QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 966
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
LT5568
DESCRIPTION
Demonstration circuit 966 is an I/Q modulator
featuring the LT
®
5568. The LT 5568 is a
700MHz to 1.05GHz 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. 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 50Ω 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 100Ω,
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 966 is designed for an RF
output frequency range from 700MHz to
1.05GHz.
Design files for this circuit board are avail-
able. Call the LTC factory.
LT is a registered trademark of Linear Technology Corporation.
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 966
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
Table 1.
Typical Performance Summary (T
A
= 25°
C)
CONDITION (f
BB
= 2MHz, f
LO
=850MHz)
V
CC
= 5V, EN = High
V
CC
= 5V, EN = Low
VALUE
4.5V to 5.25V
113 mA
230 µA
700 to 1050 MHz
DC to 380 MHz
Z
0
= 50Ω, P
LO
= 0dBm
Z
0
= 50Ω
13 dB
30 dB
-10 to +5 dBm
700 to 1050 MHz
P
RF
= -10dBm, P
LO
= 0dBm, defined as P
RF
/P
BB
2-Tone, P
RF
= -10dBm/Tone,
∆f
= 100KHz, P
LO
= 0dBm
2-Tone, P
RF
= -10dBm/Tone,
∆f
= 100KHz, P
LO
= 0dBm
P
LO
= 0dBm
P
LO
= 0dBm
P
LO
= 0dBm
-6.8 dB
+22.9 dBm
+63 dBm
+8.3 dBm
-43 dBm
-46 dBc
PARAMETER
Supply Voltage
Supply Current
Maximum Shutdown Current
RF Frequency Range
Baseband Frequency Range
LO Input Return Loss
RF Output Return Loss
LO Input Power
LO Frequency Range
Conversion Gain
Output 3 Order Intercept
Output 2 Order Intercept
Output 1dB Compression
LO leakage
Image Rejection
nd
rd
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 966
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
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 3800 MHz (3dB bandwidth).
LO TO RF LEAKAGE
The LT
®
5568 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 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 measure-
ments on the DUT, it is very important to evalu-
ate 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
harmonic output (>75 dBc) for 2-tone measure-
ments. 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 in-
ternal distortion products if they are overdriven.
Sufficient spectrum analyzer input attenuation
should be used to avoid saturating the instru-
ment. A typical input 3
rd
order intercept point for
a spectrum analyzer is +40 dBm, with 20 dB in-
put attenuation applied. This is more than 10 dB
above the intercept point of the DUT, and should
yield accurate 3
rd
order distortion results.
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 966
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
QUICK START PROCEDURE
Demonstration circuit 966 is easy to set up to
evaluate the performance of the LT
®
5568. Refer
to Figure 1 for proper measurement equipment
setup.
Follow the procedure below:
1.
2.
3.
851.9MHz and 852.2MHz.
dBm.
8.
All units are in
Connect all test equipment as shown in Figure
1.
Set the DC power supply’s current limit to
150mA, and adjust output voltage to 5V.
Connect Vcc to the 5V DC supply, and then
connect VCCEN to 5V; the modulator is
enabled (on).
Set Signal Generator #1 to provide a 850MHz,
0dBm, CW signal to the demo board LO input
port.
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.
To measure 3
rd
order distortion and conversion
gain, set the Spectrum Analyzer start and stop
frequencies to 851.8MHz and 852.3MHz,
respectively.
Sufficient spectrum analyzer
input attenuation should be used to avoid
distortion in the instrument.
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
852MHz and 852.1MHz; P
3
is the average
power level of the two 3
rd
order products at
9.
To measure 2
nd
order distortion, set the
Spectrum Analyzer start and stop frequencies
to 854MHz and 856MHz, respectively.
Sufficient spectrum analyzer input attenuation
should be used to avoid distortion in the
instrument.
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 852MHz, P
2
is the
2
nd
order product at 854.1MHz. All units are in
dBm.
4.
5.
6.
7.
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 966
HIGH LINEARITY DIRECT QUADRATURE MODULATOR
Figure 1. Proper Measurement Equipment Setup
Dual Output HP3326A (or equivalent)
A (0º)
Signal Generator
2
DC Power Supply
0°Combiner (Mini-Circuits ZFSC-2-1W or equivalent)
B (90º)
180° divider (Mini-Circuits T2-1T or equivalent)
180º
0º
Spectrum
Analyzer
Signal Generator
1
10Kohm resistor (4PL)
DC offset point (4PL)
180º
0º
180° divider (Mini-Circuits T2-1T or equivalent)
A (0º)
Signal Generator
3
0°Combiner (Mini-Circuits ZFSC-2-1W or equivalent
)
B (90º)
Dual Output HP3326A (or equivalent)
5
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