MAX2820ETM-T Maxim Integrated Products, MAX2820ETM-T Datasheet - Page 21

• Manufacturer Part Number: MAX2820ETM-T
• Description: RF Transceiver 2.4GHz 802.11b Zero-If Transceiver
• Manufacturer: Maxim Integrated Products

Specifications of MAX2820ETM-T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The transmit gain-control input provides a direct analog
control over the transmit path gain. The transmit gain is
controlled by an external voltage at pin TX_GC. The typ-
ical gain-control characteristic is provided in the Typical
Operating Characteristics graph Transmitter Gain
Control vs. Gain-Control Voltage. The input is a high-
impedance analog input designed to directly connect to
the DAC output of the baseband IC. Some local noise
filtering through a simple RC network at the input is per-
missible. However, the time constant of this network
should be kept sufficiently low so the desired response
time of the TX gain-control function is not limited.
During the TX turn-on sequence, the gain is internally
set at the minimum while the TX baseband offset cali-
bration is taking place. The RF output is effectively
“blanked” for the first 2.2µs after TX_ON is taken high.
After 2.2µs, the “blanking” is released, and the gain-
control amplifier ramps to the gain set by the external
voltage applied to the TX_GC input.
The MAX2820 family provides a programmable analog
current source output for use in biasing the RF power
amplifier, such as the MAX2242. The output is essentially
an open-drain output of a current source DAC. The output
is designed to directly connect to the bias-current pin on
the power amplifier. The value of the current is deter-
mined by the 4 bits programmed into transmit (D3:D0).
This programmability permits optimizing of the power-
amplifier idle current based on the output power level of
the PA. Care must be taken in the layout of this line. Avoid
running the line in parallel with the RF line. RF might cou-
ple 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.
The synthesizer/PLL channel frequency and reference
settings establish the divider/counter settings in the inte-
ger-N synthesizer. Both the channel frequency and ref-
erence oscillator frequency are programmable through
the serial interface. The channel frequency is pro-
grammed as a channel number 0 to 99 to set the carri-
er frequency to 2400MHz to 2499MHz (LO frequency =
channel + 2400). The reference frequency is program-
mable to 22MHz or 44MHz. These settings are intend-
Channel Frequency and Reference Frequency
______________________________________________________________________________________
2.4GHz 802.11b Zero-IF Transceivers
Transmit Gain Control
PA Bias DAC Output
Synthesizer
ed to cover only the required 802.11b channel spacing
and the two possible crystal oscillator options used in
the radios.
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
from 200mV
a 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.
A voltage reference output is provided on the MAX2821/
MAX2821A from pin 2, VREF, for use with certain base-
band ICs. The nominal output voltage is 1.2V. The ref-
erence voltage is first- order compensated over
temperature to provide a reasonably 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 refer-
ence input.
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 transceiver. The capaci-
tor and resistor values are set to provide the loop band-
width 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 turn-
around 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.
TRANSISTOR COUNT: 13,607
P-P
to 1000mV
P-P
Reference Oscillator Input
Reference Voltage Output
Chip Information
; therefore, in the case of
Loop Filter
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