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AD6402 Datenblatt(PDF) 7 Page - Analog Devices |
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AD6402 Datenblatt(HTML) 7 Page - Analog Devices |
7 / 8 page AD6402 –7– REV. 0 supply current levels when not allocated to an active transmit or receive timeslot in a TDMA system.. The respective transmit and receive blocks can be turned on only as needed thereby reducing power consumption and extending battery life of handheld terminals. The component selection in Figure 5 is explained as follows: The IF input is driven from the output of a SAW filter via an impedance matching circuit as shown. This matching minimizes the insertion loss of the filter and follows the filter manufactur- ers recommendations. The tank circuit shown uses two varactor diodes. One diode (D3) is biased by the output of the IF PLL loop filter and ensures that the IF VCO frequency is correctly centered. The second diode is provided to enable a modulation signal, which is generated at the output of the on-chip op amp (MODOUT), to be coupled into the VCO tank and thereby implement a modulation of the VCO frequency. In the case of DECT, the IF VCO control loop is opened while the VCO is being modulated by the transmit bit stream. The loop is opened by tri-stating the output of the IF VCO PLL charge pump. The exact component values used around the modulation am- plifier will be determined by the amount of attenuation required for suppression of baseband transmit spurii and images. These artifacts are usually present if the baseband FSK signal is gener- ated by a ROMDAC. In most instances a second or third order Bessel or Butterworth filter will be required. A capacitor to ground is required to be connected to COFF. This capacitor stores the demodulator charge-pump voltage required to lock the demodulator VCO to the reference fre- quency. The dynamic response of the demodulator loop is con- trolled by selection of the values for C45 and R22 which are connected in series to CFILT. These components determine the IFVCC1 IFVCC2 DOUT DFILP C71 1nF C44 100nF VRF CP REG REF CP /3,/5 PD RSSI PLLOUT PLLVCC PLLGND REXT CFILT REFSEL FMMOD2 FMMOD1 MODOUT TXOUTB TXOUT CTL1 CTL2 CTL3 VCOGND VCO SLREF VREG VBAT IFGND IFIN COFF REFIN C30 150pF R16 330 Ω C28 1nF C29 1nF L11 39nH C41 8pF C42 47pF R26 1.2k Ω C50 4.7nF C25 1nF R21 240 Ω D2 D3 C43 100nF VIF C44 1nF VCC 12pF C10 1nF L7 100nH L6 150nH 10 9 8 7 6 1 2 3 4 5 L5 150nH C69 1nF CP C52 100pF R28 330 Ω C51 180pF R32 330 Ω R70 C32 100nF C45 131nF R22 330 Ω C13 1nF C31 R17 10k Ω C11 100pF VRF R10 2k Ω R9 1k Ω C14 68pF C12 33pF R30 2k Ω C58 47pF RXBB RSSI C4 3.3nF R8 4.7k Ω AD6402 SMV 1204-37 SMV 1204-36 TXIF– TXIF+ IFC0 IFC1 IFC2 VREF VTUNE B4535 IFIN TXBB R35 4.7k Ω RCLK Figure 5. Application Circuit for DECT GFSK Transceiver transfer characteristic of the loop filter and hence the lock time, settling time and bandwidth of the loop. REXT should use the recommended value as shown. Finally, the demodulator is followed by a voltage follower, which is configured as a data filter. This data filter is used to bandlimit the FM noise generated in the demodulator. It also attenuates undesired adjacent channel interferers. The compo- nent values chosen will be a trade-off between the amount of band limiting required and attenuation of the in-band desired signal. DECT Application Circuit Notes (Figure 5) 1. Signal Description VRF: Regulated Supply Voltage; Nominal Value 2.85 V. VCC: Unregulated battery voltage; 3.1 V–4.5 V VTUNE: Synthesizer Control Voltage; Range dependent on loop filter and synth charge pump compliance. TXBB: Baseband transmit modulation voltage; typically SLREF ±0.7 V RCLK: Reference clock for PLL demodulator; 13.824 MHz (2nd IF frequency = (N/M) × Frclk where N = 3 or 5, and M = 2. Maximum 2nd IF = c.26 MHz) 2. Typical IF input sensitivity referred to the input of SAW filter for the above application will be –72 dBm. 3. TxBB filter is user configurable. In the above application, the filter is implemented to remove images generated by ROM DAC baseband signal generators. Other implementations are possible including passive pulse shaping circuits which elimi- nate the need for such filtering. |
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