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參數(shù)資料
| 型號： | LM1863 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | AM Radio System for Electronically Tuned Radios |
| 中文描述： | AM收音機系統(tǒng)的電子調(diào)諧收音機 |
| 文件頁數(shù)： | 10/14頁 |
| 文件大?。?/td> | 268K |
| 代理商： | LM1863 |
Applications Information
(Continued)
The RF AGC threshold has been carefully chosen to pre-
vent overloading the mixer, which would cause distortion
and tweet problems. However, the threshold level is suffi-
ciently large to minimize the possibility of strong adjacent
stations de-sensitizing the radio by activating the RF AGC
and thereby gain reducing the RF front end.
The RF AGC output, Pin 3, is an open collector NPN transis-
tor. This collector must be tied to a positive voltage through
a load resistor, R8. Furthermore, decoupling is required
(C11 and C12) in order to insure that the RF AGC does not
induce significant distortion in the recovered audio. Howev-
er, the tradeoff between good THD performance and fast
stop time is not too severe for the RF AGC because large
changes in the RF AGC level are unlikely when moving be-
tween adjacent channels. This is because the selectivity in
the RF stage is not great enough to cause abrupt signal
level changes at the mixer input as the radio is tuned. Thus,
since the RF AGC does not have to follow abrupt signal
level changes, the time constant on the AGC output can be
relatively long which allows for good THD performance. C12
is required in order to insure good RF decoupling of signals
at the RF AGC output, and sets the non-dominant pole.
The RF AGC 10
m
A threshold is fixed at 6 mVrms at the
mixer input. However, due to the gain of the RF stage and
losses through the RF transformers, this level may be differ-
ent when referenced to the antenna input. For the applica-
tion circuit shown the RF threshold occurs at 2 mVrms at
the dummy antenna input. Thus, the RF AGC threshold can
effectively be adjusted by altering the gain of the RF stage.
The value of R8 also has some affect on the RF AGC
threshold of the application circuit. Smaller values will tend
to increase the threshold while larger values will tend to
reduce the threshold.
GAIN DISTRIBUTION
The purpose of this section is to clarify some of the trade-
offs involved in redistributing gain from one portion of the
radio to another. An AM radio basically has three gain
blocks consisting of the RF stage, the mixer, and the IF
stage. The total gain of these three blocks must be suffi-
ciently large as to insure reception of weak stations. Given
then a fixed amount of required gain how does distributing
this gain among the three blocks affect the radio perform-
ance
Large amounts of gain in the RF stage will have the effect of
decreasing the RF AGC threshold. A decreased RF AGC
threshold means that it is more likely that strong adjacent
stations can activate the RF AGC and desensitize the radio.
Also, a lot of RF gain implies large signals across the RF
varactor diodes, which is undesirable for good tracking and
can result in overloading these varactors which can cause
cross modulation. On the other hand, high RF gain insures
good noise performance and improved THD.
High mixer gain implies large signal swings at the mixer out-
put, especially on AGC transients. These large signal
swings could cause the mixer ouput transistors to saturate
and also could overload the IF stage. On the other hand,
redistributing the gain from the IF to the mixer would im-
prove the noise performance of the radio. The gain of the
mixer can be controlled moving the tap on the mixer output
transformer, T4.
Since the output signal level of the IF is held constant by the
AGC, increasing gain in the IF has the effect of reducing the
signal level at the IF input. Noise sources at the IF input
therefore become a larger percentage of the IF input signal
thereby degrading the S/N floor of the radio. For this rea-
son, the LM1863 employs 20 dB of IF AGC. The IF gain of
the LM1863 is adjustable by changing the tap across the IF
ouput coil, or by changing the ratio of R24 to R4.
The gain distribution for the application circuit is as follows:
Gain Distribution
TL/H/5185–23
V
G
e
0 dB
V1
e b
16 dB
V2
e a
10 dB
V3
e a
33 dB
V
O
e a
84 dB
(10
m
V)
(Pin 20)
(Pin 11)
(Pin 14)
The IF gain could also be varied by changing the value of
R6 across the IF output coil. However, it is a good idea to
maintain a high Q IF tank in order to achieve good adjacent
channel rejection. In order to prevent distortion due to over-
loading the IF amplifier, it is important that the impedance
Pin 14 sees looking into the IF output tank, T5, does not go
below 3K ohms.
The above gain distribution is prior to any AGC action in the
radio. This distribution represents a good compromise be-
tween the various tradeoffs outlined previously.
LEVEL CONTROLLED LOCAL OSCILLATOR
Tracking of the RF varactors with the local oscillator varac-
tor is a serious consideration in order to insure adequate
performance of the ETR radio. Due to non-linear capaci-
tance versus voltage characteristic of the varactor, large
signals across these varactors will tend to modulate their
capacitance and cause tracking problems. This problem is
compounded further if the level of the signals across the
varactors change. In an AM radio, the local oscillator fre-
quency changes a ratio of two to one. The Q of the oscilla-
tor tank remains fairly constant over this range. Thus, since
Q
e
R
P
/
0
L
e
Constant, this implies that R
P
(R
P
e
un-
loaded parallel resistance of the tank) must change two to
one. The internal level-control loop prevents the two to one
change in AC voltage across the tank which the change in
the R
P
would otherwise cause.
Phase jitter of the local oscillator is very important in regard
to AM stereo, where L-R information is contained in the
phase of the carrier. Local oscillator jitter has the effect of
modulating the L-R channel with phase noise, thus degrad-
ing the stereo signal to noise performance. Great care has
been taken in the design of the LM1863 local oscillator to
insure that phase jitter is a minimum. In fact the dominant
source of phase jitter is the high impedance resistor drive to
the varactor. The thermal noise of the resistor modulates
the varactor voltage, thus causing phase jitter.
VARACTOR TUNED RF STAGE
Electronically tuned car radios require the use of a tuned RF
stage prior to the mixer. Many of the performance charac-
10
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| LM1863M | AM Radio System for Electronically Tuned Radios |
| LM1865 | Advanced FM IF System |
| LM1865M | Advanced FM IF System |
| LM1865N | Advanced FM IF System |
| LM1868 | Radio System |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM1863M | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:AM Radio System for Electronically Tuned Radios |
| LM1863N/A+ | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:AM Receiver Circuit |
| LM1863N/B+ | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:AM Receiver Circuit |
| LM1865 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Advanced FM IF System |
| LM1865M | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Advanced FM IF System |
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