MAX2021 datasheet(14/20 Pages) MAXIM

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MAX2021

High-Dynamic-Range, Direct Up-/Downconversion

650MHz to 1200MHz Quadrature Mod/Demod

Maxim Integrated

varying temperature, LO frequency, and baseband

drive conditions. See the

Typical Operating

Characteristics for details. Note that the resistor value is

chosen to be 100Ω with a corner frequency 1 / (2πRC)

selected to adequately filter the fLO and 2fLO leakage,

yet not affecting the flatness of the baseband response

at the highest baseband frequency. The common-

mode fLO and 2fLO signals at I+/I- and Q+/Q- effective-

ly see the RC networks and thus become terminated in

50Ω (R/2). The RC network provides a path for absorb-

ing the 2fLO and fLO leakage, while the inductor pro-

vides high impedance at fLO and 2fLO to help the

diplexing process.

RF Demodulator

The MAX2021 can also be used as an RF demodulator

(see Figure 3), downconverting an RF input signal

directly to baseband. The single-ended RF input

accepts signals from 650MHz to 1200MHz with power

levels up to +30dBm. The passive mixer architecture

produces a conversion loss of typically 9.2dB. The

downconverter is optimized for high linearity and

excellent noise performance, typically with a

+35.2dBm IIP3, a P1dB of greater than +30dBm, and a

9.3dB noise figure.

A wide I/Q port bandwidth allows the port to be used as

an image-reject mixer for downconversion to a quadra-

ture IF frequency.

The RF and LO inputs are internally matched to 50Ω.

Thus, no matching components are required, and only

DC-blocking capacitors are needed for interfacing.

Demodulator Output Port Considerations

Much like in the modulator case, the four baseband

ports require some form of DC return to establish a com-

mon mode that the on-chip circuitry drives. This can be

achieved by directly DC-coupling to the baseband ports

(staying within the ±3.5V common-mode range),

through an inductor to ground, or through a low-value

resistor to ground. Figure 4 shows a typical network that

would be used to connect to each baseband port for

demodulator operation. This network provides a com-

mon-mode DC return, implements a high-frequency

diplexer to terminate unwanted RF terms, and also pro-

vides an impedance transformation to a possible higher

impedance baseband amplifier.

The network Ca, Ra, La and Cb form a highpass/low-

pass network to terminate the high frequencies into a

load while passing the desired lower IF frequencies.

Elements La, Cb, Lb, Cc, Lc, and Cd provide a possible

impedance transformer. Depending on the impedance

being transformed and the desired bandwidth, a fewer

number of elements could be used. It is suggested that

La and Cb always be used since they are part of the

high frequency diplexer. If power matching is not a

concern then this would reduce the elements to just the

diplexer.

Resistor Rb provides a DC return to set the common

mode voltage. In this case, due to the on-chip circuitry,

the voltage would be approx 0V DC. It can also be

used to reduce the load impedance of the next stage.

Inductor Ld can provide a bit of high frequency gain

peaking for wideband IF systems. Capacitor Ce is a DC

block.

Typical values for Ca, Ra, La, and Cb would be 1.5pF,

50Ω, 11nH, and 4.7pF, respectively. These values can

change depending on the LO, RF, and IF frequencies

used. Resistor Rb is in the 50Ω to 200Ω range

The circuitry presented in Figure 4 does not allow for LO

leakage at RF port nulling. Depending on the LO at RF

leakage requirement, a trim voltage might need to be

introduced on the baseband ports to null the LO leakage.

ADC

MAX2021

DIPLEXER/

DC RETURN

MATCHING

ADC

DIPLEXER/

DC RETURN

MATCHING

Figure 3. MAX2021 Demodulator Configuration

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