A standard single-channel direct digital synthesizer (DDS) does not switch between frequencies in a phase-coherent manner. By design, DDS frequency conversion has "phase continuity" (shown in Figure 2). However, the circuit shown in Figure 1 demonstrates how to configure the AD9958 / AD9959 multi-channel DDS to achieve a stable phase-coherent FSK (frequency shift keying) modulator by superimposing the multi-channel DDS outputs.

For the same application, multi-channel DDS almost completely eliminates inter-channel temperature and timing issues compared to synchronizing multiple single-channel devices. For example, multi-channel DDS outputs, although independent of each other, can share the same system clock edge of the chip. Therefore, system clock edges on multiple chips will track temperature and power supply deviations slightly less well than integrated multi-channel DDS. Generally speaking, multi-channel DDS is more suitable for producing ideal phase coherent frequency conversion at the superposition output end.

The AD9520-X clock generator and distribution IC drives the AD9958/AD9959 with a high-performance reference clock and also clocks the data source of the FSK data stream, which is a pseudo-random sequence (PRS). The AD9520 provides multiple output logic selections and delay adjustment wipers to meet setup and hold time requirements between the FSK data stream and the multi-channel AD9958/AD9959 DDS SYNC_CLK.

The AD9958 has two built-in independent DDS channels that provide differential current outputs. In the circuit, those current outputs are connected together (superimposed) via preprogrammed frequencies (F1 and F2). To select the desired frequency, the channel outputs are equipped with an on/off function driven via the Profile pin. In this example, the Profile pins are configured to drive multipliers on each DAC input to control the output amplitude.

For this purpose, each multiplier is preprogrammed with two Profile selectable settings: zero scale and full scale. A logic low on the Profile pin turns off the sine wave at the DAC output, while a logic high on the profile pin delivers the sine wave. This operation requires two complementary input data streams to alternate between the two frequencies.

Two DDS channels operate continuously, producing frequencies F1 and F2. The shutdown function mutes the corresponding DDS output, resulting in a phase-coherent FSK signal.

A quad-channel AD9959 DDS is used to generate the unfiltered waveforms shown in Figures 3 and 4. The AD9959 has better phase-coherent switching performance because the two unused channels can be used as a phase reference for summing the two switching frequencies at the output. The upper trace represents the superimposed output of phase-coherent switching. The next two traces are the reference signals for F1 and F2. The lower trace is a pseudo-random sequence (PRS) data stream that alternates between two frequencies. Note that due to pipeline delays within the device, the PRS data stream edges are not aligned with the frequency transitions of the superimposed output.