參數(shù)資料
型號(hào)： MC13158
廠商： Motorola, Inc.
英文描述： WIDEBAND FM IF SUBSYSTEM FOR DECT AND DIGITAL APPLICATIONS
中文描述： 寬頻帶調(diào)頻中頻DECT和數(shù)字應(yīng)用子系統(tǒng)
文件頁(yè)數(shù)： 17/24頁(yè)
文件大?。?/td> 394K
代理商： MC13158
MC13158
17
MOTOROLA ANALOG IC DEVICE DATA
KMFC545) with a 3.0 dB bandwidth of
±
325 kHz and a
maximum insertion loss of 5.0 dB. The application PC board
is laid out to accommodate this filter pair (a filter pair is used
at both locations of the split IF). However, even using a series
parallel ceramic filter network yields only a maximum
bandpass of 650 kHz. In some applications a wider band IF
bandpass is necessary.
A simple LC network yields a bandpass wider than the
SAW filter but it does reduce an appreciable amount of
wideband IF noise. In the application circuit an LC network is
specified using surface mount components. The parallel LC
components are placed from the outputs of the mixer and IF
amplifier to the VCC trace; internal 330 loads are connected
from the mixer and IF amplifier outputs to DEC2 (Pin 5 and 10
respectively).This loads the outputs with the optimal load
impedance but creates a low insertion loss filter. An external
shunt resistor may be used to widen the bandpass and to
acquire the 10 dB composite loss necessary to linearize the
RSSI output. The equivalent circuit is shown in Figure 18.
Figure 18. IF LCR Filter
150
VCC
Rout
330
VCC
330 nH
680 p
DEC2
Rin
330
5, 10
3, 8
2, 7
1, 6
DEC1
4, 9
The following equations satisfy the 12 dB loss
(1:4 resistive ratio):
(Rext)(330) (Rext
Requivalent (Requivalent
330)
Requivalent
330)
1 4
Solve for Requivalent:
4(Requivalent)
3(Requivalent)
Requivalent
Requivalent
330
110
330
Substitute for Requivalent and solve for Rext:
330(Rext)
110(Rext)
Rext
(330)(110) 220
Rext
165
(330)(110)
The IF is 10.7 MHz although any IF between 10 to 20 MHz
could be used. The value of the coil is lowered from that used
in the quadrature circuit because the unloaded Q must be
maintained in a surface mount component. A standard value
component having an unloaded Q = 100 at 10.7 MHz is
330 nH; therefore the capacitor is 669 pF. Standard values
have been chosen for these components;
Rext
150
C
680 pF
L
330 nH
Computation of the loaded Q of this LCR network is
Q
Requivalent XL
where: XL = 2
π
fL and Requivalent is 103
Thus, Q
4.65
The total system loss is
20 log (103 433)
–12.5 dB
Quadrature Detector
The quadrature detector is coupled to the IF with an
internal 5.0 pF capacitor between Pins 12 and 13. For
wideband data applications, the drive to the detector can be
increased with an additional external capacitor between
these pins; thus, the recovered signal level output is
increased for a given bandwidth
The wideband performance of the detector is controlled by
the loaded Q of the LC tank circuit. The following equation
defines the components which set the detector circuit’s
bandwidth:
Q
RTXL
where RT is the equivalent shunt resistance across the LC
Tank
XL is the reactance of the quadrature inductor at the IF
frequency (XL = 2
π
fL).
The inductor and capacitor are chosen to form a resonant
LC tank with the PCB and parasitic device capacitance at the
desired IF center frequency as predicted by
(LCp)1 2]–1
[1]
fc
[2
[2]
where L is the parallel tank inductor Cp is the equivalent
parallel capacitance of the parallel resonant tank circuit.
The following is a design example for a wideband detector
at 10.7 MHz and a loaded Q of 18. The loaded Q of the
quadrature detector is chosen somewhat less than the Q of
the IF bandpass. For an IF frequency of 10.7 MHz and an IF
bandpass of 600 kHz, the IF bandpass Q is approximately
6.4.
Example:
Let the external Cext = 139 pF. (The minimum value here
should be much greater than the internal device and PCB
parasitic capacitance, Cint
≈
3.0 pF). Thus, Cp = Cint +
Cext = 142 pF.
Rewrite equation (2) and solve for L:
L = (0.159)2/(Cpfc2)
L = 1.56
μ
H; Thus, a standard value is
choosen:
L = 1.56
μ
H (tunable shielded inductor)
The value of the total damping resistor to obtain the
required loaded Q of 18 can be calculated by rearranging
equation (1):
RT
RT
Q(2 fL)
18(2 )(10.7)(1.5)
1815
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