LT5528 LINER [Linear Technology], LT5528 Datasheet - Page 14

• Manufacturer Part Number: LT5528
• Description: 1.5GHz to 2.4GHz High Linearity Direct Quadrature Modulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LT5528
internal circuit of the LT5528 is drawn. For best results,
a high ohmic source is recommended; for example, the
interface circuit drawn in Figure 3, modifi ed by pulling
resistors R5 and R6 to a –5V supply and adjusting their
values to 550Ω, with T1 omitted.
Another method to reduce current mismatch between
the currents fl owing in the BBIP and BBIM pins (or the
BBQP and BBQM pins) is to use a 1:1 transformer with
the two windings in the DC path (T1 in Figure 3). For DC,
the transformer forms a short, and for AC, the transformer
will reduce the common-mode current component, which
forces the two currents to be better matched. Alternatively,
a transformer with 1:2 impedance ratio can be used, which
gives a convenient DC separation between primary and
14
Figure 17: LO Feed-Through and Image Rejection vs Temperature
after Calibration at 25°C
–50
–55
–60
–65
–70
–75
–80
–85
–40
EN = HIGH
V
f
f
BBI
BBQ
IMAGE REJECTION
CALIBRATED WITH P
CC
= 2MHz, 0°
= 5V
U
= 2MHz, 90°
–20
TEMPERATURE (°C)
0
LO FEED-THROUGH
Figure 19: RF Two-Tone Power, IM2 and IM3 at 2140MHz vs Baseband Voltage
U
f
f
P
LO
RF
20
LO
RF
= f
= 2.14GHz
= 0dBm
= –10dBm
BB
40
+ f
LO
60
W
5528 F18
80
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
I AND Q BASEBAND VOLTAGE (V
0
0.1
U
EN = HIGH
V
f
f
IM2 = POWER AT f
IM3 = MAX POWER AT f
BBI
BBQ
CC
f
LO
= 2MHz, 2.1MHz, 0°
= 5V
= 2MHz, 2.1MHz, 90°
+ 2.2MHz
LO
+ 4.1MHz
P
1
RF
LO
secondary in combination with the required impedance
match. The secondary center tap should not be connected,
which allows some voltage swing if there is a single-
ended input impedance difference at the baseband pins.
As a result, both currents will be equal. The disadvantage
is that there is no DC coupling, so the LO feed-through
calibration cannot be performed via the BB connections.
After calibration when the temperature changes, the LO
feed-through and the Image Rejection performance will
change. This is illustrated in Figure 17. The LO feed-through
and Image Rejection can also change as a function of the
baseband drive level, as is depicted in Figure 18. The RF
output power, IM2 and IM3 vs a two-tone baseband drive
are given in Figure 19.
+ 1.9MHz OR
P-P, DIFF
Figure 18: LO Feed-Through and Image Rejection vs Baseband
Drive Voltage after Calibration at 25°C
f
f
P
LO
RF
LO
= f
= 2.14GHz
= 0dBm
EACH TONE)
IM2
BB
IM3
–40°C
25°C
85°C
+ f
5528 F19
LO
10
–20
–30
–40
–50
–60
–70
–80
–90
0
EN = HIGH
V
f
f
BBI
BBQ
CC
I AND Q BASEBAND VOLTAGE (V
= 2MHz, 0°
= 5V
= 2MHz, 90°
1
P
2
RF
f
f
P
LO
RF
LO
= f
= 2.14GHz
= 0dBm
BB
3
+ f
LO
P-P, DIFF
4
LOFT
–40°C
IR
25°C
85°C
5528 F18
)
5
10
0
–10
–20
–30
–40
–50
–60
5528f