LT5575
11
5575f
APPLICATIONS INFORMATION
The LT5575 is a direct I/Q demodulator targeting high
linearity receiver applications, such as RFID readers and
wireless infrastructure. It consists of RF transconductance
amplifi ers, I/Q mixers, a quadrature LO phase shifter, and
bias circuitry.
The RF signal is applied to the inputs of the RF
transconductance amplifi ers and is then demodulated
into I/Q baseband signals using quadrature LO signals
which are internally generated from an external LO source
by precision 90° phase-shifters. The demodulated I/Q
signals are single-pole low-pass fi ltered on-chip with a
–3dB bandwidth of 490MHz. The differential outputs of the
I-channel and Q-channel are well matched in amplitude;
their phases are 90° apart.
Broadband transformers are integrated on-chip at both
the RF and LO inputs to enable single-ended RF and LO
interfaces. In the high frequency band (1.5GHz to 2.7GHz),
both RF and LO ports are internally matched to 50Ω. No
external matching components are needed. For the lower
frequency bands (800MHz to 1.5GHz), a simple network
with series and/or shunt capacitors can be used as the
impedance matching network.
RF Input Port
Figure 4 shows the demodulator’s RF input which con-
sists of an integrated transformer and high linearity
transconductance amplifi ers. The primary side of the
transformer is connected to the RF input pin. The second-
ary side of the transformer is connected to the differential
inputs of the transconductance amplifi ers. Under no cir-
cumstances should an external DC voltage be applied to
the RF input pin. DC current fl owing into the primary side
of the transformer may cause damage to the integrated
transformer. A series blocking capacitor should be used
to AC-couple the RF input port to the RF signal source.
The RF input port is internally matched over a wide fre-
quency range from 1.5GHz to 2.7GHz with input return loss
typically better than 10dB. No external matching network is
needed for this frequency range. When the part is operated
at lower frequencies, however, the input return loss can
be improved with the matching network shown in Figure
4. Shunt capacitor C10 and series capacitor C11 can be
selected for optimum input impedance matching at the
desired frequency as illustrated in Figure 5. For lower fre-
quency band operation, the external matching component
C11 can serve as a series DC blocking capacitor.
Figure 5. RF Input Return Loss with External Matching
Figure 4. RF Input Interface
5575 F04
2
3
EXTERNAL
MATCHING
NETWORK FOR
LOW BAND AND
MID BAND
RF
INPUT
RF
C10
C11
TO I-MIXER
TO Q-MIXER
–30
–25
–20
–15
–10
–5
0
0.5
1.0
1.5 2.0 2.5 3.0
FREQUENCY (GHz)
RF PORT RETURN LOSS (dB)
5575 F05
NO EXTERNAL
MATCHING
C11 = 3.9pF;
NO SHUNT CAP
C11 = 5.6pF;
C10 = 4.7pF
