Rainbow Electronics MAX19996A - Manual (Page 31)

______________________________________________________________________________________ 31
SiGe, High-Linearity, 2000MHz to 3900MHz
Downconversion Mixer with LO Buffer
MAX19996A
High-Linearity Mixer
The core of the MAX19996A is a double-balanced,
high-performance passive mixer. Exceptional linearity is
provided by the large LO swing from the on-chip LO
buffer. When combined with the integrated IF ampli-
fiers, IIP3, 2LO-2RF rejection, and noise-figure perfor-
mance are typically +24.5dBm, 67dBc, and 9.8dB,
respectively.
Differential IF Output Amplifier
The MAX19996A 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 MAX19996A mixer is tuned for a 300MHz IF using
390nH external pullup bias inductors. Lower IF frequen-
cies would require higher inductor values to maintain a
good IF match. The differential, open-collector IF output
ports require these inductors to be connected to V
CC .
Note that these differential ports are ideal for providing
enhanced 2LO-2RF and 2RF-2LO performance. Single-
ended IF applications require a 4:1 (impedance ratio)
balun to transform the 200
Ω differential IF impedance
to a 50
Ω single-ended system. Use the TC4-1W-17 4:1
transformer 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 input provides a 50
Ω match when combined
with a series DC-blocking capacitor. Use an 8.2pF
capacitor value for RF frequencies ranging from
2000MHz to 3000MHz. A 1.5pF capacitor value should
be used to match the RF port for the 3000MHz to
3900MHz band. The LO input is internally matched to
50
Ω; use a 2pF DC-blocking capacitor to cover opera-
tions spanning the 2100MHz to 4000MHz LO range.
The IF output impedance is 200
Ω (differential). For
evaluation, an external low-loss 4:1 (impedance ratio)
balun transforms this impedance down to a 50
Ω single-
ended output (see the Typical Application Circuit ).
Reduced-Power Mode
The MAX19996A has two pins (LOBIAS, IFBIAS) that
allow external resistors to set the internal bias currents.
Nominal values for these resistors are given in Table 1.
Larger value resistors can be used to reduce power
dissipation at the expense of some performance loss.
If ±1% resistors are not readily available, substitute with
±5% resistors.
Significant reductions in power consumption can also
be realized by operating the mixer with an optional sup-
ply voltage of 3.3V. Doing so reduces the overall power
consumption by up to 57%. See the 3.3V
Supply AC
Electrical Characteristics—f RF = 2300M to 2900MHz,
High-Side Lo Injection table and the relevant 3.3V
curves in the Typical Operating Characteristics section
to evaluate the power vs. performance tradeoffs.
LEXT Inductor
Short LEXT to ground using a 0
Ω resistor. For applica-
tions requiring improved RF-to-IF and LO-to-IF isolation,
L3 can be changed to optimize performance (see the
Typical Operating Characteristics ). However, the load
impedance presented to the mixer must be so that any
capacitances from IF- and IF+ to ground do not exceed
several picofarads to ensure stable operating condi-
tions. Since approximately 90mA flows through LEXT, it
is important to use a low-DCR wire-wound inductor.
Layout Considerations
A properly designed PCB is an essential part of any
RF/microwave circuit. Keep RF signal lines as short as
possible to reduce losses, radiation, and inductance.
The load impedance presented to the mixer must be so
that any capacitance from both IF- and IF+ to ground
does not exceed several picofarads. For the best per-
formance, route the ground pin traces directly to the
exposed pad under the package. The PCB exposed
pad MUST be connected to the ground plane of the
PCB. It is suggested that multiple vias be used to con-
nect this pad to the lower-level ground planes. This
method provides a good RF/thermal-conduction path
for the device. Solder the exposed pad on the bottom
of the device package to the PCB. The MAX19996A
evaluation kit can be used as a reference for board lay-
out. Gerber files are available upon request at
www.maxim-ic.com .
Power-Supply Bypassing
Proper voltage-supply bypassing is essential for high-
frequency circuit stability. Bypass each V
CC pin with
the capacitors shown in the Typical Application Circuit
and Table 1.