Introduction; Architecture - Analog Devices AN-826 Application Note

INTRODUCTION

The purpose of this application note is to demonstrate the
Analog Devices, Inc. WiMAX 5 V transmit signal chain for
applications extending up to 2.7 GHz.
As the wireless communication industry moves toward higher
RF frequencies, and higher data rates through wider modula-
tion bandwidths, high performance linear transmit chains are
required. WiMAX wireless broadband networks reflect such a
trend. Deployment has started in the 2.5 GHz and 3.5 GHz bands
for point-to-point and point-to-multipoint fixed applications.
Data rates of up to 80 Mbps are achieved using wideband
orthogonal frequency division multiplexing (OFDM) modu-
lations.
802.16 WiMAX fixed or mobile standards are based on
N
2 -carrier OFDM modulation: 256 for 802.16d and 512 to
2048 for 802.16e. Each of the 2
with either a QPSK, a 16 QAM, or a 64 QAM data sequence.
The standards also support different signal bandwidths, from
1.25 MHz to 20 MHz to accommodate variable rates, although
the current profiles define channel bandwidths from 5 MHz to
10 MHz. The OFDM composite signal envelope amplitude can
exhibit significant peaks and valleys, with a modulation depth
close to 100% and peak-to-average ratio of about 10 dB. This
imposes severe linearity requirements on the transmit chain.
To address these challenges, direct conversion architecture has
been chosen. For this particular analysis, a full Tx signal chain,
starting from the baseband signal generation, up to the voltage
controlled amplifier and power detector functions (but excluding
the power amplifier) was evaluated. The primary focus is the
wireless broadband (WiBro) frequency band, 2.3 GHz to
2.4 GHz, used in Korea for the deployment of the 802.16d
(fixed) and 802.16e (mobile) standards. However, this signal
chain may also be used up to 2.7 GHz (see the AD9862,
ADL5373, ADL5330, ADF4153, and
for performance details).

ARCHITECTURE

The radio architecture is a direct upconversion, having the
following benefits: low number of parts, less mixing product
spurs, fewer filters, and lower current consumption.
In addition, the architecture requires only a single upconversion
operation, and thus one synthesizer. The large number of sub-
carriers within the WiMAX OFDM or orthogonal frequency
division multiple access (OFDMA) signal actually makes this
modulation quite sensitive to phase noise, as each of the N
subcarriers is modulated by the phase noise of the local
oscillator (LO). For this reason, it is important to minimize
the amount of phase error added onto the modulation.
N
subcarriers can be modulated
AD8362 data sheets
RF VGA
ADL5330
AD8362
RMS POWER
DETECTOR
This architecture includes a transmit DAC, a fixed gain IQ
modulator, an LO fractional-N synthesizer, an RF VGA, and an
rms power detector. Off-chip, low-pass filters are also required
at the DAC outputs to filter the images that lie at multiples of
the sampling frequency. The DAC and the synthesizer need a
3.3 V supply, but all other parts run off a 5 V single supply.
The specific parts used for the Tx signal chain are as follows:
•
AD9860/AD9862, 12-bit/14-bit, 128 MSPS sampling DAC,
SNR ≥ 70 dB
•
ADL5373, 3 GHz IQ modulator
•
ADF4153, 4 GHz LO fractional-N synthesizer
•
ADL5330, 2.7 GHz voltage controlled amplifier/
attenuator VGA
•
AD8362, 2.7 GHz rms power detector
Given the nature of the OFDM signal and the stringent error
vector magnitude (EVM) requirements imposed by the very
high data rates, these parts have been selected for their linearity
and noise performance of up to 2.7 GHz.
The following sections address each of the major functions
within this Tx signal chain, with a focus on the system design
rationales, implementation, and interfaces.
Rev. B | Page 3 of 16
IQ MODULATOR
ADL5373
0°
90°
FRACTIONAL-N
SYNTHESIZER
ADF4153
Figure 2. Direct Conversion Tx Chain
AN-826
AD9860/AD9862
MxFE
ADC
ADC
DAC
DAC