Rainbow Electronics MAX19993 - Manual (Page 19)

MAX19993
Dual, SiGe, High-Linearity, 1200MHz to 1700MHz
Downconversion Mixer with LO Buffer/Switch
19
Detailed Description
The MAX19993 is a dual-channel downconverter
designed to provide up to 6.4dB of conversion gain,
+27dBm input IP3, 15.4dBm 1dB input compression
point, and a noise figure of 9.8dB.
In addition to its high-linearity performance, the device
achieves a high level of component integration. It inte-
grates two double-balanced mixers for two-channel
downconversion. Both the main and diversity channels
include a balun and matching circuitry to allow 50I
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 57dB of LO-to-LO isolation and -38dBm 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 device‘s inputs
to a range of -6dBm to +3dBm. The IF ports for both
channels incorporate differential outputs for downcon-
version, which is ideal for providing enhanced 2RF - 2LO
performance.
The device is specified to operate over an RF input range
of 1200MHz to 1700MHz, an LO range of 1000MHz to
1560MHz, and an IF range of 50MHz to 500MHz. The
external IF components set the lower frequency range. See
the Typical Operating Characteristics section for details.
Operation beyond these ranges is possible; see the
Typical Operating Characteristics section for additional
information. Although this device is optimized for low-
side LO injection applications, it can operate in high-
side LO injection modes as well. However, perfor-
mance degrades as f
LO continues to increase. Contact
the factory for a variant with increased high-side LO
performance.
RF Port and Balun
The RF input ports of both the main and diversity
channels are internally matched to 50I, requiring no
external 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 better than 19dB over the 1400MHz to
1700MHz RF frequency range.
LO Inputs, Buffer, and Balun
The device is optimized for a 1000MHz to 1560MHz
LO frequency range. As an added feature, the device
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 50I, requiring only 39pF
DC-blocking capacitors.
If LOSEL is connected directly to a logic source, then
voltage MUST be applied to V
CC before digital logic
is applied to LOSEL to avoid damaging the part.
Alternatively, a 1kI 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 mixers. All interfacing
and matching components from the LO inputs to the IF
outputs are integrated on-chip.
High-Linearity Mixer
The core of the device’s dual-channel downconverter
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 amplifiers, the cascaded
IIP3, 2RF - 2LO rejection, and noise-figure performance
are typically +27dBm, 72dBc, and 9.8dB, respectively.
Differential IF
The device has a 50MHz to 500MHz IF frequency range,
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 applications require a
4:1 (impedance ratio) balun to transform the 200I
differential IF impedance to a 50I single-ended system.
After the balun, the return loss is typically 15dB. The user
can use a differential IF amplifier 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.