Introduction: The Importance of Reference Signal Selection |
In phase-sensitive detection, the choice of the reference signal source directly impacts the measurement accuracy of a lock-in amplifier. Depending on how your signal under test is modulated, you must choose between Internal Reference Mode and External Reference Mode to achieve the best noise rejection and signal integrity.
Internal Reference Mode: The Gold Standard for Precision |
Whenever possible, it is highly recommended to modulate the signal under test using the excitation source provided directly by the lock-in amplifier—typically through the Sinout (Sine Output) or TTL Output terminals.
Why it delivers higher accuracy: In this setup, the lock-in amplifier has direct, hardware-level access to the exact amplitude, frequency, and phase of the reference signal it generated. This eliminates phase jitter and tracking errors, resulting in significantly higher measurement precision.
External Reference Mode: Adapting to External Modulators |
In certain experimental setups, utilizing an internal source is impossible. A classic example is using an optical chopper to modulate an optical signal.
The Mechanism: The optical chopper modulates the physical light beam and simultaneously outputs a transistor-transistor logic (TTL) square wave that is synchronous with the chopping frequency.
How Lock-in Processing Works: This square wave is fed into the lock-in amplifier’s Ref In (Reference Input) terminal. The instrument then deploys internal Phase-Locked Loop (PLL) technology to lock onto the phase of the Ref In signal. It generates an internal sine wave of identical frequency and phase, routing it to the Phase-Sensitive Detector (PSD) for correlation computing.
Related products: |
Lock-In Amplifiers: Principle, Applications & Products | Saluki Technology
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