U2790B_06 Datasheet, PDF (13/16 Pages) ATMEL Corporation – 1000-MHz Quadrature Modulator

U2790B
8. Application Notes
8.1 Noise Floor and Settling Time
In order to reduce noise on the power-up control input and improve the wide-off noise floor of the
900-MHz RF output signal, capacitor C_PU should be connected from pin 6 to ground in the
shortest possible way.
The settling time has to be considered for the system under design. For GSM applications, a
value of C_PU = 1 nF defines a settling time, t_sPU, equal or less than 3 ms. This capacitance does
not have any influence on the noise floor within the relevant GSM mask. For mobile applications
the mask requirements can be achieved very easily without C_PU.
A significant improvement of the wide-off noise floor is obtainable with C_PU greater than 100 nF.
Such values are recommended for applications where the settling time is not critical such as in
base stations. Coupling capacitors for LO_i and RF_O also have a certain impact on the settling
time. The values used for the measurements are CLO_i = 100 pF and C_RFo = 1 nF.
8.2 Baseband Coupling
The U2790B-FP (SO16) has an integrated biasing network which allows AC coupling of the
baseband signal at a low count of external components. The bias voltage is 2.5V ±0.15V.
Figure 7-17 shows the baseband input circuitry with a resistance of 3.2 kΩ for each asymmetric
input. The internal DC offset between A and A, and B and B is typically < ±1 mV with a maximum
of ±3 mV. DC coupling is also possible with an external DC voltage of 2.5 ±0.15V.
Figure 8-1. Baseband Input Circuitry
Mixer input stage
3.2 kΩ
A_, B
A_, B
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4583D–CELL–07/06