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AD6402ARS 데이터시트(PDF) 4 Page - Analog Devices |
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AD6402ARS 데이터시트(HTML) 4 Page - Analog Devices |
4 / 8 page AD6402 –4– REV. 0 AD6402 VCO VCOGND TXOUT TXOUTB MODOUT FMMOD2 FMMOD1 1nF 1nF SMV 1204-36 39nH 8pF 47pF 1.2k Ω 4.7nF 1nF SMV 1204-37 240 Ω 150pF VCCI 330 Ω 100pF 330 Ω 180pF 330 Ω TXMOD TXIF+ TXIF– VTUNE Figure 1. OVERVIEW The AD6402 forms the basis of a highly integrated RF trans- ceiver with the benefits of increased sensitivity and wide dy- namic range that a dual-conversion architecture provides. The IC contains a low dropout voltage regulator to isolate the IF and demodulator VCOs from variation in the battery voltage, such as power-supply transients caused by the PA. The AD6402 also provides control circuitry that allows subcircuits to be turned off and on as necessary to minimize power consumption. Operation During Receive The AD6402 contains the second mixer, integrated second-IF bandpass filter, logarithmic-limiting amplifier, and PLL de- modulator. A SAW IF bandpass filter is usually required at the IF input in order to provide channel selectivity. The placement of the SAW filter in the signal path between the AD6402 and the RF section and the partitioning of the receiver’s RF and IF receive circuits minimizes the leakage around the SAW filter and maximizes the RF to IF isolation. The output of the SAW filter enters the AD6402 via the second downconversion mixer. This mixer is a high gain, doubly- balanced Gilbert-cell type. The mixer downconverts the signal to the second IF, which is 1.5 × or 2.5 × the reference frequency. This multiple is determined by the state of the REFSEL pin. An on-chip two section bandpass filter provides additional selectiv- ity to provide attenuation of adjacent channels. The VCO con- trol voltage output of the PLL demodulator tunes this filter to the second IF. The bandpass filter’s output enters a successive-detection loga- rithmic-limiting IF amplifier. The RSSI detectors are distrib- uted across the entire IF strip, including the mixer, and provide 80 dB RSSI range. The IF strip’s limiting gain also exceeds 80 dB. The RSSI signal is low-pass filtered and proceeds off-chip to the baseband subsystem. The limited output of the logarith- mic amplifier enters a PLL demodulator, which provides de- modulation of the received signal. The PLL uses an integrated VCO with no external components. Operation During Transmit The transmit signal path consists of a low-pass filter that can be user configured for antialiasing of a baseband transmit signal. An IF VCO, which should be tuned to a frequency equal to the receive IF frequency plus the desired demodulator input fre- quency, may be open-loop modulated by the transmit signal for FM and FSK schemes. The receive IF mixer uses high side mixing and therefore the IF VCO should be set to a frequency equal to the sum of the IF frequency plus the frequency of the PLL demodulator input as defined by the reference clock divider ratios. The transmit IF VCO uses an external tank circuit. This signal is upconverted to the transmit frequency in the RF mixer sec- tion of the radio. Using a transmit IF VCO prevents two prob- lems: feedback from the PA at the RF frequency does not cause distortion in the modulating circuit because the frequencies are widely separated and the IF tank circuit can be optimized for modulation linearity. The output of the transmit VCO passes through buffer amplifier and leaves the AD6402 via an optional LC filter between the RF and IF ICs. The output of the LC filter may then be fed to a transmit upconversion mixer for conversion to the final RF frequency. Onboard Voltage Regulation The AD6402 contains a low dropout voltage regulator to spe- cifically isolate the VCOs and synthesizer from the voltage “kick” that occurs when a power amplifier switches on and the battery voltage abruptly drops. The AD6402 uses an integral vertical PNP pass transistor. The regulator in the AD6402 IF IC supplies the voltage for the VCOs on both the RF section and AD6402. The other sections of the AD6402 should be powered from an independently regu- lated source at 2.85 V. Since the VCOs are isolated from this source, possible problems due to VCO supply pushing are con- siderably reduced. Frequency Control The AD6402 requires an external synthesizer to provide the control voltages for the tank circuit of the IF VCO. Normally this will be the IF section of a dual synthesizer controlling both IF and RF frequency generation. It is recommended that the VCO on the RF section implement the channel selection on transmit and receive; the VCO on the AD6402 may therefore operate at a fixed frequency. This ac- complishes two goals: first, the IF VCO being modulated can be optimized for modulation linearity and the RF VCO can be optimized for tuning range, and second, feedback from the PA at will not couple into the modulating circuit to cause spurious responses. All key sections of the AD6402 may be powered up or down as necessary to minimize power consumption and maximize battery life. |
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