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參數(shù)資料
| 型號(hào)： | LM1863M |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類(lèi)： | 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(yè)數(shù)： | 11/14頁(yè) |
| 文件大小： | 268K |
| 代理商： | LM1863M |
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Applications Information
(Continued)
teristics of the radio are determined by the design of this
stage. Generally speaking it is very difficult to design an
integrated RF stage in bipolar, as bipolar transistors do not
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.
The circuit consists of only two active devices, an N-chan-
nel JFET, Q1, which is connected in a cascode type of con-
figuration with an NPN BJT, Q2. Both Q1 and Q2 are varac-
tor tuned gain stages. Q2 also serves to gain reduce Q1
when Q2’s base is pulled low by the RF AGC circuit on the
LM1863. The gain reduction occurs because Q1 is driven
into a low gain resistive region as its drain voltage is re-
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
a
10 dB.
T2 in conjunction with D1, C21 and C26 form the 1’st tuned
circuit. C26 does not completely de-couple the RF signal at
the cathode of the varactor. In fact, the combination of C26
and C19 act to keep the gain of the whole RF stage con-
stant over the entire AM band. Without special care in this
regard the gain variation could be as high as 14 dB. This gain
variation would result from the increase in impedance at the
secondary’s of T2 and T1 as the tuned frequency is in-
creased. The increased impedance results from a constant
Q
e
Rp/(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.
C27 de-couples RF signal from the top of T2’s primary and
allows Q2 to operate properly. C18 is a coupling capacitor
which in conjunction with C19 couples the signal from the
1’st RF stage to the 2’nd RF stage. R20 acts to isolate this
signal from AC ground at C11. R19 acts in conjunction with
C12 to set a high frequency (ie: non-dominant) RF AGC
pole which is important for low distortion when the RF AGC
is active. The dominant RF AGC pole is set by R8 and C11.
Q2 is a high beta transistor allowing for little voltage drop
across R20 and R8 due to base current. This keeps the
emitter of Q2 sufficiently high (in the absence of RF AGC) to
bias Q1 in its square law region.
R13 acts to reduce the 2’nd stage gain and increase Q2’s
signal handling. R13 must not get too large, however, (ie:
R13
l
100
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
stage greatly improves the cross modulation performance of
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.
R16 and C29 decouple unwanted signals on V
a
from being
coupled into the RF stage. This front end also offers superi-
or performance with respect to varactor overload by strong
adjacent channels. This results because of the way that
gain has been distributed between the 1’st and 2’nd stages.
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
has been taken to insure that excellent cross modulation
performance, image rejection, signal to noise performance,
overload performance, and low distortion are achieved. Per-
formance characteristics for this front end in conjunction
with the LM1863 are shown in the data sheet. Also, informa-
tion with regard to the bandwidth of the front end versus
tuned frequency are given below.
TUNED FREQUENCY
530 kHz
600 kHz
1200 kHz
1500 kHz
1630 kHz
b
3 dB BANDWIDTH
6.6 kHz
7.2 kHz
20.6 kHz
26.4 kHz
36 kHz
VARACTOR ALIGNMENT PROCEDURE
The following is a procedure which will allow you to properly
align the RF and local oscillator trim capacitors and coils to
insure proper tracking across the AM band.
1. Set the voltage across the varactors
e
1 volt.
2. Set the trimmers to 50%.
3. Adjust the oscillator coil until the local oscillator is at 980
kHz.
4. Increase the varactor voltage until the local oscillator
(L0) is at 2060 kHz and check to see if this voltage is less
than 9.5 volts but greater than 7.5 volts. If it is then the
L0 is aligned. If it is not then adjust the L0 coil/trimmer
until the varactor voltage falls in this range.
5. Set the RF in to 600 kHz and adjust the tuning voltage
until the L0 is at 1050 kHz. Peak all RF coils for maxi-
mum recovered audio at low input levels.
6. Set RF in to 1500 kHz and adjust the tuning voltage until
the L0 is at 1950 kHz. Peak all RF trim capacitors for
maximum recovered audio at low input levels.
7. Go back to step 5 and iterate for best adjustment.
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.
ADDITIONAL INFORMATION
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-
tion if further reduction of the 450 kHz component is re-
quired.
11
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