2.4GHz 802.11b Zero-IF Transceivers

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21

onto the line, given the high impedance of the output.

This might result in rectified RF, altering the value of the

bias current and causing erratic PA operation.

Synthesizer

Channel Frequency and Reference Frequency

The synthesizer/PLL channel frequency and reference

settings establish the divider/counter settings in the inte-

ger-N synthesizer of the MAX2820/MAX2821. Both the

channel frequency and reference oscillator frequency

are programmable through the serial interface. The

channel frequency is programmed as a channel num-

ber 0 to 99 to set the carrier frequency to 2400MHz to

2499MHz (LO frequency = channel + 2400). The refer-

ence frequency is programmable to 22MHz or 44MHz.

These settings are intended to cover only the required

802.11b channel spacing and the two possible crystal

oscillator options used in the radios.

Reference Oscillator Input

The reference oscillator inputs ROSCP and ROSCN are

high-impedance analog inputs. They are designed to

be connected to the reference oscillator output through

a coupling capacitor. The input amplitude can range

reference oscillator with a CMOS output, the signal

must be attenuated before being applied to the ROSC

inputs. The signal can be attenuated with a resistor- or

capacitor-divider network.

Reference Voltage Output

A voltage reference output is provided from pin 2,

VREF, for use with certain baseband ICs. The nominal

output voltage is 1.2V. The reference voltage is first-

order compensated over temperature to provide a rea-

sonably low drift output, 1.1V to 1.3V over temperature,

under load conditions. The output stage is designed to

drive 2mA loads with up to 20pF of load capacitance.

The VREF output is designed to directly connect to the

baseband reference input.

Loop Filter

The PLL uses a classical charge pump into an external

loop filter (C-RC) in which the filter output connects to

the voltage tuning input of the VCO. This simple third-

order lowpass loop filter closes the loop around the

synthesizer. The Typical Application Circuit shows the

loop filter elements around the MAX2820/MAX2821.The

capacitor and resistor values are set to provide the

loop bandwidth required to achieve the desired lock

time while also maintaining loop stability. Refer to the

MAX2820/MAX2821 EV kit schematic for component

values. A 45kHz loop bandwidth is recommended to

ensure that the loop settles quickly enough to achieve

5µs TX turnaround time and 10µs RX turnaround time.

This is the loop filter on the EV kit. Narrowing the loop

bandwidth increases the settling time and results in

unacceptable TX-RX turnaround time performance.

Chip Information

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