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LTC5544IUF#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC5544
10
5544f
Introduction
The LTC5544 consists of a high linearity passive double-
balanced mixer core, IF buffer amplifier, LO buffer ampli-
fier and bias/shutdown circuits. See the Block Diagram
section for a description of each pin function. The RF and
LO inputs are single-ended. The IF output is differential.
Low side or high side LO injection can be used. The
evaluation circuit, shown in Figure 1, utilizes bandpass IF
output matching and an IF transformer to realize a 50Ω
single-ended IF output. The evaluation board layout is
shown in Figure 2.
applicaTions inForMaTion
Figure 2. Evaluation Board Layout
RF Input
The mixer’s RF input, shown in Figure 3, is connected to
the primary winding of an integrated transformer. A 50Ω
match is realized with a series capacitor (C1) and a shunt
inductor (L4). The primary side of the RF transformer
is DC-grounded internally and the DC resistance of the
primary is approximately 2.4Ω. A DC blocking capacitor
is needed if the RF source has DC voltage present.
The secondary winding of the RF transformer is internally
connected to the passive mixer. The center-tap of the
transformer secondary is connected to Pin 3 (CT) to allow
the connection of bypass capacitor, C2. The value of C
2 is
LO
frequency-dependent and can be tuned for better LO
leakage performance. When used, C2 should be located
within 2mm of Pin 3 for proper high frequency decoupling.
The nominal DC voltage on the CT pin is 1.2V.
5544 F02
For the RF input to be matched, the LO input must
be driven. A broadband input match is realized with
C1 = 0.6pF and L4 = 2.2nH. The measured RF input return
loss is shown in Figure 4 for LO frequencies of 4.4GHz,
5GHz and 5.6GHz. These LO frequencies correspond to
the lower, middle and upper values of the LO range. As
shown in Figure 4, the RF input impedance is somewhat
dependent on LO frequency.
The RF input impedance and input reflection coefficient,
versus RF frequency, is listed in Table 1. The reference
plane for this data is Pin 2 of the IC, with no external
matching, and the LO is driven at 5GHz.
LTC5544
C2
RF
IN
CT
RF
TO MIXER
2
3
5544 F03
L4
C1
Figure 3. RF Input Schematic
Figure 4. RF Input Return Loss
RF FREQUENCY (GHz)
4.0 4.2
35
5
0
4.4 6.04.6 4.8 5.0 5.2 5.4 5.6 5.8
5544 F04
RF PORT RETURN LOSS (dB)
15
10
20
25
30
LO = 4.4GHz
LO = 5GHz
LO = 5.6GHz
LTC5544
11
5544f
applicaTions inForMaTion
Table 1. RF Input Impedance and S11
(at Pin 2, No External Matching, LO Input Driven at 5GHz)
FREQUENCY
(GHz)
INPUT
IMPEDANCE
S11
MAG ANGLE
4.0 85.8 + j54.1 0.44 34.8
4.2 89.2 + j45.6 0.41 31.2
4.4 90.9 + j41.3 0.40 29
4.6 95.9 + j33.6 0.38 23.2
4.8 91.4 + j17.1 0.31 15.6
5.0 72.9 + j10.7 0.21 20.1
5.2 66.7 + j24.1 0.25 43.6
5.4 70.8 + j29.1 0.29 40.9
5.6 73.1 + j26.2 0.28 36.6
5.8 69.2 + j23.9 0.25 39.9
6.0 67.3 + j25.7 0.26 43.7
LO Input
The mixer’s LO input circuit, shown in Figure 5, consists
of a balun transformer and a two-stage high speed limiting
differential amplifier to drive the mixer core. The LTC5544’s
LO amplifiers are optimized for the 4.2GHz to 5.8GHz
LO frequency range. LO frequencies above or below this
frequency range may be used with degraded performance.
The mixer’s LO input is directly connected to the primary
winding of an integrated transformer. A 50Ω match is
realized with a series 1.2pF capacitor (C3). Measured LO
input return loss is shown in Figure 6.
The LO amplifiers are powered through V
CC1
and V
CC2
(Pin 5 and Pin 7). When the chip is enabled (SHDN =
Figure 5. LO Input Schematic
low), the internal bias circuit provides a regulated 4mA
current to the amplifier’s bias input, which in turn causes
the amplifiers to draw approximately 90mA
of DC current.
