LTC5551
16
5551fa
For more information www.linear.com/LTC5551
applicaTions inForMaTion
IF Output
The IF amplifier, shown in Figure 7, has differential open-
collector outputs (IF
+
and IF
–
), and a pin for modifying the
internal bias (IFBIAS). The IF outputs must be biased at the
supply voltage (V
CC
), which is applied through matching
inductors L1 and L2. Alternatively, the IF outputs can be
biased through the center tap of a transformer. Each IF
output pin draws approximately 67mA of DC supply cur
-
rent (134mA total).
For the highest performance, high-Q
wire-wound chip inductors are recommended for L1 and
L2. Low cost multilayer chip inductors may be substituted,
with a slight degradation in performance.
Figure 7. IF Amplifier Schematic with
Transformer-Based Bandpass Match
Figure 8. IF Output Small-Signal Model
Table 3. IF Output Impedance vs Frequency
FREQUENCY (MHz)
DIFFERENTIAL OUTPUT
IMPEDANCE (R
IF
|| X
IF
(C
IF
))
90 954 || –j1442 (1.2pF)
140 950 || –j848 (1.2pF)
190 945 || –j681 (1.2pF)
240 942 || –j539 (1.2pF)
380 938 || –j338 (1.2pF)
456 926 || –j281 (1.2pF)
Transformer-Based Bandpass IF Matching
The IF output can be matched using the bandpass IF
matching shown in Figures 1 and 7. L1 and L2 resonate
with the internal IF output capacitance at the desired IF
frequency. The value of L1, L2 is calculated as follows:
L1, L2 = 1/[(2 π f
IF
)
2
• 2 • C
IF
]
where C
IF
is the internal IF capacitance (listed in Table 3).
Values of L1 and L2 are tabulated in Figure 1 for various
IF frequencies.
For IF Frequency below 80MHz, the inductor values become
unreasonably high and the high pass impedance matching
network described in a later section is preferred, due to
its lower inductor values.
Table 4 summarizes the optimum IF matching inductor
values vs IF center frequency, to be used in the standard
downmixer test circuit shown in Figure 1. The inductor
values listed are less than the ideal calculated values
due to the additional capacitance of the 4:1 transformer.
Measured IF output return losses are shown in Figure 9.
For optimum single-ended performance, the differential IF
outputs must be
combined through an external IF trans-
former
or discrete IF balun circuit. The evaluation board
(see
Figures 1 and 2) uses a 4:1 ratio IF transformer for
impedance transformation and differential to single-ended
transformation. It is also possible to eliminate the IF trans
-
former and drive differential filters or amplifiers directly.
The
IF output impedance can be modeled as 950Ω in
parallel with 1.2pF at IF frequencies. An equivalent small-
signal model is shown in Figure 8. Frequency-dependent
differential IF output impedance is listed in Table 3. This
data is referenced to the package pins (with no external
components) and includes the effects of IC and package
parasitics.
15
14
IF
+
IF
–
R
IF
C
IF
LTC5551
5551 F08
4:1
T1
IF
OUT
V
CC
C8C9
L2L1
C4
141516
IF
AMP
BIAS
4mA
LTC5551
IFBIAS IF
–
IF
+
R3
(OPTION TO
REDUCE
DC POWER)
5551 F07
R2R1
V
CC
