If the signal source used to modulate the signal under test is derived from the lock-in amplifier’s sinusoidal or TTL output, the system operates in internal reference mode. In this mode, the lock-in amplifier directly accesses the amplitude and phase of the reference signal, resulting in higher measurement accuracy.
Therefore, whenever possible, the signal source provided by the lock-in amplifier itself—i.e., the internal reference mode—should be used to modulate the signal under test.
In certain situations, however, external reference mode is required. For instance, when using an optical chopper to modulate an optical signal, the chopper outputs a square-wave signal—synchronized in phase with the modulation—which is fed into the lock-in amplifier’s reference input.
The lock-in amplifier employs phase-locked loop (PLL) technology to lock onto the phase of this reference signal, generating a sine wave of the same frequency and phase to supply to the phase detector for correlation processing.