This
4mA reference current is also connected to LOBIAS
(Pin 6) to allow modification of the amplifier’s DC bias
current for special applications. The recommended ap-
plication circuits require no LO amplifier bias modification,
so this pin should be left open-circuited.
The nominal LO input level is +2dBm although the limiting
amplifiers will deliver excellent performance over a ±3dB
input power range. LO input power greater than +5dBm
may be used with slightly degraded performance.
The LO input impedance and input reflection coefficient,
versus frequency, is shown in Table 2.
Table 2. LO Input Impedance vs Frequency
(at Pin 10, No External Matching)
FREQUENCY
(GHz)
INPUT
IMPEDANCE
S11
MAG ANGLE
4.0 22.7 + j14.7 0.42 140.2
4.2 24.4 + j18.6 0.41 129.9
4.4 28.2 + j22.5 0.39 118.1
4.6 33.2 + j25.3 0.35 106.7
4.8 39.7 + j26.4 0.30 95
5.0 47.4 + j24.3 0.24 82.1
5.2 52.2 + j16.9 0.16 73.3
5.4 52 + j9.4 0.09 72.7
5.6 49.9 + j3.8 0.04 88.8
5.8 47.7 – j1 0.03 –156.5
6.0 44.2 – j6.2 0.09 –129.4
Figure 6. LO Input Return Loss
LO
IN
V
CC1
V
CC2
LO BUFFER
TO
MIXER
LTC5544
LO
LOBIAS
5544 F05
10
7
5
BIAS
6
C3
4mA
LO FREQUENCY (GHz)
4.0 4.2
5
0
4.4 6.04.6 4.8 5.0 5.2 5.4 5.6 5.8
5544 F06
LO PORT RETURN LOSS (dB)
15
10
20
25
30
LTC5544
12
5544f
applicaTions inForMaTion
IF Output
The IF amplifier, shown in Figure 7, has differential
open-collector outputs (IF
+
and IF
–
), a DC ground return
pin (IFGND), and a pin for modifying the internal bias
(IFBIAS). The IF outputs must be biased at the supply
voltage (V
CCIF
), which is applied through matching induc-
tors L1 and L2. Alternatively, the IF outputs can be biased
through the center tap of a transformer. The common
node of L1 and L2 can be connected to the center tap of
the transformer. Each IF output pin draws approximately
49mA of DC supply current (98mA total). IFGND (Pin 13)
must be grounded or the amplifier will not draw DC current.
For the highest conversion gain, high-Q wire-wound chip
inductors are recommended for L1 and L2, especially when
using V
CCIF
= 3.3V. Low cost multilayer chip inductors may
be substituted, with a slight degradation in performance.
Grounding through inductor L3 may improve LO-IF and
RF-IF leakage performance in some applications, but is
otherwise not necessary. High DC resistance in L3 will
reduce the IF amplifier supply current, which will degrade
RF performance.
Figure 7. IF Amplifier Schematic with
Transformer-Based Bandpass Match
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 332Ω in
parallel with 1.7pF 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.
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
))
90 351 || –j707 (2.5pF)
140 341 || –j494 (2.3pF)
190 334 || –j441 (1.9pF)
240 332 || –j390 (1.7pF)
300 325 || –j312 (1.7pF)
380 318 || –j246 (1.7pF)
456 304 || –j205 (1.7pF)
Transformer-Based Bandpass IF Matching
The IF output can be matched for IF frequencies as low
as 40MHz, or as high as 500MHz, 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
15
14
IF
+
IF
–
R
IF
C
IF
LTC5544
5544 F08
4:1
T1
IF
OUT
V
CC
C10
L2L1
C8
L3
(OR SHORT)
V
CCIF
13141516
IF
AMP
BIAS
98mA
4mA
IFGND
LTC5544
IFBIAS IF
–
IF
+
R1
(OPTION TO
REDUCE
DC POWER)
5544 F07
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
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LTC5544IUF#PBF

Mfr. #:
Buy LTC5544IUF#PBF
Manufacturer:
Analog Devices Inc.
Description:
RF Mixer 4GHz - 6GHz High Dynamic Range Downconverting Mixer
Lifecycle:
New from this manufacturer.
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