Rainbow Electronics MAX19998 - Manual (Page 25)

SiGe, High-Linearity, 2300MHz to 4000MHz
Downconversion Mixer with LO Buffer
MAX19998
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Detailed Description
The MAX19998 provides high linearity and low noise
figure for a multitude of 2300MHz to 4000MHz WiMAX,
LTE, and MMDS base-station applications. This device
operates over a 2600MHz to 4300MHz LO range and
a 50MHz to 500MHz IF range. Integrated baluns and
matching circuitry allow 50I single-ended interfaces to
the RF and LO ports. The integrated LO buffer provides a
high drive level to the mixer core, reducing the LO drive
required at the MAX19998’s input to a range of -3dBm
to +3dBm. The IF port incorporates a differential output,
which is ideal for providing enhanced 2RF - 2LO and
2LO - 2RF performance.
RF Input and Balun
The MAX19998 RF input provides a 50I match when
combined with a series DC-blocking capacitor. This
DC-blocking capacitor is required as the input is inter-
nally DC shorted to ground through the on-chip balun.
When using an 8.2pF DC-blocking capacitor, the RF port
input return loss is typically 17dB over the RF frequency
range of 3200MHz to 3900MHz. See Table 1 for lower
band tuning.
LO Inputs, Buffer, and Balun
The LO input is internally matched to 50I, requiring
only a 2pF DC-blocking capacitor. A two-stage internal
LO buffer allows for a -3dBm to +3dBm LO input power
range. The on-chip low-loss balun, along with an LO
buffer, drives the double-balanced mixer. All interfacing
and matching components from the LO inputs to the IF
outputs are integrated on-chip.
High-Linearity Mixer
The core of the MAX19998 is a double-balanced, high-
performance passive mixer. Exceptional linearity is pro-
vided by the large LO swing from the on-chip LO buffer.
When combined with the integrated IF amplifier, IIP3,
2RF - 2LO rejection, and noise-figure performance are
typically +24.3dBm, 67dBc, and 9.7dB, respectively,
for low-side LO injection architectures covering the
3000MHz to 4000MHz RF band.
Differential IF Output Amplifier
The MAX19998 has a 50MHz to 500MHz IF frequency
range, where the low-end frequency depends on the
frequency response of the external IF components. The
MAX19998 mixer is tuned for a 300MHz IF using 390nH
external pullup bias inductors. Lower IF frequencies
would require higher L1 and L2 inductor values to main-
tain a good IF match. The differential, open-collector IF
output ports require that these inductors be connected
to V
CC .
Note that these differential ports are ideal for provid-
ing enhanced 2RF - 2LO performance. Single-ended
IF applications require a 4:1 (impedance ratio) balun to
transform the 200I differential IF impedance to a 50I
single-ended system. Use the TC4-1W-17 4:1 transform-
er for IF frequencies above 200MHz and the TC4-1W-7A
4:1 transformer for frequencies below 200MHz. The user
can use a differential IF amplifier or SAW filter on the
mixer IF port, but a DC block is required on both IF+/
IF- ports to keep external DC from entering the IF ports
of the mixer.
Applications Information
Input and Output Matching
The RF and LO inputs provide 50I matches when
combined with the proper tuning. Use an 8.2pF capaci-
tor value on the RF port for frequencies ranging from
3000MHz to 4000MHz. Use a 3.3nH series inductor and
a 0.3pF shunt capacitor on the RF port for frequencies
ranging from 2300MHz to 2900MHz. On the LO port, use
a 2pF DC-blocking capacitor to cover operations span-
ning the 2600MHz to 4300MHz range.
The IF output impedance is 200I (differential). For evalu-
ation, an external low-loss 4:1 (impedance ratio) balun
transforms this impedance down to a 50I single-ended
output (see the Typical Application Circuit).
Reduced-Power Mode
The MAX19998 has two pins (LOBIAS, IFBIAS) that allow
external resistors to set the internal bias currents. See
Table 1 for nominal values for these resistors. Larger
value resistors can be used to reduce power dissipa-
tion at the expense of some performance loss. If Q1%
resistors are not readily available, substitute with Q5%
resistors.
Significant reductions in power consumption can also
be realized by operating the mixer with an optional
supply voltage of 3.3V. Doing so reduces the overall
power consumption by 57% (typ). See the 3.3V Supply
AC Electrical Characteristics table and the relevant 3.3V
curves in the Typical Operating Characteristics section
to evaluate the power vs. performance trade-offs.