Modulating a Reference Allows Maximum-value Search for Phase Detection

A lock-in amplifier is a useful tool for measuring a signal’s amplitude and phase. A data-acquisition card can digitize the amplifier’s output signal, and software can calculate amplitude and phase. A DSP or a general-purpose processor can also calculate amplitude and phase. The circuit in this Design Idea integrates two Analog Devices AD9954 DDS (direct digital synthesizers) and an AD630 balanced modulator/demodulator into a low-cost lock-in amplifier (Figure 1).


FIGURE 1

One AD9954 generates a reference signal and a measurement signal for the target. The AD630 then modulates the reference signal and the target signal. A low-pass filter removes the 2ωt component that you calculate from the signal modulation. When the modulator function of the AD630 operates at frequencies higher than 50kHz, however, the output waveform of the AD630 causes a measurement error and thus a signal-analysis error in the phase measurement.

When the reference and measurement signals are in phase, the result of modulation is at its maximum. A maximum-value-search method improves the operating frequency of the AD630’s modulation function. Using a flow chart, you can control two AD9954s to generate the reference and target signals (Figure 2). The AD630 then modulates the two signals, digitizes the results, and stores them in registers. Later, you shift the phase of the reference signal and repeat the procedure until the circuit sweeps through all possible differences in phase.


FIGURE 2


TABLE 1

You then search for the maximum value in the data set. When you modulate the signal at different phases, the phase shift of the reference signal indicates the phase shift of the target. Table 1 represents the real case with 30° phase shift between the reference and the target signals. At 50kHz, the error of lock-in calculation is 19 percent, which is larger than the 5 percent phase shift at 10kHz. Those results indicate weak performance at high frequencies. Using the search results, you can increase the accuracy to 3 percent at 10kHz and 7 percent at 50kHz.