LT5558 데이터 시트보기 (PDF) - Linear Technology
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LT5558 Datasheet PDF : 16 Pages
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LT5558
APPLICATIONS INFORMATION
the LO input and the internal in-phase LO and quadra-
ture LO signals is fixed, 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 significantly 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 PLO < –2dBm, the
gain, OIP2, OIP3, dynamic-range (in dBc/Hz) and image
rejection will degrade, especially at TA = 85°C.
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
S11 on the LO port can be improved at lower frequencies
by adding a shunt capacitor.
Table 1. LO Port Input Impedance vs Frequency for EN = High
and PLO = 0dBm
FREQUENCY
(MHz)
INPUT IMPEDANCE (Ω)
S11
MAG ANGLE
500
50.5 + j10.3
0.101 81.3
600
63.8 + j4.6
0.127 16.0
700
70.7 – j6.9
0.180 –15.2
800
70.7 – j20.3
0.237 –34.9
900
63.9 – j30.6
0.285 –50.5
1000
56.7 – j32.2
0.295 –61.4
1100
52.1 – j31.3
0.295 –69.1
1200
46.3 – j32.0
0.318 –78.0
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.
Table 2. LO Port Input Impedance vs Frequency for EN = Low
and PLO = 0dBm
FREQUENCY
(MHz)
INPUT IMPEDANCE (Ω)
S11
MAG ANGLE
500
37.3 + j43.4
0.464 79.7
600
72.1 + j74.8
0.545 42.1
700
184.7 + j77.8
0.630 11.7
800
203.6 – j120.8
0.696 –12.7
900
75.9 – j131.5
0.737 –32.6
1000
36.7 – j99.0
0.760 –48.8
1100
23.4 – j77.4
0.768 –62.4
1200
17.8 – j62.8
0.764 –74.3
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 PLO = 0dBm
FREQUENCY
(MHz)
OUTPUT IMPEDANCE (Ω)
S22
MAG ANGLE
500
22.8 + j4.9
0.380 165.8
600
30.2 + j11.4
0.283 141.9
700
42.7 + j12.9
0.159 111.8
800
53.7 + j3.0
0.045 37.2
900
52.0 – j10.1
0.101 –73.2
1000
44.8 – j15.2
0.168 –99.7
1100
39.1 – j15.1
0.206 –116.1
1200
35.7 – j13.1
0.224 –128.9
5558fa
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