Q DACs where they become analog
signals. They are filtered to set the
bandwidth somewhere in the 1.5 to
28 MHz range. The signals are
amplified and fed to the mixer. The
transmitter is the direct conversion
type that upconverts the signals to
the desired output frequency. An
internal PLL synthesizer drives the
mixer. The output is then amplified to
+ 3 dBm that is sufficient to drive an
external power amplifier.
The dual receivers are at the
bottom of Figure 4. Three LNAs
accept inputs from the antenna by
way of separate tuned circuits or
filters. The mixer then downconverts
these signals to baseband. Again, a
direct conversion architecture is used
where the receiver PLL is set to the
receive frequency. The baseband
signals are amplified and low pass
filtered, amplified again, and fed to
the I and Q ADCs. Some DSP
operations occur on-chip before the
digital signals go to the LimeLight
interface bus for transport to the
external DSP/FPGA.
As with the AD9361, the
LMS7002M is fully programmable. It
uses the SPI interface to send digital
codes to set the operating
frequencies, bandwidths, gains, and
other parameters. The LMS7002M
also has an onboard 8051
microcontroller to aid in the
programming and configuration of
the chip.
The LMS7002M comes in an
11. 5 mm x 11. 5 mm QFN package
with 261 pins. It operates from a 1.8
volt supply.
Both of the radio chips make
designing complex wireless devices
relatively fast and easy. Because there
is so much circuitry inside, it lessens
the number of discrete external
components needed, while cutting
costs and minimizing printed circuit
board space.
Radio design with chips like
these becomes more of a
programming exercise than hardware
design. That is the nature of software-defined radios. NV
April 2014 55