AD9174 datasheet(61/164 Pages) AD

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Data Sheet

AD9174

Rev. A | Page 61 of 164

In phase coherent mode, a bank of 31 additional phase

accumulators is enabled, which maintains count regardless of

which accumulator value is applied to the main accumulator.

The result is that the phase of all 32 FTWs is known at all times

for each carrier frequency, thus allowing to maintain phase

coherency between hops, relative to Time 0 (the time when the

NCO was last reset). Therefore, it is recommended to preload

all FTWs and only then select the phase coherent switch mode

so that the NCO phase accumulators start at the same time.

To conserve power, each of the 31 additional NCOs is enabled only

when its corresponding FTW is programmed to a value other than

0x0. All NCO FTWs have a default value of 0x0. Note that the main

phase accumulator (corresponding to the 48-bit FTW, namely

FTW0), is concurrently enabled by the DDSM_NCO_EN bit in the

DDSM_DATAPATH_CFG register (Register 0x112, Bit 3 = 0b1).

Each of the 32-bit NCOs can be powered down at the convenience

of the user. If a 32-bit NCO is initially powered up, it is

recommended to first program the FTW to 0x0001 to flush the

accumulator from any residual values, and only then program

the FTW to 0x0000, which powers down the NCO output, but

not the accumulator. This method avoids the possibility of any

residual spurious tones appearing at the output of adjacent

NCOs during power-down, however unlikely.

Calibration NCO

In addition to the 48-bit NCO and the 31, 32-bit NCOs, there is

a 32-bit calibration NCO, which is also part of the main datapath

NCO block, shown in Figure 74. This NCO is separate from the

48-bit NCO, allowing a convenient method for generating a

calibration tone without the need to modify the configuration of

the main datapath. Similar to all other NCOs, this NCO can be

used in NCO only mode, or used to translate incoming data to a

new carrier frequency. Register 0x1E6, Bit 0 controls whether

the 32-bit calibration NCO is connected to the main datapath,

or whether the normal 48-bit main NCO is connected instead.

To use the 32-bit calibration NCO, first enable the calibration

NCO accumulator by setting Register 0x1E6, Bit 2 = 1. Then,

program the calibration NCO FTW in Register 0x1E2 to

be used instead of the main NCO by setting Register 0x1E6,

Bit 0 = 1. Similar to other NCOs, the calibration NCO can be

configured to operate in NCO only mode, which is enabled by

setting Register 0x1E6, Bit 1 = 1.

Set the amplitude of the tone can be set in Registers 0x148 and

details.

NCO ONLY MODE

The AD9174 NCOs can operate in standalone mode, where the

JESD204B link is disconnected (or disabled) and one or more NCO

tones are output from DAC0 and/or DAC1. The correct JESD204B

mode must still be selected to configure the corresponding

channelizer and/or main datapath clock domains. In NCO only

mode, a single-tone sine wave is generated by each NCO by

modulating the NCO output with dc samples that are generated

internally. The amplitude of the dc samples directly corresponds

to the amplitude of the NCO tone output by the DAC core. All

NCOs are modulated by the same set of dc samples and, therefore,

their amplitude cannot be independently controlled as shown in

Figure 72. If independent amplitude control is desired, consider

establishing a JESD204B link to provide dc samples from an

external data source.

The data source of the digital datapaths in NCO only mode is the

dc data word, meaning that whether the JESD204B link is

initially brought up or not, the data from the link is not passed

to the datapaths. However, the input to the datapaths is easily

switched between the dc data input and SERDES block input, by

either Register 0x130 or Register 0x1E6, depending on the data-

path. The connection can be made on-the-fly, assuming that a

JESD204B link is previously configured and proper data samples

are supplied. During the transition, sensitive external components

can be protected using the PA protection block as described

previously.

To use any of the NCOs in NCO only mode, the user can elect to

configure the AD9174 to operate in JESD204B Mode 0, Mode 1,

or Mode 2, depending on the desired number of channel NCOs.

Any other JESD204B mode can be selected instead, as long as

the NCO is not bypassed (interpolation = 1). To enable the

NCOs that connect to DAC1, a dual-link JESD204B mode can

be configured. It is not necessary to establish the JESD204B link

with an external source, such as an FPGA, and instead only a

few SPI register writes are needed to enable the necessary

JESD204B mode, to set up the clock domains corresponding to

each NCO.

In general, NCO only mode is a useful to bring up a transmitter

radio signal chain without requiring a digital data source initially,

or in applications where a sine wave output is all that is required

(also known as DDS mode), such as in LO or radar applications.

There is an additional optional calibration NCO block that can

be used as part of the initial system calibration without otherwise

making changes to the configuration of the digital datapath.

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