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:
(2)
fc = (2π
√(LC
p))–1
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(2πfl)
RT = 5(2π)(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.
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