Rainbow Electronics MAX19985A - Manual (Page 19)

MAX19985A
Dual, SiGe, High-Linearity, 700MHz to 1000MHz
Downconversion Mixer with LO Buffer/Switch
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Detailed Description
The MAX19985A is a dual-channel downconverter
designed to provide 8.7dB of conversion gain,
+25.5dBm of IIP3, +12.6dBm typical input 1dB com-
pression point, and a 9.0dB noise figure.
In addition to its high-linearity performance, the
MAX19985A achieves a high level of component inte-
gration. The device integrates two double-balanced
mixers for two-channel downconversion. Both the main
and diversity channels include a balun and matching
circuitry to allow 50
Ω single-ended interfaces to the RF
ports and the two LO ports. An integrated single-pole/
double-throw (SPDT) switch provides 50ns switching
time between the two LO inputs with 46dB of LO-to-LO
isolation and -40dBm of LO leakage at the RF port.
Furthermore, the integrated LO buffers provide a high
drive level to each mixer core, reducing the LO drive
required at the MAX19985A’s inputs to a range of
-3dBm to +3dBm. The IF ports for both channels incor-
porate differential outputs for downconversion, which is
ideal for providing enhanced 2LO-2RF performance.
Specifications are guaranteed over broad frequency
ranges to allow for use in WCDMA, GSM/EDGE, iDEN,
cdma2000, and LTE/WiMAX cellular and 700MHz band
base stations. The MAX19985A is specified to operate
over an RF input range of 700MHz to 1000MHz, an LO
range of 900MHz to 1300MHz, and an IF range of
50MHz to 500MHz. The external IF components 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 high-side LO injection applica-
tions, it can operate in low-side LO injection modes as
well. However, performance degrades as f
LO continues
to decrease. For increased low-side LO performance,
refer to the MAX19985 data sheet.
RF Port and Balun
The RF input ports of both the main and diversity chan-
nels are internally matched to 50
Ω, requiring no exter-
nal matching components. A 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 20dB over the RF frequency range of
770MHz to 915MHz.
LO Inputs, Buffer, and Balun
The MAX19985A is optimized for a 900MHz to
1300MHz LO frequency range. As an added feature,
the MAX19985A includes an internal LO SPDT switch
for use in frequency-hopping applications. The switch
selects one of the two single-ended LO ports, allowing
the external oscillator to settle on a particular frequency
before it is switched in. LO switching time is typically
50ns, which is more than adequate for typical GSM
applications. If frequency hopping is not employed,
simply set the switch to either of the LO inputs. The
switch is controlled by a digital input (LOSEL), where
logic-high selects LO1 and logic-low selects LO2. LO1
and LO2 inputs are internally matched to 50
Ω, requiring
only an 82pF DC-blocking capacitor. To avoid damage
to the part, voltage MUST be applied to V
CC
before
digital logic is applied to LOSEL. Alternatively, a 1k
Ω
resistor can be placed in series at the LOSEL to limit
the input current in applications where LOSEL is
applied before V
CC .
The main and diversity channels incorporate a two-
stage LO buffer that allows for a wide-input power
range for the LO drive. The on-chip low-loss baluns,
along with LO buffers, drive the double-balanced mix-
ers. All interfacing and matching components from the
LO inputs to the IF outputs are integrated on-chip.
High-Linearity Mixer
The core of the MAX19985A dual-channel downcon-
verter consists of two double-balanced, high-
performance passive mixers. Exceptional linearity is
provided by the large LO swing from the on-chip LO
buffers. When combined with the integrated IF ampli-
fiers, the cascaded IIP3, 2LO-2RF rejection, and noise
figure performance are typically +25.5dBm, 76dBc,
and 9.0dB, respectively.
Differential IF
The MAX19985A has an IF frequency range of 50MHz
to 500MHz, where the low-end frequency depends on
the frequency response of the external IF components.
Note that these differential ports are ideal for providing
enhanced IIP2 performance. Single-ended IF applica-
tions require a 4:1 (impedance ratio) balun to transform
the 200
Ω differential IF impedance to a 50Ω single-
ended system. After the balun, the return loss is
typically 18dB. The user can use a differential IF ampli-
fier on the mixer IF ports, but a DC block is required on
both IFD+/IFD- and IFM+/IFM- ports to keep external
DC from entering the IF ports of the mixer.