QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
1
LT5568-2
DESCRIPTION
Demonstration circuit 1178 is an I/Q modulator featuring
the LT
®
5568-2. The LT
5568-2 is a 700MHz to 1.05GHz
direct I/Q modulator designed for high performance
wireless applications, including wireless infrastructure.
It is optimized for image rejection in the 850 to 960 MHz
frequency range. 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. It 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 50
Ω
single-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
100
Ω
, making them 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 1178 is designed for an RF output
frequency range from 700MHz to 1.05GHz.
Design files for this circuit board are available. Call
the LTC factory.
LT is a registered trademark of Linear Technology Corporation.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
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Table 1.
Typical Performance Summary (T
A
= 25°C)
PARAMETER CONDITION (f
BB
= 2MHz, f
LO
=850MHz) VALUE
Supply Voltage 4.5V to 5.25V
Supply Current V
CC
= 5V, EN = High 110 mA
Maximum Shutdown Current V
CC
= 5V, EN = Low 245 µA
RF Frequency Range 700 to 1050 MHz
Baseband Frequency Range DC to 380 MHz
LO Input Return Loss Z
0
= 50
Ω,
P
LO
= 0dBm 15 dB
RF Output Return Loss Z
0
= 50
Ω
18 dB
LO Input Power -10 to +5 dBm
LO Frequency Range 700 to 1050 MHz
Conversion Gain
P
RF
= -10dBm, P
LO
= 0dBm, defined as P
RF
/P
BB
-6.8 dB
Output 3
rd
Order Intercept 2-Tone, P
RF
= -10dBm/Tone,
∆
f = 100KHz, P
LO
= 0dBm
+22.9 dBm
Output 2
nd
Order Intercept 2-Tone, P
RF
= -10dBm/Tone,
∆
f = 100KHz, P
LO
= 0dBm
+59 dBm
Output 1dB Compression P
LO
= 0dBm +8.6 dBm
LO leakage P
LO
= 0dBm -43 dBm
Image Rejection P
LO
= 0dBm -52 dBc
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
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APPLICATION NOTE
RF FREQUENCY RANGE
No tuning is required for operation over the RF frequency
range of 700 MHz to 1.05 GHz.
BASEBAND FREQUENCY RANGE
The baseband frequency range extends from DC to 380
MHz (3dB bandwidth).
LO TO RF LEAKAGE
The LT
®
5568-2 offers excellent LO to RF leakage
performance, typically -43 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, for example, using the DC
offset points shown in figure 1.
SIDETONE TO RF LEAKAGE
The LT
®
5568-2 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 performance to
ensure that: 1) distortion-free input signals are applied
and 2) the spectrum analyzer internal distortion is mini-
mized. 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 intermodula-
tion products. Attenuators on the outputs of these gen-
erators can also be used to increase the effective port-to-
port isolation.
Spectrum analyzers can produce significant internal dis-
tortion products if they are overdriven. Sufficient spec-
trum analyzer input attenuation should be used to avoid
saturating the instrument. A typical input 3
rd
order inter-
cept 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.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
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QUICK START PROCEDURE
Demonstration circuit 1178 is easy to set up to evaluate
the performance of the LT
®
5568-2. 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 Signal Generator #1 to provide a 900MHz, 0dBm,
CW signal to the demo board LO input port.
5.
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.
6.
To measure 3
rd
order distortion and conversion gain,
set the Spectrum Analyzer start and stop frequencies to
901.8MHz and 902.3MHz, respectively. Sufficient
spectrum analyzer input attenuation should be used to
avoid distortion in the instrument.
7.
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 902MHz and 902.1MHz;
P
3
is the average power level of the two 3
rd
order
products at 901.9MHz and 902.2MHz. All units are in
dBm.
8.
To measure 2
nd
order distortion, set the Spectrum
Analyzer start and stop frequencies to 904MHz and
906MHz, respectively. Sufficient spectrum analyzer
input attenuation should be used to avoid distortion in
the instrument.
9.
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 902MHz, P
2
is the 2
nd
order product at
904.1MHz. All units are in dBm.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
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Figure 1.
Proper Measurement Equipment Setup
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
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