The Lock-in Technique

The Lock-in technique is used to detect and measure very small ac signals. A Lock-in amplifier can make accurate measurements of small signals even when the signals are obscured by noise sources which may be a thousand times larger. Essentially, a lock-in is a filter with an arbitrarily narrow bandwidth which is tuned to the frequency of the signal. Such a filter will reject most unwanted noise to allow the signal to be measured. A typical lock-in application may require a center frequency of 10 KHz and a

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bandwidth of 0.01 Hz. This 'filter' has a Q of 10- well beyond the capabilities of passive electronic filters.

In addition to filtering, a lock-in also provides gain. For example, a 10 nanovolt signal can be amplified to produce a 10 V output--a gain of one billion.

All lock-in measurements share a few basic principles. The technique requires that the experiment be excited at a fixed frequency in a relatively quiet part of the noise spectrum. The lock-in then detects the response from the experiment in a very narrow bandwidth at the excitation frequency.

Applications include low level light detection, Hall probe and strain gauge measurement, micro-ohm meters, C-V testing in semiconductor research, electron spin and nuclear magnetic resonance studies, as well as a host of other situations which require the detection of small ac signals.

A Measurement Example

Suppose we wish to measure the resistance of a material, and we have the restriction that we must not dissipate very much power in the sample. If the resistance is about 0.1Ω and the current is restricted to 1 µA, then we would expect a 100 nV signal from the resistor. There are many noise signals which would obscure this small signal -- 60Hz noise could easily be 1000 times larger, and dc potentials from dissimilar metal junctions could be larger still.

In the block diagram shown below we use a 1Vrms sine wave generator at a frequency wr as

our reference source. This source is current limited by the 1 MΩ resistor to provide a 1 µA ac excitation to our 0.1Ω sample.

Two signals are provided to the lock-in. The 1VAC reference is used to tell the lock-in the exact frequency of the signal of interest. The lock-in's Phase-Lock Loop (PLL) circuits will track this input signal frequency without any adjustment by the user. The PLL has two outputs, cos(wrt) and

sin(wrt).

The signal, Vs cos(wst+Ø), from the sample under

test is amplified by a high gain ac coupled differential amplifier. The output of this amplifier is multiplied by the PLL outputs in two Phase- Sensitive Detectors (PSD1 and PSD2). This multiplication shifts each frequency component of the input signal, ws, by the reference frequency,

wr, so that the output of the PSD's are given by:

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SRS Labs Lock-In Amplifier, SR530 manual Lock-in Technique, Measurement Example
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SR530, Lock-In Amplifier specifications

The SRS Labs Lock-In Amplifier, model SR530, is a powerful tool designed for high-precision measurements in the realm of scientific research and industrial applications. This state-of-the-art instrument excels in extracting small signals from noisy environments, making it an invaluable asset for experiments in fields such as physics, engineering, and materials science.

One of the main features of the SR530 is its ability to perform synchronous detection, which is key to improving signal-to-noise ratios. By utilizing a reference signal, the device correlates the incoming signal with the reference to effectively filter out noise, allowing for the accurate measurement of weak signals that might otherwise be obscured. This process of phase-sensitive detection is fundamental to the operation of the Lock-In Amplifier.

The SR530 offers a wide frequency range, covering from 0.1 Hz to 100 kHz. This broad frequency response allows it to handle a diverse array of signals, making it suitable for various applications including optical detection, capacitance measurements, and in many cases, voltammetry. The device is also equipped with multiple inputs and outputs, facilitating the integration with other laboratory equipment and enabling complex experimental setups.

Precision is further enhanced with its adjustable time constant, which allows users to optimize the response time based on experimental needs. The user can choose time constants from 10 microseconds to 10 seconds, accommodating fast dynamic measurements as well as those requiring stability over longer durations.

Another remarkable characteristic of the SR530 is its digital processing capabilities. The device features a highly accurate digital voltage measurement system, minimizing drift and ensuring long-term stability. Additionally, the use of microprocessors enhances data handling and allows for features such as programmable settings, facilitating automated measurements.

Moreover, the SR530 includes a range of output options, including analog outputs, which can be used for direct signal processing, as well as digital interfaces for integration with computers. This ensures that users can not only capture high-fidelity data but also analyze and display it efficiently.

In conclusion, the SRS Labs SR530 Lock-In Amplifier stands out due to its sophisticated technology, versatile features, and robust performance. Its precision, flexibility, and ease of use make it an ideal choice for researchers and engineers looking to unlock the potential of weak signal measurement in complex environments.
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