4%
Tweet k2% at all input levels
Typical time for valid stop indication k 50 ms
TL H 5185–16
Note Tweet is an audio tone produced by the 2nd and 3rd harmonic of the IF beating against the received
signal It is measured as an equivalent modulation level ie 30% tweet has the same amplitude at the
detector as a desired signal with 30% modulation
DataShee
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Applications Information (Continued)
teristics of the radio are determined by the design of this
or performance with respect to varactor overload by strong
stage Generally speaking it is very difficult to design an
adjacent channels This results because of the way that
integrated RF stage in bipolar as bipolar transistors do not
gain has been distributed between the 1’st and 2’nd stages
have good overload characteristics Thus the RF stage is
usually designed using discrete components Because of
this there is a great deal of concern with minimizing the
number of discrete components without severely sacrificing
performance The applications circuit RF stage does just
this
In summary this front end offers two stages of RF gain with
the 2’nd stage acting to gain reduce the 1’st stage when RF
AGC is active Furthermore a unique coupling scheme is
employed from the output of the 1’st stage to the input of
the 2’nd stage This coupling scheme equalizes the gain
from one end of the AM band to the other Additional care
The circuit consists of only two active devices an N-chan-
has been taken to insure that excellent cross modulation
nel JFET Q1 which is connected in a cascode type of con-
performance image rejection signal to noise performance
figuration with an NPN BJT Q2 Both Q1 and Q2 are varac-
overload performance and low distortion are achieved Per-
tor tuned gain stages Q2 also serves to gain reduce Q1
formance characteristics for this front end in conjunction
when Q2’s base is pulled low by the RF AGC circuit on the
with the LM1863 are shown in the data sheet Also informa-
LM1863 The gain reduction occurs because Q1 is driven
tion with regard to the bandwidth of the front end versus
into a low gain resistive region as its drain voltage is re-
tuned frequency are given below
duced R10 and C15 set the gain of the 1’st RF stage which
is kept high (about 19 dB) for good low signal signal noise
performance The gain of the front end to the mixer input
referenced to the generator output is about a10 dB
TUNED FREQUENCY
530 kHz
600 kHz
1200 kHz
b3 dB BANDWIDTH
6 6 kHz
7 2 kHz
20 6 kHz
T2 in conjunction with D1 C21 and C26 form the 1’st tuned
circuit C26 does not completely de-couple the RF signal at
1500 kHz
1630 kHz
26 4 kHz
36 kHz
the cathode of the varactor In fact the combination of C26
and C19 act to keep the gain of the whole RF stage con-
VARACTOR ALIGNMENT PROCEDURE
stant over the entire AM band Without special care in this
The following is a procedure which will allow you to properly
regard the gain variation could be as high as 14 dB This gain
align the RF and local oscillator trim capacitors and coils to
variation would result from the increase in impedance at the
insure proper tracking across the AM band
secondary’s of T2 and T1 as the tuned frequency is in-
1 Set the voltage across the varactorse1 volt
creased The increased impedance results from a constant
QeRp (wL) of the tanks over the AM band With C26 and
C19 the gain is held constant to within 6 dB (including the
tracking error) over the entire AM band
2 Set the trimmers to 50%
3 Adjust the oscillator coil until the local oscillator is at 980
kHz
C27 de-couples RF signal from the top of T2’s primary and
4 Increase the varactor voltage until the local oscillator
allows Q2 to operate properly C18 is a coupling capacitor
(L0) is at 2060 kHz and check to see if this voltage is less
which in conjunction with
1’st RF stage to the 2’nd
RCF19stacgoeupRle2s0thaectssigtonaisl ofrlaotDme atthhtiesaSheet4tLUh0a.nics 9oalm5igvnoeldts
but
If it
greater than 7 5 volts
is not then adjust the
If
L0
it is
coil
then the
trimmer
signal from AC ground at C11 R19 acts in conjunction with
until the varactor voltage falls in this range
C12 to set a high frequency (ie non-dominant) RF AGC
5 Set the RF in to 600 kHz and adjust the tuning voltage
pole which is important for low distortion when the RF AGC
is active The dominant RF AGC pole is set by R8 and C11
until the L0 is at 1050 kHz Peak all RF coils for maxi-
mum recovered audio at low input levels
Q2 is a high beta transistor allowing for little voltage drop
6 Set RF in to 1500 kHz and adjust the tuning voltage until
across R20 and R8 due to base current This keeps the
the L0 is at 1950 kHz Peak all RF trim capacitors for
emitter of Q2 sufficiently high (in the absence of RF AGC) to
maximum recovered audio at low input levels
bias Q1 in its square law region
7 Go back to step 5 and iterate for best adjustment
R13 acts to reduce the 2’nd stage gain and increase Q2’s
signal handling R13 must not get too large however (ie
R13l100 X) or low level signal noise will be degraded T3
in conjunction with C20 C27 and D2 form the 2’nd RF tuned
circuit The output of Q2 is capacitively coupled through C28
to the mixer input The output of Q2 is loaded not only by
the reflected secondary impedance but also by R22 R22 is
carefully chosen to load the 2’nd stage tuned circuit and
broaden its bandwidth The increased bandwidth of the 2’nd
8 Check the radio gain at 530 kHz and 750 kHz to make
sure that the gain is about the same at these two fre-
quencys If it is not then slightly adjust the RF coils until
it is
The above procedure will insure perfect tracking at 600 kHz
950 kHz and 1500 kHz The amount of gain variation across
the AM band using the above procedure should not exceed
6 dB
stage greatly improves the cross modulation performance of
ADDITIONAL INFORMATION
the front end In the absence of this increased bandwidth
the relatively large AC signals across varactor D2 result in
cross modulation R22 also reduces the total gain of the
2’nd stage R22 does slightly degrade (by about 6 dB) the
image rejection especially at the high end of the AM band
However the image rejection of this front end is still excel-
lent and 6 dB is a small price to pay for the greatly increased
immunity to cross modulation
R5 and C7 act as a low pass filter to remove most of the
residual 450 kHz IF signal from the audio output Some re-
sidual 450 kHz signal is still present however and may
need to be further removed prior to audio amplification This
need becomes more important when the LM1863 is used in
conjunction with a loopstick antenna which might pick up an
amplified 450 kHz signal An additional pole can be added
to the audio output after R5 and C7 prior to audio amplifica-
R16 and C29 decouple unwanted signals on Va from being
tion if further reduction of the 450 kHz component is re-
coupled into the RF stage This front end also offers superi-
quired
11
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| PDF Descargar | [ Datasheet LM1863.PDF ] | |
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