QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 889
900MHZ DIRECT CONVERSION QUADRATURE DEMODULATOR
LT5516
DESCRIPTION
Demonstration circuit 889 is a 900MHz Direct Con-
version Quadrature Demodulator featuring the
®
LT5516. The LT 5516 is an 800MHz to 1.5GHz direct
conversion quadrature demodulator optimized for
high linearity receiver applications including PA lin-
earization. It is suitable for communications receiv-
ers where an RF signal is directly converted into I
and Q baseband signals with bandwidth up to
260MHz. The LT5516 incorporates balanced I and Q
mixers, LO buffer amplifiers and a precision, high
frequency quadrature generator.
In an RF receiver, the high linearity of the LT5516
provides excellent spur-free dynamic range. This
direct conversion receiver can eliminate the need for
intermediate frequency (IF) signal processing, as
well as the corresponding requirements for image
filtering and IF filtering. Channel filtering can be per-
formed directly at the outputs of the I and Q chan-
nels. These outputs can interface directly to chan-
nel-select filters (LPFs) or to a baseband amplifier.
Demonstration circuit 889 is designed for operations
in the frequency range between 800MHz and 1.5GHz.
Design files for this circuit board are available.
Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation
Table 1. Typical Performance Summary (T
A
= 25°C)
PARAMETER
Supply Voltage
Supply Current
Maximum Shutdown Current
Frequency Range
LO Input Power
Conversion Gain
Noise Figure
Input 3 Order Intercept
Input 2 Order Intercept
Input 1dB Compression
Baseband Bandwidth
I/Q Gain Mismatch
I/Q Phase Mismatch
Output Impedance
LO to RF leakage
RF to LO Isolation
P
RF
= -5dBm, P
LO
= -5dBm, output frequency = 1MHz
P
RF
= -5dBm, P
LO
= -5dBm, output frequency = 1MHz
Differential
P
LO
= -5dBm
P
RF
= -10dBm
nd
rd
CONDITION (f
RF
= 900MHz, f
LO
=901MHz)
VCC = 5V, EN = High
VCC = 5V, EN = Low
VALUE
4V to 5.25V
117mA
20µA
800MHz to 1.5GHz
-13dBm to -2dBm
Voltage Gain, Load Impedance = 1k•••P
RF
= -10dBm, P
LO
= -5dBm
P
P
LO
= -5dBm, R1 = 8.2Ohm
2-Tone, -10dBm/Tone,
•f
= 200kHz, P
LO
= -5dBm, R1 = 3.3Ohm
f
2-Tone, -10dBm/Tone,
•f
= 200kHz, P
LO
= -5dBm, R1 = 3.3Ohm
f
P
LO
= -5dBm, R1 = 3.3Ohm
4.3dB
11.4dB
21.5dBm
51dBm
9dBm
260MHz
0.3dB
1°
120•
-65dBm
57dB
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 889
900MHZ DIRECT CONVERSION QUADRATURE DEMODULATOR
APPLICATION NOTE
FREQUENCY RANGE
Demonstration circuit 889 is optimized for the fre-
quency range from 800MHz to 1.5GHz, particularly for
the 900MHz Cellular band. This frequency range is
limited by the bandwidth of the RF and LO transform-
ers on the PCB. Outside this range, appropriate exter-
nal RF and LO matching transformers designed for the
intended frequency are required to maintain best per-
formance.
CURRENT CONSUMPTION
If lower power consumption is required, the LT5516’s
supply current can be reduced by increasing the DC
return resistor, R1. However, lowering supply current
will degrade linearity.
OUTPUT FILTERING
Proper filtering of unwanted high frequency mixing
products at the I- and Q- outputs is important to main-
tain superior linearity. The most convenient method is
to terminate each output with a shunt capacitor to
ground. The capacitor value should be optimized de-
pending upon the operating frequency. However, the
capacitors may reduce baseband output bandwidth.
QUICK START PROCEDURE
Demonstration circuit 889 is easy to set up to evaluate
the performance of the LT5516. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
NOTE:
a.
4.
5.
Set Signal Generator #1 to provide a 901MHz,
-5dBm, CW signal to the demo board LO input port.
Set the Signal Generators #2 and #3 to provide two -
10dBm CW signals to the demo board RF input
port—one at 899.9MHz, and the other at 900.1MHz.
Set the Spectrum Analyzer’s start frequency to
100kHz and stop frequency to 1400kHz. Perform
input 2
nd
order and 3
rd
order distortion measurement.
Sufficient spectrum analyzer input attenuation
should be used to avoid saturating the instrument.
IIP2 = P1 – P2 + Pin, IIP3 = (P1 – P3) / 2 + Pin.
Where P1 is the lowest power level of the two fun-
damental output tones at either 900kHz or 1100kHz,
P2 is the 2
nd
order product at 200kHz, P3 is the larg-
est 3
rd
order product at either 700kHz or 1300kHz,
and Pin is the input power (in this case, -10dBm).
All units are in dBm.
Voltage conversion gain can also be measured. But
beware that the gain is reduced by combiner loss
plus 6.85dB, because the load impedance to each
output pin is
50Ω
in this setup. Please refer to the
LT5516 data sheet for more detailed explanation
Use high performance signal generators with low
harmonic output for 2-tone measurements. Oth-
erwise, low-pass filters at the signal generator
outputs should be used to suppress higher-order
harmonics.
High quality combiners that provide 50-ohm ter-
mination on all ports and have good port-to-port
isolation should be used. Attenuators on the out-
puts of the signal generators are recommended to
further improve source isolation and to reduce re-
flection into the sources.
6.
b.
7.
1.
2.
3.
Connect all test equipment as shown in Figure 1.
Set the DC power supply’s current limit to 170mA,
and adjust output voltage to 5V.
Connect Vcc to the 5V DC supply, and then connect
EN to 5V; the demodulator is enabled (on).
8.
2
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