Rainbow Electronics MAX19996 - Manual (Page 16)

MAX19996
SiGe High-Linearity, 2000MHz to 3000MHz
Downconversion Mixer with LO Buffer
16 ______________________________________________________________________________________
Detailed Description
The MAX19996 high-linearity downconversion mixer
provides 8.7dB of conversion gain and +24.5dBm of
IIP3, with a typical 9.6dB noise figure. The integrated
baluns and matching circuitry allow for 50Ω single-
ended interfaces to the RF and the LO port. The inte-
grated LO buffer provides a high drive level to the
mixer core, reducing the LO drive required at the
MAX19996’s input to a -3dBm to +3dBm range. The IF
port incorporates a differential output, which is ideal for
providing enhanced 2RF-2LO performance.
Specifications are guaranteed over broad frequency
ranges to allow for use in WCS, LTE, WiMAX, and
MMDS base stations. The MAX19996 is specified to
operate over an RF input range of 2000MHz to
3000MHz, an LO range of 1800MHz to 2550MHz, and
an IF range of 50MHz to 500MHz. The external IF com-
ponents set the lower frequency range (see the
Typical
Operating Characteristics for details). Operation
beyond these ranges is possible (see the Typical
Operating Characteristics for additional information).
Although this device is optimized for low-side LO injec-
tion applications, it can operate in high-side LO injec-
tion modes as well. However, performance degrades
as f
LO continues to increase. For increased high-side
LO performance, refer to the MAX19996A data sheet.
RF Port and Balun
The MAX19996 RF input provides a 50Ω match when
combined with a series 8.2pF DC-blocking capacitor.
This DC-blocking capacitor is required as the input is
internally DC shorted to ground through the on-chip
balun. The RF port input return loss is typically 15dB
over the RF frequency range of 2300MHz to 2800MHz.
LO Inputs, Buffer, and Balun
The MAX19996 is optimized for low-side LO injection
applications with an 1800MHz to 2550MHz LO frequen-
cy range. The LO input is internally matched to 50Ω,
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 MAX19996 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 ampli-
fiers, the performance of IIP3, 2RF-2LO rejection, and
noise-figure is typically +24.5dBm, 69dBc, and 9.6dB,
respectively.
Differential IF Output Amplifier
The MAX19996 has an IF frequency range of 50MHz to
500MHz, where the low-end frequency depends on the
frequency response of the external IF components. The
MAX19996 mixer is tuned for a 450MHz IF using 120nH
external pullup bias inductors. Lower IFs of 350MHz
and 300MHz require higher inductor values of 270nH
and 470nH, respectively. The differential, open-collec-
tor IF output ports require these inductors to be con-
nected to V
CC
.
Note that these differential ports are ideal for providing
enhanced 2RF-2LO performance. Single-ended IF
applications require a 4:1 (impedance ratio) balun to
transform the 200Ω differential IF impedance to a 50Ω
single-ended system. Use the TC4-1W-17 4:1 trans-
former 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 ports are designed to operate in a
50Ω system. Use DC blocks at the RF and LO inputs to
isolate the ports from external DC while providing some
reactive tuning. The IF output impedance is 200Ω (dif-
ferential). For evaluation, an external low-loss 4:1
(impedance-ratio) balun transforms this impedance
down to a 50Ω single-ended output (see the
Typical
Application Circuit ).
Externally Adjustable Bias
Bias currents for the LO buffer and the IF amplifier are
optimized by fine-tuning resistors R1 and R2. The val-
ues for R1 and R2, as listed in Table 1, represent the
nominal values which yield the highest level of linearity
performance. Larger value resistors can be used to
reduce power dissipation at the expense of some per-
formance loss. Contact the factory for details concern-
ing recommended power reduction vs. performance
tradeoffs. If ±1% resistors are not readily available,
±5% resistors can be substituted.
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 up to 57%. 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 tradeoffs.