U2790B Datasheet, PDF (12/15 Pages) ATMEL Corporation – 1000-MHz QUADRATURE MODULATOR

Figure 19. Demo Board Layout
Application Notes
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 applica-
tions, 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 with-
out 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.
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 2.5 V±
0.15 V.
Figure 19 shows the baseband input circuitry with a resistance of 3.2 kW for each asym-
metric 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 ± 0.15 V.
12 U2790B
4583A–CELL–01/03