參數(shù)資料
型號(hào)： LM2893N
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 通信及網(wǎng)絡(luò)
英文描述： LM1893/LM2893 Carrier-Current Transceiver
中文描述： SPECIALTY TELECOM CIRCUIT, PDIP20
封裝： 0.300 INCH, PLASTIC, DIP-20
文件頁(yè)數(shù)： 9/24頁(yè)
文件大?。?/td> 576K
代理商： LM2893N
Component Selection
(Continued)
be made above 10 pF so that parasitic capacitance is not
dominant. Excessive or unbalanced common-mode-to-
ground capacitance should be avoided. A low temperature
coefficient (TC) of capacitance (
k
100 PPM/
§
C), such as a
monolithic NPO ceramic multilayer type, preserves low TC
of F
O
. Figure 6 finds a C
O
value given F
O
.
R
O
Resistor R
O
is used by the IC to generate a V
BE
/R related
current that is multiplied by 2 to produce the 200
m
A ICO
control current that sets F
O
. The control current TC ‘‘bucks’’
the V
BE
related tri-wave amplitude across C
O
to effect a low
TC of F
O
. Vary R
O
to trim F
O
, within limits. Raising F
O
more
than 20% above its untrimmed value by means of decreas-
ing R
O
more than 20% is not recommended. Low R
O
, and
so high control current, risks ICO saturation and poor TC
under worst-case conditions. Raising R
O
reduces the de-
modulated signal amplitude from the phase detector; raising
R
O
bymorethanafactorof2(1octave)isnotrecommended.
Since lower TC pots are relatively costly, it is recommended
that R
O
be made up of a 5.6 k fixed (
k
100 PPM/
§
C) resistor
with a 2 k
X
(
k
250 PPM/
§
C) series pot.
C
A
and R
A
Components C
A
and R
A
control the dynamic characteristics
of the transmitter output envelope. Their values are not crit-
ical. Use the values given in Figure 5. C
A
and R
A
are func-
tions of loaded T
1
tank Q, R
O
, f
DATA
, and line impulse
noise. Any changes made in C
A
and R
A
should be made
based on empirical measurements of a CCT on the line.
Roughly, C
A
acts as an ALC pole and R
A
an ALC zero.
T
1
At this point, the CCT system designer may choose to use
one of the recommended transformers or to design custom
T
1
. Consult ‘‘The Coupling Transformer’’ section to help
with the design of T
1
if a new or boost-capable transformer
is needed. The recommended 125 kHz transformer func-
tions with an I
O
of up to 600 mApp.
It is recommended that CCT systems use the recommended
transformers, described inFigure 7, for T
1
. The 3 transform-
ers are optimized for use in the ranges of 50–100 kHz, 100–
200 kHz, and 200–400 kHz with unloaded Q’s (Q
U
) of about
35, and loaded Q’s (Q
L
) of about 12. Three secondary taps
are supplied with nominal 7.07, 10, and 14.1 turns ratios (N)
to drive industrial and residential power line impedances of
3.5, 7, and 14
X
respectively. All are inexpensive, all have
the same pin-outs for easy exchange in a PC board, and all
are small - on the order of 10 mm diameter at the base.
C
Q
Tank resonant frequency F
Q
must be correct to allow pas-
sage of transmitter signal to the line. Use Figure 8 to find
C
Q
’s value. Trimming F
Q
to equal F
O
is done with T
1
’s trim-
ming slug. The inductance of T
1
has a TC of
a
150 PPM/
§
C
which may be cancelled by using a
b
150 PPM/
§
C cap such
as polystyrene. Since circulating current in the tank is
(/4
A
RMS
, C
Q
should have a low series resistance (a 1
X
series
resistance is too much). Polypropelene caps are excellent,
‘‘orange drop’’ mylars are adequate, while many other my-
lars are inadequate. A 100V rating is needed for transient
protection.
TL/H/6750–5
FIGURE 6. Find C
O
’s value knowing F
O
TL/H/6750–10
FIGURE 8. Find C
O
’s value given F
O
Bottom View
TL/H/6750–6
TL/H/6750–7
125 kHz
Toko 707VX-A042YUK
TL/H/6750–8
50 kHz
Toko 707VX-A043YUK
TL/H/6750–9
300 kHz
Toko 161XN-A207YUK
FIGURE 7. The recommended T
1
transformers, available through:
Toko America, 1250 Feehanville Drive, Mount Prospect, IL, 60056, (312) 297-0070
9
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