DN418 - High Linearity Components Simplify Direct Conversion Receiver Designs

High Linearity Components Simplify Direct Conversion
Receiver Designs – Design Note 418
Cheng-Wei Pei
Introduction
A direct conversion radio receiver takes a high frequency
input signal, often in the 800MHz to 3GHz frequency range,
and utilizes one mixer/demodulator stage to convert the
signal to baseband without going through an intermediate
frequency (IF) stage. The resulting low frequency (baseband) signal spectrum has useful information at frequencies from DC to typically a few tens of MHz. Designing
these receivers requires the use of very high performance
analog ICs. High performance direct conversion radio
receiver signal chains for applications such as cellular
infrastructure and RFID readers require high linearity,
low noise figure (NF), and good matching between the
in-phase and quadrature (I and Q) channels.
The Right Components for the Job
Linear Technology’s LT® 5575 direct conversion demodulator has a combination of excellent linearity and noise
performance. The most important linearity specification
for direct conversion mixers is the 2nd order intercept
point (IIP2) due to the 2nd order distortion product falling
within the baseband output spectrum, and the LT5575
boasts 54.1dBm at 900MHz (60dBm at 1900MHz). The
LT5575 also has high 3rd order linearity and a low noise
figure of 12.8dB.
The LTC® 6406 is a fully differential amplifier with low noise
(1.6nV/√Hz at the input) and high linearity (+44dBm OIP3
at 20MHz) in a small 3mm × 3mm QFN package. External
resistors set the gain, giving the user maximum design
flexibility. The low power consumption (59mW with a 3.3V
supply) means that using two amplifiers for I and Q has
minimal effect on the system power budget. The LTC6406
maintains high linearity up to 50MHz, which is perfect for
WCDMA receivers and other wideband applications.
A Basic Receiver Design
One common design challenge when using active demodulators is level-shifting the outputs, which can have a DC
level close to VCC, to a usable DC level within the input
range of the analog-to-digital converter (ADC). Fortunately, the LTC6406’s rail-to-rail inputs make interfacing
with the outputs of the LT5575 simple and direct. The
LTC6406 also includes an extra feedback loop (controlled
, LTC, LT and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
C5
1.8pF
5V
LT5575
5V
5pF
I
RF IN
900MHz
0dBm
65Ω
DIFF OUTPUT Z
130Ω 2.5pF
65Ω
5pF
65Ω
5V
C1
10pF
65Ω
IDENTICAL
Q CHANNEL
DC LEVEL
1.25V
3.3V
R1
75Ω
R3
75Ω
R2
75Ω
C3
R4
12pF 75Ω
+
LTC6406
–
C2
10pF
R7
49.9Ω
R8
49.9Ω
C8
22pF
R9
10Ω +10dBm
C6 R10
22pF 10Ω
3.3V
1/2 LTC2299
DUAL 14-BIT ADC
C7
22pF
VOCM
1.25V
R6
475Ω
5pF
GAIN: 0dB
INPUT NF: 12.8dB
OIP3: 31dBm
R5
475Ω
DC LEVEL
3.9V
5pF
0dBm
5V
Q
5V
DC LEVEL
3.3V
C4
1.8pF
SYSTEM SPECS
NOISE FIGURE: 19dB
OIP3: 39dB
GAIN: 10dB
DN418 F01
80MHz
SAMPLE
CLOCK
GAIN: +10dB
INPUT NF: 18dB
OIP3: 44dBm
Figure 1. The LT5575 Demodulator and LTC6406 Amplifier Driving an LTC2299 14-Bit ADC.
System Bandwidth is Approximately 40MHz. Overall System OIP3 was Measured to be 39dBm
06/07/418
by an external VOCM voltage) that independently sets the
output common mode DC level, regardless of the input
voltage.
Figure 1 shows a basic receiver circuit with the LT5575
demodulator and the LTC6406 followed by an LTC2299
14-bit ADC. An RC lowpass filter at the output of the
demodulator filters undesired out-of-band signals, and
another RC lowpass filter before the ADC antialiases and
limits noise bandwidth. The DC voltage at the LTC6406
inputs is 3.3V, the same as the supply voltage.
