W3030 Datasheet, PDF (12/22 Pages) Agere Systems – W3030 3 V Dual-Mode IF Cellular Receiver

W3030 3 V Dual-Mode IF Cellular Receiver
Quadrature Detector (continued)
Data Sheet
April 1999
40 kΩ
dc
QUAD
PIN 3
150 pF
18 pF 4 pF—25 pF 680 µF
R
0.1 µF
Figure 5. L/C Tank Equivalent Circuit
f1= ¹ =
1
= 450 kHz
( ) ( ) ^{2π LC 2π 680 * 6−10 * 184 * 10−12}
( ) ^{( )} ( ) Q = 2π * f * RC = 2π * 450 * 10³ * ^{40 * 103 * 33 * 103} * 184 * 10 ^−12 = 9.4
( ) 40 * 10³ + 33 * 10³
Equation (1)
Equation (2)
The W3030 evaluation board is designed with a 450 kHz IF frequency, as shown in our example. The Q of the
tank circuit is set to 10 by the external resistor.
Quad Tank S-Curves
One method of determining if the Q of the tank is too large or too small is to produce an S-curve of the quad
tank. An S-curve is a plot of the dc audio output voltage versus IF input frequency. With small deviations from
center frequency, there is a proportional change in the dc audio output voltage. The overall linearity of the curve
is determined by the Q of the tank circuit; therefore, the Q determines how much deviation is allowed before
distortion of the audio signal occurs. The L/C tank circuit has a shunt resistor to set the Q of the tank. The
procedure to produce these plots is as follows:
1. Remove the 450 kHz IF filter and drive the input of the limiting amplifier with a signal generator capable of
FM modulation.
2. Apply FM modulation and adjust the tank capacitor for maximum audio out and minimal distortion.
3. Remove the FM modulation and sweep the IF frequency above and below center frequency while monitoring
the dc voltage at the audio output.
The following S-curves were produced with the value of the quad tank resistor varied from 18 kΩ, to 30 kΩ, to
removing the resistor. The resistor value of 33 kΩ, which corresponds to a Q of 10, was chosen as the optimal
resistor value.
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Lucent Technologies Inc.