lt5557 Linear Technology Corporation, lt5557 Datasheet - Page 12

• Manufacturer Part Number: lt5557
• Description: 400mhz To 3.8ghz 3.3v High Linearity Downconverting Rf Mixer
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT5557
Transformer-Based Bandpass IF Matching
The standard evaluation board (shown in Figure 1) uses an
L-C bandpass IF matching network, with an 8:1 transformer
connected across the IF pins. The L-C network maximizes
mixer performance at the desired IF frequency. The
transformer performs impedance transformation and
provides a single-ended 50Ω output.
The value of L1 is calculated as:
where C
(listed in Table 3). The value of C3 is selected such that L1
falls on a standard value, while satisfying the desired IF
bandwidth. The IF bandwidth can be estimated as:
where R
the transformer and inductor loss, is approximately 200Ω.
Below 40MHz, the magnitude of the internal IF reactance
is relatively high compared to the internal resistance. In
this case, L1 (and C3) can be eliminated, and the 8:1
transformer alone is adequate for IF matching.
The LT5557 was characterized with IF frequencies of
70MHz, 140MHz, 240MHz, 360MHz and 450MHz. The
values of L1 and C3 used for these frequencies are
tabulated in Figure 1 and repeated in Figure 9. In all cases,
L1 is a high-Q 0603 wire-wound chip inductor, for highest
conversion gain. Low-cost multi-layer chip inductors can
be substituted, with a slight reduction in conversion gain.
The measured IF output return losses are plotted in
Figure 9.
12
L1 = 1/[(2πf
IF
BW
EFF
is the sum of C3 and the internal IF capacitance
, the effective IF resistance when loaded with
IF
= 1/(2πR
U
IF
)
2
EFF
• C
U
C
IF
IF
)
]
W
U
Discrete IF Balun Matching
For many applications, it is possible to replace the IF
transformer with the discrete IF balun shown in Figure 2.
The values of L1, L2, C6 and C7 are calculated to realize a
180 degree phase shift at the desired IF frequency and
provide a 50Ω single-ended output, using the equations
listed below. Inductor L3 is calculated to cancel the
internal 2.6pF capacitance. L3 also supplies bias voltage to
the IF
equate for L1, L2 and L3. C3 is a DC blocking capacitor.
L L
C C
L
1 2
3
6 7
,
,
+
=
pin. Low cost multilayer chip inductors are ad-
ω
X
=
=
–10
–20
–30
IF
IF
Figure 9. IF Output Return Loss with
Transformer-Based Bandpass Matching
0
ω
50
R
A: 70MHz, L1 = 270nH, C3 = 15pF
B: 140MHz, L1 = 180nH, C3 = 3.9pF
C: 240MHz, L1 = 82nH, C3 = 2.2pF
D: 360MHz, L1 = 47nH, C3 = 1.2pF
E: 450MHz, L1 = 39nH, C3 = 0pF
IF
A
IF
ω
•
•
B
IF
150
R
R
OUT
1
IF
IF FREQUENCY (MHz)
•
C
250
R
OUT
350
D
450
E
5557 G09
550
5557fa