MC13158FTB View Datasheet(PDF) - LANSDALE Semiconductor Inc.
Part Name
Description
Manufacturer
MC13158FTB Datasheet PDF : 23 Pages
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ML13158
LANSDALE Semiconductor, Inc.
Legacy Applications Information
IF FILTERING/MATCHING
In wideband data systems the IF bandpass needed is greater than
can be found in low cost ceramic filters operating at 10.7 MHz. It
is necessary to bandpass limit with LC networks or series–parallel
ceramic filter networks. Murata offers a series–parallel resonator
pair (part number KMFC545) with a 3.0 dB band width 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 par-
allel ceramic filter network yields only a maximum bandpass of
650 kHz. In some applications a wider band IF bandpass is neces-
sary.
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
330Ω loads are connected from the mixer and IF amplifier outputs
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.
Substitue for Requivalent and solve for Rext
330(Rext) = 110 (Rext) + (330)(110)
Rext = (330)(110)/220
Rext - 165 Ω
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 Q
C = 680 pF
L = 330 nH
Computation of the loaded Q of the is LCR network is
Q = Requivalent/XL
where XL = 2πfl and Requivalent is 103 Ω
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 exter-
nal 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 = RT/XL
[1]
where RT is the equivalent shunt resistance across the LC Tank
XL is the reactance of the quadrature inductor at the IF fre-
quency (XL = 2πfl).
The inductor and capacitator are chosen to form a resonant LC tank
with the PCB and parasitic device capacitance at the desired IF
center frequency as predicted by
fc = [2π (LCp)1/2]–1
[2]
where L is the parallel tank inductor Cp is the equivalent parallel
capacitance of the parallel resonant tank circuit.
The following equations satisfy the 12 dB loss
(1:4 resistive ratio):
(Rext)(330)/(Rext + 330) = Requivalent
Requivalent/Requivalent + 330) = 1/4
Solve for Requivalent:
4(Requivalent) = Requivalent + 330
3(Requivalent) = 330
Requivalent = 110
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 detec-
tor 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.
Page 16 of 23
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