According to Maxim Integrated Products,
the MAX2065 fully programmable,
multistate, analog and digital
IF/RF variable-gain amplifier (VGA)
aims to solve a number of automatic
gain control (AGC) design problems in
GSM/EDGE, CDMA, WCDMA, LTE, and
WiMAX receiver applications (see the
figure). But what does that mean?
In explaining the thinking that went
into the design of the chip, Maxim says
its engineers started by looking at today’s
gain trim and
AGC circuits
in those apps.
Typically, they
involve
a lot of separate
chips—analog
voltage and
digitalcontrolled
attenuators,
RF/IF amplifiers, and associated control
circuitry. In lieu of all that, Maxim’s engineers
packed a linearly controlled 31-dB
voltage-variable attenuator, a 31-dB
digital step-attenuator, a 22-dB gain
driver amplifier, an on-chip 8-bit control
digital-to-analog converter (DAC), and
a simple SPI-compatible interface into
one chip.
Designers can use the MAX2065 as
either an IF or RF all-purpose VGA,
interfacing directly with 50-O systems
operating anywhere between 50 and
1000 MHz. Each of the three independent
RF stages has its own RF input and
RF output, so the chip can be configured
to optimize either noise figure or
linearity.
If a designer were to follow the chip’s
digital attenuator with its driver amplifier,
thereby optimizing linearity, the
total gain range would be on the order
of 62 dB, with a maximum gain of 19.4
dB, and a 6.5-dB noise figure. The thirdorder
intercept would be +42 dBm,
second order, +63 dBm, and first order,
+19 dBm. Harmonic distortion would
be –67 dBc (second order) and –83 dBc
(third order).
The SPI can be used to control the
5-bit digital attenuator and the analog
attenuator through the on-chip 8-bit
DAC. With that combination, it’s possible
to adjust the analog attenuation in
0.12-dB increments. It’s also possible to
disable the DAC.
The MAX2065 is designed for fastattack
AGC functionality as well by
limiting amplitude overshoot/undershoot
during attenuation transitions
to a maximum of 0.05 dB between any
two attenuation states over an elapsed
time of only 40 ns. To capitalize on this capability, Maxim included a supplemental 5-bit, parallel-control interface that
avoids SPI latencies.
Beyond that, an even faster “rapid-fire” gain selection enables even faster toggling
among four preprogrammed attenuation steps. “Assume that the AGC application
requires a static attenuation adjustment to trim out gain inconsistencies within a
receiver lineup. The same AGC circuit may also be required to dynamically attenuate
an unwanted blocker signal that could de-sense the receiver and lead to an ADC overdrive
condition,” Maxim explains.
“In this example, the MAX2065 would be preprogrammed (through the SPI bus)
with two customized attenuation states: one state to address the static gain-trim
adjustment, and the other state to counter the unwanted blocker condition,” the company
continues.
“The user can also program two additional attenuation states by using the
STATE_B control bit as a second I/O pin. These additional attenuation settings are
useful for software-defined radio applications where multiple static gain settings may
be needed to account for different operating frequencies, or where multiple dynamic
attenuation settings are needed to account for different blocker levels (as defined by
multiple wireless standards),” Maxim says.
The MAX2065 is available in a 40-TQFN (thin quad flat no-lead) lead-free package.
Prices start at $7.48 in quantities of 1000 and up.
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MAXIM INTEGRATED PRODUCTS • www.maxim-ic.com