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ADF7901BRU-REEL 데이터시트(PDF) 9 Page - Analog Devices |
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ADF7901BRU-REEL 데이터시트(HTML) 9 Page - Analog Devices |
9 / 12 page ADF7901 Rev. A| Page 9 of 12 CIRCUIT DESCRIPTION Table 4. Frequency (MHz) FSK3 FSK2 FSK1 OOK_SEL 369.5 0 0 0 0 371.1 0 0 1 0 375.3 0 1 0 0 376.9 0 1 0 0 384.0 Don’t care Don’t care Don’t care 1 388.3 1 0 0 0 391.5 1 0 1 0 394.3 1 1 0 0 395.9 1 1 1 0 LOOP FILTER The loop filter integrates the current pulses from the charge pump to form a voltage that tunes the output of the VCO to the desired frequency. It also attenuates spurious levels generated by the PLL. The recommended loop filter design for this circuit is 297 kHz. This is based on the trade-off between attenuation of beat note spurs and the need to minimize chirp when the PA is turned on. CHARGE PUMP OUT VCO R2 = 6.2k Ω C3 = 10pF C1 = 33pF C2 = 390pF R1 = 3k Ω Figure 7. Improved spurious performance in FSK mode can be achieved by using a narrower loop bandwidth. For a data rate of 20 kbps, a loop bandwidth of roughly 50 kHz would be suitable. The following components give a loop bandwidth of 51.1 kHz: C1 = 680 pF C2 = 15 nF C3 = 180 pF R1 = 510 Ω R2 = 6.2 kΩ ADIsimPLL is a free software tool offered by Analog Devices for assistance in designing with ADI’s frequency synthesizers and ISM band transmitters. To select the correct loop filter components for use with the ADF7901, open a project for the ADF7012 device. Then, enter the desired output carrier frequency and loop bandwidth, and use the 870 μA charge pump current setting. ADIsimPLL can be downloaded from www.analog.com. CHANNEL FREQUENCIES The nine channel frequencies listed in Table 4 are obtainable from a single 9.8304 MHz crystal reference by changing the value of the N and F numbers in the fractional PLL, using control lines FSK1, FSK2, and FSK3. The channel frequency is given by FCHANNEL = FREF × (N + F) However, the VCO is tuned to operate over a frequency range of 344 MHz to 401 MHz (typically). Therefore, any channel frequency within this range can be obtained if the required reference frequency is used. The N and F numbers for each channel are listed in Table 5, together with the corresponding channel frequencies for 9.8304 MHz and, for example purposes, frequencies for 10 MHz. With the 10 MHz reference, the two largest N settings give channel frequencies above the maximum VCO output frequency and are therefore invalid. Frequency deviation is also dependent on reference frequency. The relationship is given by FDEV = 58 × (9.8304 MHz)/214 Therefore, the frequency deviation is 34.8 kHz when the 9.8304 MHz reference is used and 35.4 kHz when the 10 MHz reference is used. Table 5. Channel Frequency (MHz) N F 9.8304 MHz Ref 10 MHz Ref 37 2406/4096 369.5 375.9 37 3073/4096 371.1 377.5 38 727/4096 375.3 381.8 38 1374/4096 376.9 383.4 39 256/4096 384.0 390.6 39 2048/4096 388.3 395 39 3381/4096 391.5 398.3 40 452/4096 394.3 N/A 40 1118/4096 395.9 N/A |
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