
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]
02773-014
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
()
3
2π
4
2π
π
sin
4π
sin
4
1
4
1
=
=
f
e
H(f)
f)
j(
f
j
(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
amplitude error of no more than 1 dB, signals are restricted to a
bandwidth of no more than approximately 60% of fNYQ.