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Page 11 of 16

Issue A

ML13155

LANSDALE Semiconductor, Inc.

QUADRATURE DETECTOR

The quadrature detector is coupled to the IF with internal 2.0

pF. capacitors between Pins 7 and 8 and Pins 9 and 10. For

wideband data applications, such as FM video and satellite

receivers, the drive to the the detector can be increased with

additional external capacitors between these pins, thus, the

recovered video signal level output is increased for a given

bandwidth (see Figure 11A and Figure 11B).

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 band-

width:

Q=RT/XL

(1)

where: RT is the equivalent shunt resistance across the LC

Tank and 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:

fc = (2

π √(LCp))–1

(2)

where: L is the parallel tank inductor and Cp is the equivalent

parallel capacitance of the parallel resonant tank circuit.

The following is a design example for a wideband detector at

70 MHz and a loaded Q of 5. The loaded Q of the quadrature

detector is chosen somewhat less than the Q of the IF band-

pass. For an IF frequency of 70 MHz and an IF bandpass of

10.9 MHz, the IF bandpass Q is approximately 6.4.

Example:

Let thE external Cext = 20 pF. (The minimum value here

should be greater than 15 pF making it greater than the inter-

nal device and PCB parasitic capacitance. Cint

≈ 3.0 pF).

Cp = Cint + Cext = 23 pF

Rewrite Equation 2 and solve for L:

L = (0.159)2/(Cp fc2)

L = 198 nH, thus, a standard value is chosen.

L = 0.22 µH (tunable shielded inductor).

The value of the total damping resistor to obtain the required

loaded Q of 5 can be calculated by rearranging Equation 1:

RT = Q(2

πfl)

RT = 5(2

π)(70)(0.22) - 483.8 Ω

The internal resistance, Rint between the quadrature tank Pins

8 and 9 is approximately 3200 Ω and is considered in deter-

mining the external resistance, Rext which is calculated from:

Rext = ((RT)(Rint))/(Rint–RT)

Rext = 570, thus, choose the standard value

Rext = 560 Ω

SAW FILTER

In wideband video data applications, the IF occupied band-

width may be several MHz wide. A good rule of thumb is to

choose the IF frequency about 10 or more times greater than

the IF occupied bandwidth. The IF bandpass filter is a SAW

filter in video data applications where a very selective

response is needed (i.e., very sharp bandpass response). The

evaluation PCB is laid out to accommodate two SAW filter

package types: 1) A five–leaded plastic SIP package.

Recommended part numbers are Siemens X6950M which

operates at 70 MHz; 10.4 Mhz 3 dB passband, X6951M

(X252.8) which operates at 70 Mhz; 9.2 MHz 3 dB passband;

and X6958M which operates at 70 MHz, 6.3 MHz 3 dB pass-

band, and 2) A four–leaded TO–39 metal can package.

Typical insertion loss in a wide bandpass SAW filter is 25 dB.

The above SAW filters require source and load impedances of

50 Ω to assure stable operation. On the PC board layout,

space is provided to add a matching network, such as a 1:4

surface mount transformer between the SAW filter output and

the input to the ML13155. A 1:4 transformer, made by

Coilcraft and Mini Circuits, provides a suitable interface (see

Figures 16, 17 and 18). In the circuit and layout, the SAW fil-

ter and the ML13155 are differentially configured with inter-

connect traces which are equal in length and symmetrical.

This balanced feed enhances RF stability, phase linearity, and

noise performance.