Swept sine oscillator has quadrature output


Posted on Feb 6, 2014

Signals with a know frequency but unknown phase may be detected using an in-phase and a 90 ° shifted reference signal, as frequently used in lock-in amplifiers, synchronous detectors, and frequency-response analyzers. The 90 °-shifted signal is called the quadrature signal. By multiplying the received signal with the reference signals, the low-pas


Swept sine oscillator has quadrature output
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s filtered outputs will represent the real and imaginary components of the received signal. Once the two components are known, this complex number can be converted into amplitude and phase. When used for frequency response analysis, the system is excited with a sine signal and the output of the system is demodulated using the two references. Neglecting harmonics, the multiplication can be performed using square-wave reference signals, which control amplifiers with a digitally controlled gain of either +1 or ’1. The frequency must be swept over the range of interest. Designing a swept-sine generator with quadrature output isn`t a trivial task, though. For low-frequency purposes, an undocumented feature of Maxim`s MAX038 oscillator chip makes this easy. The MAX038 is a sine, triangle, and square-wave generator whose frequency can be swept over 2 to 3 decades by a control current. By switching the frequency-determining capacitor, this range can be considerably extended. The MAX038 has an OUT signal output and a SYNC logic level output (when powered by ±5 V). The SYNC output always has a 50% duty cycle, but the duty cycle of the output signal can be adjusted. For quadrature purposes, the output signal should be adjusted to 50% duty. The chip also contains a phase detector that can be used to generate the quadrature. Figure 1 indicates the phase relations of SYNC, OUT, and the phasedetector output (PDO). By setting a constant...




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