13
LT5521
5521f
APPLICATIO S I FOR ATIO
WUUU
Table 6. Matching Values Using M/A-COM ETC1.6-4-2-3
Output Transformer
f
OUT
L1, L2 C3 C12 ∆f (10dB RL)
2.4GHz 0nH 82pF 82pF 450MHz
2.2GHz 1nH 82pF 82pF 430MHz
2.0GHz 2.7nH 82pF 82pF 400MHz
1.7GHz 4.7nH 82pF 82pF 400MHz
1.3GHz 10nH 82pF 82pF 400MHz
1.0GHz 10nH 3.9pF 1nF 500MHz
For applications with LO and output frequencies below
1GHz, the M/A-COM MABAES0054 is recommended for
the output component T2. This transformer maintains
better low frequency output symmetry. Table 7 lists com-
ponents necessary for a 750MHz output match using the
M/A-COM MABAES0054.
Table 7. Matching Values Using M/A-COM MABAES0054
Output Transformer
f
OUT
L1, L2 C3 C12 ∆f (10dB RL)
750MHz 33nH 82pF 1nF 500MHz
Hybrid baluns provide a low cost alternative for differen-
tial to single-ended conversion. The critical performance
parameters of conversion gain, IIP3, noise figure and LO
suppression are largely unaffected by these transform-
ers. However, their limited bandwidth and reduced sym-
metry outside the frequency of operation degrades the
suppression of higher order LO harmonics, particularly
2xLO. Murata LBD21 series hybrid balun transformers,
for ex
ample, can be used for output frequencies as low as
840MHz and as high as 2.4GHz.
Figure 10. Output Return Loss vs Frequency
FREQUENCY (GHz)
0.7
–10
–5
1GHz
5
2.2
5521 F10
–15
–20
1.2 1.7
–25
–30
0
RETURN LOSS (dB)
2GHz
Johanson Technology supplies the 3700BL15B100S hy-
brid balun for use between 3.4GHz and 4GHz. With addi-
tional matching, this transformer can be used for
applications between 3.3GHz and 3.7GHz. Example LT5521
performance is shown in Figure 11.
FREQUENCY (GHz)
3.2
10
8
6
4
2
0
–2
–4
22
IIP3
NF
HS
HS
HS
G
C
20
18
16
14
12
10
8
3.5 3.7
5521 F11
3.3 3.4
3.6 3.8
CONVERSION GAIN (dB)
IIP3 (dBm) AND NOISE FIGURE (dB)
LS
LS
LS
T
A
= 25°C
f
IF
= 300MHz
Figure 11. LT5521 Performance for an Application Tuned to
3.5GHz with Low Side (LS) and High Side (HS) LO Injection
LO Interface
The LO input pin is internally matched to 50Ω. It has an
internal DC bias of 960mV. External AC coupling is re-
quired. Figure 12 shows a simplified schematic of the LO
input. Overdriving the LO input will dramatically reduce
the performance of the mixer. The LO input power should
not exceed +1dBm for normal operation. Select C1 (Figure
12) only large enough to achieve the desired LO input
return loss. This reduces external low frequency signal
amplification through the LO buffer.
For applications with LO frequency in the range of 2.1GHz
to 2.4GHz, the LT5521 achieves improved distortion and
Figure 12. Simplified LO Input Circuit
15
V
CC
8Ω
C1
5521 F12
60Ω
LT5521
LO
IN
50Ω
60Ω
