Linear Technology LT5572 User manual
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
1
LT5572
DESCRIPTION
Demonstration circuit 945 is an I/Q modulator
featuring the LT
®
5572. The LT
5572 is a 1.5GHz to
2.5GHz direct I/Q modulator designed for high
performance wireless applications, including wireless
infrastructure. It may also be configured as an image
reject upconverting mixer, by applying 90°phase-
shifted signals to the I and Q inputs.
A high-speed, internally matched LO amplifier drives
two double-balanced mixer cores, allowing the use of
a low power, single-ended LO source. It allows direct
modulation of an RF signal using differential
baseband I and Q signals. The LT
5572 supports
GSM, EDGE, CDMA, CDMA2000, W-CDMA, 64-QAM,
OFDM and other modulation formats.
The I/Q baseband inputs consist of voltage-to-current
converters that in turn drive double-balanced mixers.
The outputs of these mixers are summed and applied
to an on-chip RF transformer, which converts the
differential mixer signals to a 50Ωsingle-ended
output. The four balanced I and Q base-band input
ports are intended for DC coupling from a source with
a common-mode voltage level of about 0.5V. The
differential input impedance of the baseband inputs is
approximately 90KΩ. The input termination
impedance is easily set for a specific application
using external resistors. On the DC945
demonstration circuit, this is set to 100Ωdifferential,
making it ideally suited for current drive applications.
The LO path consists of an LO buffer with single-
ended input, and precision quadrature generators
which produce the LO drive for the mixers.
Demonstration circuit 945 is designed for an RF
output frequency range from 1.5GHz to 2.5GHz.
Design files for this circuit board are available. Call
the LTC factory.
LT is a registered trademark of Linear Technology Corporation.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
2
Table 1.
Typical Performance Summary (TA= 25°C)
PARAMETER CONDITION (f
BB
= 2MHz, f
LO
=2000MHz) VALUE
Supply Voltage 4.5V to 5.25V
Supply Current V
CC
= 5V, EN = High 120 mA
Maximum Shutdown Current V
CC
= 5V, EN = Low 230 µA
RF Frequency Range 1.5 to 2.5 GHz
Baseband Frequency Range DC to 460 MHz
LO Input Return Loss Z
0
= 50
Ω,
P
LO
= 0dBm 15 dB
RF Output Return Loss Z
0
= 50
Ω
13.5 dB
LO Input Power -10 to +5 dBm
LO Frequency Range 1.5 to 2.5 GHz
Conversion Gain
P
RF
= -10dBm, P
LO
= 0dBm, defined as P
RF
/P
BB
-2.5dB
Output 3
rd
Order Intercept 2-Tone, P
RF
= -10dBm/Tone,
∆
f = 100KHz, P
LO
= 0dBm
+21.6dBm
Output 2
nd
Order Intercept 2-Tone, P
RF
= -10dBm/Tone,
∆
f = 100KHz, P
LO
= 0dBm
+53.2 dBm
Output 1dB Compression P
LO
= 0dBm +9.3 dBm
LO leakage P
LO
= 0dBm -39.4 dBm
Image Rejection P
LO
= 0dBm -41.2 dBc
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
3
APPLICATION NOTE
RF FREQUENCY RANGE
No tuning is required for
o
peration over the RF
frequency range of 1.5 GHz to 2.5 GHz.
BASEBAND FREQUENCY RANGE
The baseband frequency range extends from DC to 460
MHz (3dB bandwidth).
LO TO RF LEAKAGE
The LT
®
5572 offers excellent LO to RF leakage
performance, typically -39.4 dBm.
The LO leakage may be further reduced by the
introduction of small differential DC offsets (V
BBIP
–
V
BBIM
,V
BBQP
– V
BBQM
) at the
baseband inputs, typically less than 10mV. These DC
offsets may be introduced by applying slightly different
DC bias voltages to each of the four 10KΩresistors
shown in Figure 1.
SIDETONE TO RF LEAKAGE
The LT
®
5572 also offers very good image rejection
(sidetone suppression) at the RF port. The image
rejection may be further enhanced by the introduction
of small differential phase and amplitude offsets at
the baseband inputs.
