lt5558 Linear Technology Corporation, lt5558 Datasheet - Page 10

• Manufacturer Part Number: lt5558
• Description: 600mhz To 1100mhz High Linearity Direct Quadrature Modulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT5558
the LO input and the internal in-phase LO and quadra-
ture LO signals is fi xed, and is independent of start-up
conditions. The phase shifters are designed to deliver
accurate quadrature signals for an LO frequency near
900MHz. For frequencies signifi cantly below 750MHz
or above 1.1GHz, the quadrature accuracy will diminish,
causing the image rejection to degrade. The LO pin input
impedance is about 50Ω and the recommended LO input
power window is –2dBm to + 2dBm. For P
gain, OIP2, OIP3, dynamic-range (in dBc/Hz) and image
rejection will degrade, especially at T
Harmonics present on the LO signal can degrade the image
rejection, because they introduce a small excess phase shift
in the internal phase splitter. For the second (at 1.8GHz)
and third harmonics (at 2.7GHz) at –20dBc level, the in-
troduced signal at the image frequency is about –61dBc
or lower, corresponding to an excess phase shift much
less than 1 degree. For the second and third harmonics at
–10dBc, still the introduced signal at the image frequency
is about –51dBc. Higher harmonics than the third will have
less impact. The LO return loss typically will be better than
10dB over the 750MHz to 1GHz range. Table 1 shows the
LO port input impedance vs. frequency. The return loss
S
by adding a shunt capacitor.
Table 1. LO Port Input Impedance vs Frequency for EN = High
and P
The input impedance of the LO port is different if the part
is in shutdown mode. The LO input impedance for EN =
Low is given in Table 2.
10
11
FREQUENCY
on the LO port can be improved at lower frequencies
(MHz)
LO
1000
1100
1200
500
600
700
800
900
= 0dBm
INPUT IMPEDANCE (Ω)
50.5 + j10.3
70.7 – j20.3
63.9 – j30.6
56.7 – j32.2
52.1 – j31.3
46.3 – j32.0
63.8 + j4.6
70.7 – j6.9
A
= 85°C.
LO
0.101
0.127
0.180
0.237
0.285
0.295
0.295
0.318
MAG
< –2dBm, the
S
11
ANGLE
–15.2
–34.9
–50.5
–61.4
–69.1
–78.0
81.3
16.0
Table 2. LO Port Input Impedance vs Frequency for EN = Low
and P
RF Section
After up-conversion, the RF outputs of the I and Q mixers are
combined. An on-chip balun performs internal differential
to single-ended output conversion, while transforming the
output signal impedance to 50Ω. Table 3 shows the RF
port output impedance vs frequency.
Table 3. RF Port Output Impedance vs Frequency for EN = High
and P
FREQUENCY
FREQUENCY
(MHz)
(MHz)
LO
LO
1000
1100
1200
1000
1100
1200
500
600
700
800
900
500
600
700
800
900
= 0dBm
= 0dBm
OUTPUT IMPEDANCE (Ω)
INPUT IMPEDANCE (Ω)
203.6 – j120.8
184.7 + j77.8
75.9 – j131.5
37.3 + j43.4
72.1 + j74.8
36.7 – j99.0
23.4 – j77.4
17.8 – j62.8
30.2 + j11.4
42.7 + j12.9
52.0 – j10.1
44.8 – j15.2
39.1 – j15.1
35.7 – j13.1
22.8 + j4.9
53.7 + j3.0
0.464
0.545
0.630
0.696
0.737
0.760
0.768
0.764
0.380
0.283
0.159
0.045
0.101
0.168
0.206
0.224
MAG
MAG
S
S
11
22
ANGLE
ANGLE
–116.1
–128.9
–12.7
–32.6
–48.8
–62.4
–74.3
165.8
141.9
111.8
–73.2
–99.7
79.7
42.1
11.7
37.2
5558fa