Two DDS ICs implement amplitude-shift keying
Many communications systems, including RFID systems and cable modems, use AM (amplitude modulation). This Design Idea shows how two DDS (direct-digital-synthesis) devices can implement AM and ASK (amplitude-shift keying) over a range of frequencies. The AD9834 complete 50-MHz DDS IC (Figure 1) has a current output, so you can easily sum the outputs of two or more of them by connecting them to a common termination resistor. Each AD9834 has two internal phase registers, P0 and P1, and two internal frequency registers, F0 and F1 (not shown). With each AD9834 generating a sine wave at the same frequency, the amplitude of the summed signal depends on the phase of each signal. You can achieve four preset amplitude levels—or any on-the-fly level—by summing the outputs of two AD9834s.
Table 1 shows two AD9834s, IC1 and IC2, configured to give four output levels. P0A is phase register 0 for IC1, P1A is phase register 1 for IC1, and so on. You can select the desired output level with either the PSEL pins or with the PSEL bits in the control register. Figure 2 shows the waveforms at the RTERM summing junction for the phases used in Table 1. You can achieve any signal level from 0V to a full-scale voltage of approximately 600 mV by programming the phase registers with the appropriate values. Both devices use the same MCLK (master clock), and you need to synchronize them to get the correct signal levels at the RTERM output. You achieve synchronization by simultaneously applying a Reset signal to both parts after programming both parts with the correct phase and frequency. You can accomplish the synchronization by applying a positive pulse to the Reset pins, or you can implement a software synchronization by setting the reset bit in the control register to one, stopping MCLK, setting the reset bit in the control register to zero, and then starting MCLK. Either method ensures that both parts simultaneously exit the reset state.
You can easily implement 100% AM with a single AD9834 by toggling the Reset pin or the reset bit in the control register. When the part is in reset, the DAC's output is at midscale. The predetermined sine wave is available at IOUT when the DDS exits reset. To calculate the magnitude of the sum of the two signals from the AD9834s, represent each signal as a rotating vector (Figure 3). You can easily calculate the magnitude and phase of the resulting summed vector as follows: If the length of each vector is 1, then:
Magnitude of resulting vector: =0.765;
Phase of summed vector: 112.5°(180°–Tan–1(y3/x3)).
The maximum output-voltage level that any one AD9834 can develop across the 100Ω termination resistor with RSET=6.8 kΩ is 320 mV p-p. Therefore, the voltage level this example achieves is 320 mV×0.765=244.8 mV. This example shows that the phase of the resulting summed vector depends on the phase of two input vectors and may result in a phase discontinuity as the phases of the input vector change. To avoid phase discontinuity at the transition, you can set the resultant phase, P3, to a fixed angle, say 180°≥p2=360°–p1.
Desired amplitude: A=2Cos [0.5(p1–p2)]; p1–p2=2Cos–1(A/2).
Resultant phase: P3=(p2+p2)/2.
Therefore, p1=180°+Cos–1(A/2), and p2=180°–Cos–1(A/2) gives amplitude A with no phase shift at the transition.