Lock-in amplifiers are a useful tool for measuring signal amplitude and phase. The data acquisition card can digitize the output signal of the amplifier, and the software can calculate the amplitude and phase. A DSP or general-purpose processor can also be used to calculate amplitude and phase. The circuit in this article integrates two AD9954 DDS (direct digital synthesizer) and AD630 balanced modulator / demodulator of Analog Devices company in a low-cost lock-in amplifier (Figure 1).
An AD9954 generates a reference signal and measurement signal for the target. Then AD630 modulates this reference signal and target signal. The low-pass filter eliminates the 2t component calculated from the signal modulation. However, when the AD630 modulator function is operated at a frequency higher than 50 kHz, the output waveform of the AD630 will produce a measurement error, which will cause a signal analysis error in the phase measurement.
When the reference signal and the measurement signal are in phase, the modulation result is the maximum value. The maximum search method can increase the working frequency of the AD630 modulation function. Through a flow chart, two AD9954 can be controlled to generate a reference signal and a target signal (Figure 2). Then, AD630 modulates the two signals, digitizes the result, and saves them in the register. Next, phase shift the reference signal and repeat the above steps until the circuit sweeps through all possible phase differences.
Then, search for the maximum value in the data set. When modulating signals of different phases, the phase shift of the reference signal indicates the phase shift of the target signal. Table 1 is the real situation when the reference signal and the target signal are shifted by 30 °. At 50 kHz, the phase-locked calculation error is 19%, which is greater than the 5% phase shift value at 10 kHz. These results indicate that peak performance occurs at high frequencies. Using search results, you can increase the accuracy of 10 kHz to 3% and 50 kHz to 7%.
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