What's the difference between external and external (PLL) mode?
Written by Paul Cracknell
Updated at July 18th, 2024
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External:
The External mode directly multiplies the signals from input 1 and input 2, which is particularly useful when the modulation signal is not sinusoidal. For instance, if the modulation signal is a low-duty-cycle pulse, direct multiplication significantly enhances the demodulation spectral coverage compared to sinusoidal demodulations. However, in this mode, the lock-in amplifier can only demodulate the X (in-phase) component and cannot calculate R or theta.
External (PLL):
When the modulation signal is a single-tone signal (sine wave), the External demodulation mode performs worse than the External (PLL) mode because it includes noise from both inputs in the final measured results. The External (PLL) establishes a phase-locked loop with input 2 and generates a pair of noiseless sinusoidal waves at the same frequency for dual-phase demodulation. This allows the user to access the X, Y, R, and theta directly from the lock-in amplifier. The user can also fine-tune the phase of the local oscillator with this mode. This is the recommended external reference mode for most use cases.