TEST EQUIPMENT AND SETUP
Refer to Figure 1 for proper measurement equipment
setup. Before performing measurements on the DUT, it
is very important to evaluate the test system perform-
ance to ensure that: 1) distortion-free input signals are
applied and 2) the spectrum analyzer internal distortion
is minimized. Follow the guidelines below to do this.
Use high performance signal generators with low har-
monic output (>75 dBc) for 2-tone measurements.
The signal generators must provide 2 equal amplitude
outputs in quadrature with one another. High quality
combiners that provide broadband 50 ohm termination
on all ports should be used. The combiners should
have good port-to-port isolation (>30 dB) to prevent
the signal generators from modulating each other and
generating intermodulation products. Attenuators on
the outputs of these generators can also be used to
increase the effective port-to-port isolation.
Spectrum analyzers can produce significant internal
distortion products if they are overdriven. Sufficient
spectrum analyzer input attenuation should be used to
avoid saturating the instrument. A typical input 3
rd
or-
der intercept point for a spectrum analyzer is +40 dBm,
with 20 dB input attenuation applied. This is more
than 10 dB above the intercept point of the DUT, and
should yield accurate 3
rd
order distortion results.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
4
QUICK START PROCEDURE
Demonstration circuit 945 is easy to set up to evaluate
the performance of the LT
®
5572. Refer to Figure 1 for
proper measurement equipment setup.
Follow the procedure below:
1.
Connect all test equipment as shown in Figure 1.
2.
Set the DC power supply’s current limit to 150mA,
and adjust output voltage to 5V.
3.
Connect Vcc to the 5V DC supply, and then connect
VCCEN to 5V; the modulator is enabled (on).
4.
Set the baseband bias supply’s output voltage to
0.5V.
5.
Set Signal Generator #1 to provide a 2000MHz,
0dBm, CW signal to the demo board LO input port.
6.
Set the Signal Generators #2 and #3 to provide two
-10dBm CW signals to the combiner ports - one at
2MHz, and the other at 2.1MHz. For both
generators, output B should lead output A by 90
degrees. These 2 signals must be in quadrature to
drive the demo board properly. This is most
conveniently accomplished by using signal
generators with dual outputs with adjustable phase.
An example is the
HP3326A shown in Figure 1.
7.
To measure 3
rd
order distortion and conversion gain,
set the Spectrum Analyzer start and stop
frequencies to 2001.8MHz and 2002.3MHz,
respectively. Sufficient spectrum analyzer input
attenuation should be used to avoid distortion in the
instrument.
8.
The 3
rd
order intercept point is equal to (P
1
– P
3
) / 2
+ P
1
, where P
1
is the average power level of the two
fundamental output tones at 2002MHz and
2002.1MHz; P
3
is the average power level of the two
3
rd
order products at 2001.9MHz and 2002.2MHz.
All units are in dBm.
9.
To measure 2
nd
order distortion, set the Spectrum
Analyzer start and stop frequencies to 2001MHz and
2005MHz, respectively. Sufficient spectrum
analyzer input attenuation should be used to avoid
distortion in the instrument.
10.
The 2
nd
order intercept point is equal to 2*P
1
– P
2
,
where P
1
is the power level of the fundamental
output tone at 2002MHz, P
2
is the 2
nd
order product
at 2004.1MHz. All units are in dBm.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 945
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
5
Figure 1.
Proper Measurement Equipment Setup
Dual Output HP3326A (or equivalent)
A (0º) 0°Combiner (Mini-Circuits ZFSC-2-1W or equivalent)
B (90º)
180° divider (Mini-Circuits T2-1T or equivalent)
180º 0º
Baseband bias supply
180º 0º 10Kohm resistor (4PL)
180° divider (Mini-Circuits T2-1T or equivalent)
A (0º)
0°Combiner (Mini-Circuits ZFSC-2-1W or equivalent
)
B (90º)
Dual Output HP3326A (or equivalent)
Spe
c
trum
Analyzer
Signal Gener
a
tor
2 A)
Signal Gener
a
tor
1
Signal Gener
a
tor
3
DC Power Supply
DC Power Supply
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