Rainbow Electronics MAX2701 - Manual (Page 23)

MAX2700/MAX2701
1.8GHz to 2.5GHz Direct Downconversion
Receivers
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to operate without degrading its performance over the
LO power range of -10dBm to -16dBm. The quadrature
generator consists of a wideband polyphase network.
Each output of the polyphase filter is buffered, ampli-
fied, and then fed to the mixer’s differential LO port.
Variable Gain Amplifier (VGA)
The AGC in each baseband channel I/Q is implement-
ed by two variable gain amplifiers with equal gains.
Each amplifier provides about 40dB voltage gain at the
maximum setting and 30dB of gain control. The first
baseband VGA (VGA1) is a cascaded wideband ampli-
fier with differential input and single-ended output. It is
optimized for low noise in the high-gain state and has
low-power dissipation and sufficient linearity in all gain
settings to ensure desired compression performance.
The second baseband VGA (VGA2) is a multistage
wideband amplifier with differential inputs and a single-
ended output. In each channel, connect a baseband
lowpass filter between VGA1 and VGA2 to provide
additional channel selectivity at the adjacent channel. If
the VGA amplifiers are driven single ended, the com-
plementary input of VGA should be AC-coupled to
ground through a matched source impedance.
VGA Offset Correction
An internal offset correction feedback amplifier associ-
ated with each VGA removes the DC offsets present in
the VGAs. Offset correction preserves maximum output
compression performance during maximum gain condi-
tions. Each offset correction loop effectively AC-cou-
ples the associated VGA signal path. Each VGA1
network yields a highpass corner frequency according
to the following:
f -3dB (Hz) = 5300 / C DC (nF) (V AGC = 2.0V)
f -3dB (Hz) = 700 / C DC (nF) (V AGC = 0.5V)
where C DC is the value of the capacitors, in nano-
farads, across DCI1+, DCI1- and DCQ1+, DCQ1-. Note
that the corner frequency is a function of the gain set-
ting, increasing with increasing gain. Each VGA2 net-
work provides a highpass corner frequency predicted
by the following:
f -3dB (Hz) = 145 / C DC (µF)
where C DC is the value of the capacitors, in micro-
farads, across DCI2+, DCI2- and DCQ2+, DCQ2-.
The time constants associated with the offset correction
networks limit turn-on time. For applications where the
turn-on time is critical, the offset correction networks
can be disabled by shorting the corresponding pins
together (DCI1+ to DCI1-, DCQ1+ to DCQ1-, DCI2+ to
DCI2-, and DCQ2+ to DCQ2-).
VGA2 I/Q Gain Mismatch Correction
The signal amplitudes at the outputs of the I- and Q-
channel VGA2 amplifiers are compared, and any differ-
ence is corrected by a differential feedback network
associated with the gain control circuitry. Differential
amplitude information is extracted by use of a single
external capacitor across pins 1 and 2 (CEXT- and
CEXT+). The residual difference signal is amplified and
fed back to the gain control network, increasing the
gain of the channel with the smaller signal while
decreasing the gain of the larger signal’s channel. This
network will correct amplitude mismatches generated
by gain mismatches in the previous stages of the
receiver (the mixer and VGA1), as well as insertion-loss
mismatch. The correction network is capable of
decreasing up to 2dB of amplitude mismatch at the
inputs of the I/Q VGA2 amplifiers to <0.5dB amplitude
mismatch. The gain correction network can be disabled
by shorting CEXT- to CEXT+.
Bias Circuit
Operate the MAX2700/MAX2701 in shutdown mode by
connecting SHDN to GND, reducing current consump-
tion to 20µA. In shutdown mode, bias current to all the
blocks is turned off through a master shutdown circuit.
In applications where the LNA is not used, turn off the
LNA by connecting the LNAIN to ground.
Applications Information
LNA Matching
The MAX2700/MAX2701 are designed to operate from
1.8GHz to 2.1GHz and 2.1GHz to 2.5GHz, respectively.
The LNAs in Figures 1 and 2 are optimized for noise
figure and gain centered around 1960MHz and
2400MHz, respectively. Operation at other frequencies
in the band requires reoptimization of the input and out-
put matching circuits. The noise figure is sensitive to
input matching and losses in the input traces. LNA
input matching should be optimized for desired noise
figure, gain, and VSWR performance. High Q matching
elements should be used at the LNA input. Proper
board layout is essential to increase the isolation
between LO and the LNA input. This minimizes LO
leakage and thus DC offset.
I/Q Demodulator Input Matching
The RF input match of the I/Q demodulator in Figure 1
and 2 are optimized for 1960MHz and 2400MHz opera-
tion, respectively. For operation at a different frequency,
the matching circuit should be reoptimized. Single-
ended operation at the demodulator is achieved through
the use of an off-chip balun transformer. In Figure 1, the
balun, inductors, and capacitors constitute the matching
circuit of the differential I/Q demodulator input.