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RF2689

W-CDMA/GSM/DCS RECEIVE AGC AND DEMODULATOR

RF Micro Devices 

RF2689

Preliminary

Since the power gain to the input of the Op-Amp GP’=αGP, where GP is the power gain of demodulator for matched load.

Therefore, the demodulator power gain is 65+15 = 80 dB.

AC Coupling in Evaluation Board

The output I and Q baseband signal is AC coupled for evaluation purposes only. The high-pass corner frequency is at

1/(2π RC)=1/(6.28*30kΩ*100nF)=56Hz.

I and Q Output DC Voltage and Its Offset

Although the I and Q output is AC coupled on the evaluation board, in most applications, it would be DC coupled to the

ADC input buffer. The DC voltage at the IC output is VCC-1.6V with a possible variation of ±0.3V due to temperature and

tolerance. The differential circuit asymmetry would cause common mode DC offset to the extent of ±40mV.

Baseband Filter Calibration Process

The BB (baseband) filter calibration process is same for both WCDMA and GSM/DCS. After calibration is done, the

WCDMA mode sets the circuitry to have a 3dB bandwidth of 2.5MHz, the GSM/DCS mode (if the chip has GSM/DCS

mode) sets the circuitry to have a 3dB bandwidth of 250kHz.

The BB filter in the I and Q path needs to be calculated every time after power down. When the FCLK pin is connected to

a signal generator with 0dBm output level at 13.0MHz, a logic high at CALEN pin for 200µs will calibrate the filter to have

2.5MHz bandwidth with 10% accuracy when WCDMA mode is set, or to 250kHz bandwidth with 10% accuracy when

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GSM mode is set. The calibration is done when the chip is powered on only. Calibration is independent from all other

conditions, e.g. the chip enable could be off.

The calibration circuitry consumes 400µA. When the calibration sequence is complete after 200µs, the ICC drops to

0 mA.

The 3dB bandwidth is defined to be from the reference level at 1MHz for WCDMA and at 50kHz for GSM/DCS. The 3dB

bandwidth is independent of VGC and VCC.

The filter can also be calibrated with different clock frequencies from 10MHz to 30MHz to tune the bandwidth over -40%

to +60% from its default 3dB bandwidth (2.5MHz for WCDMA and 250kHz for GSM). The 3dB bandwidth is linear with

clock frequency.

7-46

Rev A4 010815

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