AD9879-EB datasheet(22/32 Pages) AD

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AD9879

Rev. A | Page 22 of 32

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 14. Transmit Path Timing

TRANSMIT TIMING

The AD9879 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 (in 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-four 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

The CIC filter is configured as a programmable interpolator

and provides a sample rate increase by a factor of 4. The

frequency response of the CIC filter is given by

()

2π

sin

4π

sin

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

⎥

⎦

⎤

⎢

⎣

⎡

−

⎟

⎠

⎞

⎜

⎝

⎛

−

H(f)

(15)

The frequency response in this form has f 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(f/R)

yields the frequency response with respect to the input sample

of the CIC filter.

COMBINED FILTER RESPONSE

The combined frequency response of HBF 1, HBF 2, and

CIC puts a limit on the input signal bandwidth that can be

propagated through the AD9879.

The usable bandwidth of the filter chain puts a limit on the

maximum data rate that can be propagated through the

AD9879. A look at the pass-band detail of the combined filter

response (Figure 15 and Figure 16) indicates that to maintain an

amplitude error of no more than 1 dB, signals are restricted to a

bandwidth of no more than approximately 60% of fNYQ.

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