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參數(shù)資料
| 型號： | LM1863M |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 接收器 |
| 英文描述： | AM Radio System for Electronically Tuned Radios |
| 中文描述： | AM, AUDIO SINGLE CHIP RECEIVER, PDSO20 |
| 封裝： | PLASTIC, SOP-20 |
| 文件頁數(shù)： | 9/14頁 |
| 文件大小： | 268K |
| 代理商： | LM1863M |
Applications Information
(Continued)
to the center frequency of the incoming station. Both the
signal strength threshold and the center tune window are
externally adjustable.
The signal strength threshold is set by resistor R1. Increas-
ing the value of this resistor will reduce the signal level
threshold. There is no difficulty in setting the signal strength
threshold, either above or below the AGC threshold.
Resistor R2 sets the center tune window. The incoming sta-
tion is considered to be center tuned whenever the frequen-
cy of the signal at the IF output falls within the center tune
window. Increasing the value of R2 will narrow the window,
while decreasing R2 will widen the window. Since there is
some interaction between R2 and R1, R2 should be chosen
before R1. In the United States, stations within the AM band
are spaced no closer than 10 kHz apart. Consequently, the
controller should be set up to stop every 10 kHz within the
AM band when the ETR is in scan mode. A center tune
window anywhere less than
g
10 kHz is therefore adequate
in determining the center tune condition, though a narrower
stop window is desirable in order to minimize the chance
that side bands from a strong adjacent channel will fall with-
in the stop window.
Because of asymmetry in the resonator amplitude charac-
teristic, the center tune stop window will not be symmetric
about the center frequency of the resonator. This is not a
problem as long as the stop window brackets the center
frequency of the IF and does not extend into the next chan-
nel. However, in order to avoid any problems in this regard it
is recommended that the resonator center frequency devi-
ate no more than
g
1 kHz from the center frequency of the
IF.
The stop output, Pin 9, is an open collector NPN transistor.
This output must be taken to a positive voltage through a
load resistor, R3. A valid stop condition is indicated by a
high output level on Pin 9 (i.e., the NPN is turned off). The
voltage on this pin should not exceed 16 volts.
STOP DETECTOR STOP TIME
The amount of time required for the LM1863 to output an
accurate stop indication on Pin 9 is defined as the stop time.
The stop time determines how quickly the ETR can scan
across the AM band. There are several factors that influ-
ence the stop time. Since the signal level stop function op-
erates in conjunction with the Automatic Gain Control
(AGC), the AGC settling time is a critical factor. This settling
time is dominated by the low frequency AGC pole which is
set by C1 and internal IC resistances. Decreasing C1 will
decrease the AGC settling time but increase total harmonic
distortion, THD, of the recovered audio. A good compromise
between AGC settling time and THD is very difficult to reach
with a single pole AGC system. Consequently, the LM1863
has been designed with a second, higher frequency, AGC
pole. This non-dominant pole is externally set by capacitor
C2. As a result, C1 can be made much smaller than it other-
wise could for an equivalent amount of THD. Reducing C1
will reduce the stop time. The combination of C1 and C2 as
shown in the applications circuit results in a stop time of less
than 50 ms for most input conditions, while at the same time
the circuit achieves .9% THD at 80% modulation with 400
Hz modulation frequency at 10 mV input signal strength.
Had C2 not been present the stop time would still be 50 ms
but the THD for similar input conditions would be 8%. By
decreasing both C1 and C2 (keeping the ratio of C1/C2
constant) the stop time can be reduced at the expense of
THD, while the converse is also true.
The addition of a second pole to the AGC response does
add some ringing to the AGC voltage following signal tran-
sients. The frequency, duration and amount of ringing are
dependent on where both AGC poles are placed and to
some extent the input signal conditions. The amount of ring-
ing should be kept to a minimum in order to insure proper
stop indications. The amount of ringing can be reduced by
either reducing C2 (this will increase THD) or by increasing
C1 (this will improve THD but increase stop time).
If the ratio of C1/C2 is made too small, an increase in low
frequency noise may be noticed resulting from the peaking
that a closed loop two pole system exhibits near the unity
gain frequency. The extent of this peaking can be observed
by examining the amount of recovered audio at various low
frequency modulations. In general, the values shown reach
a good compromise between THD, stop time, ringing and
low frequency noise.
The center tuning detector on the LM1863 passes the sig-
nal at the IF output through a limiting amplifier which re-
moves most of the modulation from the IF waveform. The
output of this limiter is then applied to the resonator on Pin
7. Unfortunately, large modulation peaks are not completely
removed by the limiting amplifier. Without C3, these large
modulation peaks would cause glitches on the stop output
when the LM1863 was tuned to a valid station. C3 acts to
reduce these glitches by filtering the output of the center
tune circuit. C3, however, also affects the stop time and
cannot be made arbitrarily large. A time constant of about
30 ms on Pin 5 gives the best compromise. R21 biases Pin
5 to about .4 volts, which is below the stop threshold at this
point. This biasing results in a shorter stop time.
Extra precaution can be taken within the software of the
controller IC to further insure accurate stop detector per-
formance over a wide variety of input signal conditions. A
typical controller IC stop algorithm is as follows:
The controller waits the first 10 ms after the LM1863 is
tuned to the next channel. The controller then samples
the LM1863 stop output 10 times within the next 40 ms.
If no high output is sensed within that time the control-
ler concludes there is no valid station at the frequency
and moves to the next channel. If, however, at least
one high output is detected within the first 50 ms the
controller waits an additional 200 ms and at the end of
that time re-samples the stop output in order to make
its final stop determination.
RF AGC
The RF AGC detector is designed to control the gain of an
external RF amplifier which is placed between the antenna
and the mixer input. The RF AGC operates by detecting
when the input signal to the mixer reaches 6 mVrms, the RF
AGC threshold. When the mixer input signal reaches this
level the RF AGC is activated and will hold the mixer input
level relatively constant at the level of the RF AGC thresh-
old. The gain of the RF AGC determines how constant the
RF AGC can control the RF output. The LM1863 RF AGC is
high gain and consequently the RF AGC output, Pin 3, will
transition from high to low over a very narrow input range to
the mixer when the LM1863 is examined in an OPEN LOOP
condition. However, in a radio where the RF AGC controls
the RF gain, a CLOSED LOOP negative feedback system is
established. In this application the RF AGC output will tran-
sition from high to low over a large range of signal levels to
the input of the RF stage.
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