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ADF7021 Datasheet(PDF) 18 Page - Analog Devices |
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ADF7021 Datasheet(HTML) 18 Page - Analog Devices |
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18 / 44 page  ADF7021 Preliminary Technical Data Rev. PrI | Page 18 of 44 RECEIVER SECTION RF FRONT END The ADF7021 is based on a fully integrated, low IF receiver architecture. The low IF architecture facilitates a very low external component count and does not suffer from power- line-induced interference problems. Figure 19 shows the structure of the receiver front end. The many programming options allow users to trade off sensitivity, linearity, and current consumption against each other in the way best suited for their applications. To achieve a high level of resilience against spurious reception, the low noise amplifier (LNA) features a differential input. Switch SW2 shorts the LNA input when transmit mode is selected (R0_DB27 = 0). This feature facilitates the design of a combined LNA/PA matching network, avoiding the need for an external Rx/Tx switch. See the LNA/PA Matching section for details on the design of the matching network. SW2 LNA RFIN RFINB Tx/Rx SELECT [R0_DB27] LNA MODE [R9_DB25] LNA CURRENT [R9_DB(26:27)] MIXER LINEARITY [R9_DB28] LO I (TO FILTER) Q (TO FILTER) LNA GAIN [R9_DB(20:21)] LNA/MIXER ENABLE [R8_DB6] Figure 19. ADF7021 RF Front End The LNA is followed by a quadrature downconversion mixer, which converts the RF signal to the IF frequency of 100 kHz. An important consideration is that the output frequency of the synthesizer must be programmed to a value 100 kHz below the center frequency of the received channel. The LNA has two basic operating modes: high gain/low noise mode and low gain/low power mode. To switch between these two modes, use the LNA Mode Bit, R9_DB25. The mixer is also configurable between a low current and an enhanced linearity mode using the Mixer Linearity Bit, R9_DB28. Based on the specific sensitivity and linearity requirements of the application, it is recommended to adjust control bits LNA mode (R9_DB25) and mixer linearity (R9_DB28) as outlined in Table 8. The gain of the LNA is configured by the LNA gain field, R9_DB[20:21], and can be set by either the user or the automatic gain control (AGC) logic. IF Filter Settings Out-of-band interference is rejected by means of a fifth-order Butterworth polyphase IF filter centered on a frequency of 100 kHz. The bandwidth of the IF filter can be programmed between 12.5 kHz and 25 kHz by Control Bits R4_DB[30:31], and should be chosen as a compromise between interference rejection, attenuation of the desired signal, and the AFC pull-in range. Coarse/Fine Filter Calibration To compensate for manufacturing tolerances, the IF filter should be calibrated once after power-up. Coarse and fine calibration schemes are provided in order to offer a choice between fast calibration with the coarse scheme and high filter centering accuracy with fine calibration. Coarse calibration is enabled by setting Bit R5_DB4 high. Fine calibration is enabled by setting Bit R6_DB4 high. It is necessary to do a coarse cali- bration before doing a fine calibration. If the IF_FINE_CAL bit, R6_DB4, has already been configured high, it is possible to do a fine calibration by writing only to Register 5. Once initiated by writing to the part, the calibration is performed automatically without any user intervention. Calibration time is 200 μs for coarse calibration and a few milliseconds for fine calibration, during which the ADF7021 should not be accessed. The IF filter calibration logic requires that the IF filter divider in Bits R5_DB[5:13] be set depending on the crystal frequency. It is important not to initiate the calibration cycle before the crystal oscillator has fully settled. If the AGC loop is disabled, the gain of IF filter can be set to three levels using the Filter Gain Field, R9_DB[22:23]. The filter gain is adjusted automatically if the AGC loop is enabled. Table 8. LNA/Mixer Modes Receiver Mode LNA Mode (R9_DB25) LNA Gain Value (R9_DB[20:21]) Mixer Linearity (R9_DB28) Sensitivity (DR = 9.6 kbps, FDEV = 4.8 kHz) Rx Current Consumption (mA) Input IP3 (dBm) High Sensitivity Mode (default) 0 30 0 −117 22 TBD Rx Mode2 1 10 0 TBD 20 TBD Low Current Mode 1 3 0 TBD 19 TBD Enhanced Linearity Mode 1 3 1 TBD 19 TBD Rx Mode5 1 10 1 TBD 20 TBD Rx Mode6 0 30 1 TBD 21 TBD |
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Similar Description - ADF7021 |
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