QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1048A
1.5GHZ TO 2.7GHZ DIRECT CONVERSION I/Q RECEIVER
1
LT5575
DESCRIPTION
Demonstration circuit 1048A is a 1.5GHz to 2.7GHz Di-
rect Conversion I/Q Receiver featuring the LT5575.
The LT
®
5575 is an 800MHz to 2.7GHz direct conversion
quadrature demodulator optimized for high linearity
receiver applications. It is suitable for communications
receivers where an RF signal is directly converted into I
and Q baseband signals with bandwidth up to 490MHz.
The LT5575 incorporates balanced I and Q mixers, LO
buffer amplifiers and a precision, high frequency quad-
rature phase shifter. The integrated on-chip broadband
transformers provide 50
Ω
ΩΩ
Ω
single-ended interfaces at
the RF and LO inputs. Only a few external capacitors
are needed for its application in an RF receiver system.
Demonstration circuit 1048A is designed for evaluating
the LT5575 in the frequency range between 1.5GHz and
2.7GHz. It can be optimized for input frequencies below
1.5GHz by installing external shunt capacitors at RF in-
put and LO input ports. Refer to the LT5575 data sheet
for details.
Design files for this circuit board are available. Call
the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corp.
Table 1.
Typical Performance Summary (T
A
= 25°C, V
CC
= 5V, EN = High, f
RF
= 1900MHz (
∆
∆∆
∆
f = 200kHz for 2-tone IIP2 and IIP3 tests),
P
RF
= -10dBm (-10dBm/tone for 2-tone IIP2 and IIP3 tests), f
LO
= 1901MHz, P
LO
= 0dBm, unless otherwise noted.)
PARAMETER CONDITIONS VALUE
Supply Voltage
4.5V to 5.25V
Supply Current 132mA
Maximum Shutdown Current EN = Low 100µA
Output DC Offset Voltage
|
I
OUT
+
- I
OUT
-
|
,
|
Q
OUT
+
- Q
OUT
-
|
< 9mV
RF/LO Input Frequency Range
No External Matching (High Band)
With External Matching (Low Band, Mid Band)
1.5GHz to 2.7GHz
0.8GHz to 1.5GHz
Baseband Frequency Range DC to 490MHz
Baseband I/Q Output Impedance Single-Ended
65
Ω
// 5pF
RF/LO Input Return Loss Z
0
= 50
Ω
, 1.5GHz to 2.7GHz, Internally Matched > 10dB
LO Input Power -13dBm to 5dBm
Conversion Gain Voltage Gain, R
LOAD
= 1k
Ω
4.2dB
Noise Figure
Double-Side Band
12.7dB
Input 3
rd
-Order Intercept
2 RF tones, -10dBm/Tone,
∆
f = 200kHz
22.6dBm
Input 2
nd
-Order Intercept
2 RF tones, -10dBm/Tone,
∆
f = 200kHz
60dBm
Input 1dB Compression
11.2dBm
I/Q Gain Mismatch 0.01dB
I/Q Phase Mismatch 0.4°
LO to RF leakage -64.6dBm
RF to LO Isolation 57.1dB
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1048A
1.5GHZ TO 2.7GHZ DIRECT CONVERSION I/Q RECEIVER
2
APPLICATION NOTE
ABSOLUTE MAXIMUM INPUT RATING
Power Supply Voltage ..........................................5.5V
Enable Voltage .............................. –0.3V to V
CC
+ 0.3V
RF Input Power................................................ 20dBm
LO Input Power................................................ 10dBm
RF Input DC Voltage ...........................................±0.1V
LO Input DC Voltage ...........................................±0.1V
FREQUENCY RANGE
Demonstration circuit 1048A is designed for evaluating
the LT5575 in the frequency range between 1.5GHz and
2.7GHz. In this frequency range, both of the LT5575’s
RF and LO ports are internally matched to 50
Ω
ΩΩ
Ω
. Below
1.5GHz, a shunt capacitor to ground placed close to the
RF and LO pins of the IC can be used to provide imped-
ance matching to maintain best performance. Matching
capacitor values for different frequency bands are listed
in Table 2.
Table 2.
RF and LO match and Baseband Output Filtering
RF Match LO Match Baseband
Frequency Range
C10 C12 C1-C4
Low Band:
800 to 1000MHz
4.7pF 3.9pF 10pF
Mid Band:
1000MHz to 1500MHz
2pF 2pF 2.2pF
High Band:
1500MHz to 2700MHz
None None None
OUTPUT FILTERING
Proper filtering of unwanted high frequency mixing
products at the I and Q outputs is important to maintain
superior linearity. The most convenient method is to
terminate each output with a shunt capacitor to ground.
The capacitor value should be optimized depending
upon the operating frequency. However, the capacitors
may reduce baseband output bandwidth. Filtering ca-
pacitor values for different frequency bands are listed in
Table 2.
TEST EQUIPMENT AND SETUP
Refer to Figure 1 for proper measurement equipment
setup.
Use high performance signal generators with low
harmonic output for 2-tone measurements. Other-
wise, low-pass filters at the signal generator outputs
should be used to suppress higher-order harmonics.
High quality combiners that provide broadband 50Ω
ΩΩ
Ω
termination on all ports and have good port-to-port
isolation should be used. Attenuators on the outputs
of the signal generators are recommended to further
improve source isolation, to prevent the sources
from modulating each other and generating inter-
modulation products.
Spectrum analyzers can produce significant internal
distortion products if they are overdriven. Generally,
spectrum analyzers are designed to operate at their best
with about –30dBm to -40dBm at their input.
Sufficient
spectrum analyzer input attenuation should be used
to avoid saturating the instrument.
Before performing measurements on the DUT, the
system performance should be evaluated to ensure
that: 1) a clean input signal is obtained and 2) the
spectrum analyzer internal distortion is minimized.