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AD9878

Rev. A | Page 21 of 36

tSU

tHU

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 24. Tx Timing Diagram

TRANSMIT PATH

The transmit path contains an interpolation filter, a complete

quadrature digital upconverter, an inverse sinc filter, and a 12-bit

current output DAC. The maximum output current of the DAC is

set by an external resistor. The Tx output PGA provides additional

transmit signal level control. The transmit path interpolation

filter provides an upsampling factor of 16 with an output signal

bandwidth as high as 4.35 MHz for <1 dB droop. Carrier

frequencies up to 65 MHz with 26 bits of frequency tuning

resolution can be generated by the direct digital synthesizer

(DDS). The transmit DAC resolution is 12 bits, and it can run at

sampling rates of up to 232 MSPS. Analog output scaling from

0 dB to 7.5 dB in 0.5 dB steps is available to preserve SNR when

reduced output levels are required.

DATA ASSEMBLER

The AD9878 data path operates on two 12-bit words, the I and Q

components, that form a complex symbol. The data assembler

builds the 24-bit complex symbol from four consecutive 6-bit

words read over the TxIQ [5:0] bus. These words are strobed

into the data assembler synchronous to the master clock (MCLK).

A high level on TxSYNC signals the start of a transmit symbol.

The first two 6-bit words of the symbol form the I component;

the second two 6-bit words form the Q component. Symbol

components are assumed to be in twos complement format. The

timing of the interface is fully described in the Transmit Timing

section. The I/Q sample rate fIQCLK puts a bandwidth limit on the

maximum transmit spectrum. This is the familiar Nyquist limit

(hereafter referred to as fNYQ) and is equal to half fIQCLK.

TRANSMIT TIMING

The AD9878 has a master clock and expects 6-bit, multiplexed

TxIQ data upon each rising edge (see Figure 24). 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).

INTERPOLATION FILTER

Once through the data assembler, the IQ data streams are fed

through a 4× FIR low-pass filter and a 4× cascaded integrator

comb (CIC) low-pass filter. The combination of these two filters

results in the sample rate increasing by a factor of 16. In addition

to the sample rate increase, the half-band filters provide the

low-pass filtering characteristics necessary to suppress the spectral

images between the original sampling frequency and the new

(16× higher) sampling frequency.

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 increase the sampling rate by a factor of 4

(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 dis-

tortion is generally intolerable in a data transmission system.

CASCADE 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:

()

sin

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

⎥

⎦

⎤

⎢

⎣

⎡

−

⎟

⎠

⎞

⎜

⎝

⎛

−

COMBINED FILTER RESPONSE

The combined frequency response of the HBF and CIC filters

limits the input signal bandwidth that can be propagated through

the AD9878.The usable bandwidth of the filter chain limits the

maximum data rate that can be propagated through the AD9878.

A look at the pass-band detail of the combined filter response

(Figure 25) indicates that to maintain an amplitude error of

1 dB or less, signal bandwidth is restricted to about 60% or less

of fNYQ.

Max BW (1dB droop) = 0.60 * fMCLK/8

Thus, in order to keep the bandwidth of the data in the flat

portion of the filter pass band, the user must oversample the

baseband data by at least a factor of two prior to presenting it to

the AD9878. Note that without oversampling, the Nyquist

bandwidth of the baseband data corresponds to fNYQ. As such,

the upper end of the data bandwidth suffers 6 dB or more of

attenuation due to the frequency response of the digital filters.

Furthermore, if the baseband data applied to the AD9878 has

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