Adding Free Gain to the System
For signal chains that require more gain, the LTC6401-8
differential amplifier/ADC driver is a good complement
to the LT5575 and LTC6406. The LTC6401-8 has higher
linearity (50dBm OIP3 at 20MHz) and 2.5nV/√Hz of input
noise in a 3mm × 3mm QFN package. It contributes gain
and linearity without significantly impacting the noise
figure. Figure 2 adds the LTC6401-8 (also available in
14dB, 20dB and 26dB flavors) to the signal chain to
drive the LTC2299. The higher linearity of the LTC6401-8
increases the combined system OIP3 to 45dBm. In addition, 8dB of gain is added with no significant degradation
to the noise figure. The 400Ω input impedance of the
LTC6401-8 is not a heavy load for the LTC6406, which
enables direct coupling of the two amplifiers with minimal
signal loss (from series resistors, etc.).
5V
5pF
I
RF IN
900MHz
–8dBm
65Ω
5pF
Q
DC LEVEL
3.9V
5pF
65Ω
65Ω
5V
DC LEVEL
3.3V
DIFF OUTPUT Z
130Ω 2.5pF
5V
–8dBm
5V
A concern when designing the LC network is the need to
preserve the I and Q gain/phase matching of the LT5575
(0.04dB/0.4° mismatch), which necessitates using low
tolerance LC components (±2% inductors and ±5% capacitors). The frequency response and group delay of the
system are almost entirely determined by the LC filter.
Conclusion
Signal chain devices that offer high linearity and excellent noise specifications can greatly simplify the design
of high frequency receivers—speeding up entire design
cycles.
C5
1.8pF
5V
LT5575
A More Selective Filter
There are three places where a filter can be implemented
in the circuit of Figure 2: after the mixer, in between the
two amplifier stages, and prior to the ADC. Each has its
trade-offs, but the simplest design places the filter after
the mixer. This topology attenuates unwanted signals
earlier in the signal chain, which preserves the IP3 of
the following stages and allows for more gain through
the system. An LC filter at the demodulator output
minimally affects the distortion and noise figure of the
system, whereas LC lowpass filters can present a heavy
load impedance to a feedback amplifier output near their
resonant frequencies. For reasons outside the scope of
this article, it is tricky to design LC networks at the input
of a high speed sampling ADC.
C1
100pF
C10
33pF
R1
150Ω
L2
680nH
C3
R2
33pF 150Ω
+
LTC6406
65Ω
IDENTICAL
Q CHANNEL
GAIN: 0dB
INPUT NF: 12.8dB
OIP3: 31dBm
DIFF OUTPUT Z
25Ω
–
3.3V
+2dBm
+
LTC6401-8
–
C9
33pF
R7
40.2Ω
R8
40.2Ω
3.3V
C8
22pF
R9
10Ω +10dBm
C6 R10
22pF 10Ω
1/2 LTC2299
DUAL 14-BIT ADC
C7
22pF
VOCM
1.25V
R6
475Ω
5pF
DC LEVEL
1.25V
3.3V
L1
680nH
C2
100pF
R5
475Ω
C4
1.8pF
VOCM
1.25V
DIFF
INPUT Z
400Ω GAIN: +8dB
INPUT NF: 15dB
OIP3: 50dBm
SYSTEM SPECS
NOISE FIGURE: 19dB
OIP3: 45dBm
GAIN: 18dB
DN418 F02
80MHz
SAMPLE
CLOCK
GAIN: +10dB
INPUT NF: 18dB
OIP3: 44dBm
Figure 2. The LT5575 Demodulator with a 20MHz Lowpass Filter followed by LTC6406 and LTC6401-8. System OIP3
is Measured to be 45dBm at 920MHz RF with a 900MHz Local Oscillator. System Noise Figure (NF) Adds up to
Approximately 19dB
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2525
www.linear.com
Linear Technology Corporation
dn418f LT/TP 0607 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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