lt5568 Linear Technology Corporation, lt5568 Datasheet - Page 14

• Manufacturer Part Number: lt5568
• Description: 700mhz To 1050mhz High Linearity Direct Quadrature Modulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT5568-2
the same (but of opposite sign). The current will enter
the LT5568-2’s common-base stage, and will fl ow to the
mixer upper switches. This can be seen in Figure 1 where
the internal circuit of the LT5568-2 is drawn.
feedthrough and the image rejection performance will
14
After calibration when the temperature changes, the LO
Figure 14. LO Feedthrough and Image Rejection
vs Temperature after Calibration at 25°C
–50
–60
–70
–80
–90
–40
CALIBRATED WITH P
EN = High
V
f
f
BBI
BBQ
CC
LO FEED-
THROUGH
= 2MHz, 0°
–20
= 5V
= 2MHz, 90°
U
IMAGE REJECTION
TEMPERATURE (°C)
0
U
20
RF
= –10dBm
Figure 16. GSM EVM and Noise vs RF Output Power at 900MHz
f
f
P
LO
RF
40
LO
= f
= 900MHz
= 0dBm
W
BB
60
+ f
LO
55682 F14
4
2
0
5
3
1
80
–10
U
–8
GSM RF OUTPUT POWER (dBm)
–6
NOISE
–4
–2
change. This is illustrated in Figure 14. The LO feedthrough
and image rejection can also change as a function of the
baseband drive level, as is depicted in Figure 15. In Figure
16 the GSM EVM and noise performance vs RF output
power is drawn.
0
2
Figure 15. LO Feedthrough and Image Rejection
vs Baseband Drive Voltage after Calibration at 25°C
EVM
4
55682 F16
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
0
6
0
f
f
V
EN = High
–96
–98
–100
–102
–104
–106
25°C
BBI
BBQ
CC
P
I AND Q BASEBAND VOLTAGE (V
RF
= 2MHz, 0°
= 5V
= 2MHz, 90°
1
85°C
–40°C
25°C
2
25°C
f
f
P
LO
RF
–40°C
LO
= f
= 900MHz
= 0dBm
LOFT
3
BB
+ f
85°C
LO
–40°C
P-P, DIFF
4
85°C
IR
)
5
55682f