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MC13158

WIDEBAND FM IF SUBSYSTEM FOR DECT AND DIGITAL APPLICATIONS

Motorola => Freescale 

MC13158

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

Rout

330

1, 6

150 330 nH

680 p

2, 7 VCC

3, 8

DEC1

4, 9

Rin

330

5, 10

DEC2

VCC

The following equations satisfy the 12 dB loss

(1:4 resistive ratio):

ń ) + (Rext)(330) (Rext 330) Requivalent

ń ) + ń Requivalent (Requivalent 330) 1 4

Solve for Requivalent:

+ ) 4(Requivalent) Requivalent 330

+ 3(Requivalent) 330

+ Requivalent 110

Substitute for Requivalent and solve for Rext:

+ ) 330(Rext) 110(Rext)

+ ń Rext (330)(110) 220

+ Rext 165 W

(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 W

+ C 680 pF

+L 330 nH

Computation of the loaded Q of this LCR network is

+ ń Q Requivalent XL

+ where: XL = 2πfL and Requivalent is 103 Ω

Thus, Q 4.65

The total system loss is

ń + 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:

+ ń 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

frequency (XL = 2πfL).

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

+ ń p fc [2 (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 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 µH; Thus, a standard value is

choosen:

L = 1.56 µH (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(2pfL)

p 18(2 )(10.7)(1.5)

1815 W

MOTOROLA ANALOG IC DEVICE DATA

17

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