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AD9877-EB Datasheet(PDF) 25 Page - Analog Devices |
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AD9877-EB Datasheet(HTML) 25 Page - Analog Devices |
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25 / 37 page  AD9877 Rev. B | Page 24 of 36 TRANSMIT PATH (Tx) tSU tHD MCLK TxSYNC TxIQ TxI[11:6] TxI[5:0] TxQ[11:6] TxQ[5:0] TxI[11:6] TxI[5:0] TxQ[11:6] TxQ[5:0] TxI[11:6] TxI[5:0] Figure 26. Transmit Timing Diagram TRANSMIT TIMING The AD9877 provides a master clock, MCLK, and expects 6-bit multiplexed TxIQ data upon each rising edge. Transmit symbols are framed with the TxSYNC input. TxSYNC high indicates the start of a transmit symbol. Four consecutive 6-bit data packages form a symbol (I MSB, I LSB, Q MSB, and Q LSB). DATA ASSEMBLER The input data stream is representative complex data. Two 6-bit words form a 12-bit symbol component (twos complement format). Four input samples are required to produce one I/Q data pair. The I/Q sample rate, fIQCLK, at the input to the first half-band filter is a quarter of the input data rate, fMCLK. The I/Q sample rate, fIQCLK, puts a bandwidth limit on the maximum transmit spectrum. This is the familiar Nyquist limit and is equal to one-half fIQCLK, hereafter referred to as fNYQ. HALF-BAND FILTERS (HBFs) HBF 1 and HBF 2 are both interpolating filters, each of which doubles the sampling rate. Together, HBF 1 and HBF 2 have 26 taps and provide a factor of 4 increase in the sampling rate (4 × fIQCLK or 8 × fNYQ). In relation to phase response, both HBFs are linear phase filters. As such, virtually no phase distortion is introduced within the pass band of the filters. This is an important feature, because phase distortion is generally intolerable in a data transmission system. CASCADED INTEGRATOR-COMB (CIC) FILTER A CIC filter is unlike a typical FIR filter in that it offers the flexibility to handle differing input and output sample rates in any integer ratios. In the AD9877, the CIC filter is configured as a programmable interpolator and provides a sample rate increase by a factor of R = 3 or R = 4. In addition to the ability to provide a change in the sample rate between the input and output, a CIC filter has an intrinsic low-pass frequency response characteristic. The frequency response of a CIC filter is dependent on three factors: • The rate change ratio, R. • The order of the filter, n. • The number of unit delays per stage, m. It can be shown that the system function H(z) of a CIC filter is given by n Rm k k Rm z R z z R Hz ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = ∑ − = − − − 1 0 1 1 1 1 1 The form on the far right has the advantage of providing a result for z = 1 (corresponding to zero frequency or dc). The alternate form yields an indeterminate form (0/0) for z = 1, but is otherwise identical. The only variable parameter for the CIC filter of the AD9877 is R; m and n are fixed at 1 and 3, respectively. Thus, the CIC system function for the AD9877 simplifies to 3 1 0 3 1 1 1 1 1 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = ∑ − = − − − Rm k k R z R z z R Hz The transfer function is given by () () () () 3 3 ) 2 ( / 2 sin sin 1 1 1 1 ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ π π ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = π − π − f fR R e e R f H f j R j The frequency response in this form is such that f is scaled to the output sample rate of the CIC filter. That is, f = 1 corresponds to the frequency of the output sample rate of the CIC filter. H(z) yields the frequency response with respect to the input sample of the CIC filter. |
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