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MAX2700/MAX2701

1.8GHz to 2.5GHz Direct Downconversion

Receivers

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Detailed Description

The MAX2700/MAX2701 consist of five major blocks:

LNA, I/Q direct demodulator, VGAs, gain correction,

and bias circuits.

Low-Noise Amplifier

The LNA is a two-gain-level amplifier with low noise fig-

ure and high IIP3. Connect GAIN_SET to GND to switch

the amplifier to a low-gain mode that provides an accu-

rate gain step. High IIP3 minimizes the cross-modula-

tion between TX power leakage and close-in interferers

at the RX input. The LNA can be turned off independent

of the other functional blocks by connecting LNAIN to

GND. External matching is required to match the input

and output to 50Ω. The LNA in Figures 1 and 2 is

matched to 1960MHz and 2400MHz over a narrow

bandwidth.

I/Q Demodulator

The direct I/Q demodulator downconverts the RF signal

directly to baseband I and Q signals. This architec-

ture’s main advantage is that the received signal is

amplified and filtered at baseband rather than at some

high intermediate frequency. This eliminates the need

for an expensive IF SAW filter and the IF oscillator.

Furthermore, the direct conversion scheme eliminates

the need for image rejection, thereby relaxing the

bandpass filter selectivity requirements following the

LNA. The direct downconverter consists of highly linear

double-balanced I/Q mixers, an LO frequency doubler

option, an LO quadrature generator, and baseband I/Q

buffer amplifiers driven by the mixers’ outputs.

In a direct downconversion receiver, I/Q mixers have

more stringent requirements on mixer output linearity

since they need to handle large voltage swings at

baseband due to close-in interferers. The RF signal is

applied to the differential input (RFIN+, RFIN-) of the

direct downconversion receiver through an off-chip

balun. The differential input structure results in a higher

common-mode rejection for second-order nonlinearity

generated in the receiver’s front end. The differential

input requires matching to appropriate impedance of

the balun. Some applications may require a bandpass

filter between the LNA and the mixer, as shown in

Figures 1 and 2, to attenuate the residual transmit

power leakage and out-of-band spurious signals.

The mixer baseband buffers amplify the mixer I and Q

differential outputs and convert them to single-ended

outputs (MIX_I, MIX_Q). These buffer amplifiers have

PIN NAME FUNCTION

31 RFIN-

I/Q Mixers Noninverting Input. For narrow frequency bands between 1.8GHz and

2.5GHz, port must be matched using external matching components.

33 IOUT2 I-Channel VGA2 Baseband Output

34 DCI2- Inverting Offset Correction Input for I-Channel VGA2 Amplifier

35 DCI2+ Noninverting Offset Correction Input for I-Channel VGA2 Amplifier

36 IIN2- Inverting VGA2, I-Channel Baseband Input

37 IIN2+ Noninverting VGA2, I-Channel Baseband Input

39 IOUT1 I-Channel VGA1 Amplifier Baseband Output

40 DCI1- Inverting Offset Correction Input for I-Channel VGA1 Amplifier

41 DCI1+ Noninverting Offset Correction Input for I-Channel VGA1 Amplifier

42 IIN1- Inverting VGA1, I-Channel Baseband Input

43 IIN1+ Noninverting VGA1, I-Channel Baseband Input

45 MIX_I

Mixer I-Channel Baseband Output. Connect external series capacitor to AC-couple the

output to the load.

47 LO LO Input. Internally matched to 50Ω.

Pin Description (continued)