What is Lock-in amplifier: Definition and 16 Discussions

A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment. Depending on the dynamic reserve of the instrument, signals up to 1 million times smaller than noise components, potentially fairly close by in frequency, can still be reliably detected. It is essentially a homodyne detector followed by low-pass filter that is often adjustable in cut-off frequency and filter order. Whereas traditional lock-in amplifiers use analog frequency mixers and RC filters for the demodulation, state-of-the-art instruments have both steps implemented by fast digital signal processing, for example, on an FPGA. Usually sine and cosine demodulation is performed simultaneously, which is sometimes also referred to as dual-phase demodulation. This allows the extraction of the in-phase and the quadrature component that can then be transferred into polar coordinates, i.e. amplitude and phase, or further processed as real and imaginary part of a complex number (e.g. for complex FFT analysis).
The device is often used to measure phase shift, even when the signals are large, have a high signal-to-noise ratio and do not need further improvement.
Recovering signals at low signal-to-noise ratios requires a strong, clean reference signal with the same frequency as the received signal. This is not the case in many experiments, so the instrument can recover signals buried in the noise only in a limited set of circumstances.
The lock-in amplifier is commonly believed to have been invented by Princeton University physicist Robert H. Dicke who founded the company Princeton Applied Research (PAR) to market the product. However, in an interview with Martin Harwit, Dicke claims that even though he is often credited with the invention of the device, he believes that he read about it in a review of scientific equipment written by Walter C. Michels, a professor at Bryn Mawr College. This could have been a 1941 article by Michels and Curtis, which in turn cites a 1934 article by C. R. Cosens, while another timeless article was written by C. A. Stutt in 1949.

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  1. A

    Cannot lock reference signal on lock-in amplifier

    Hello everyone, I am using a lock-in amplifier SR860 with the aim of reading an optical pulse from a laser break-out board operating at 30 kHz as the reference. The reference signal is not being picked up by the lock-in amplifier. However, it is being read by the oscilloscope. I have used a...
  2. fatima ece

    Lock-in amplifier with pulsed sinewave

    Hi, I am trying to use the SR865A lock-in amplifier to measure the frequency of a pulse of a sine wave( example: waveform below), - the sinewave signal is only ON during finite time not continuously-, I am using an external reference signal that is also a pulse of a sinewave with the same...
  3. Sotiris

    Problem with Lock-in Amplifier (cannot lock the signal)

    Hello, I'm using a lock in amplifier (model 5210 from signal recovery) for some experiments with photodiodes. Firstly, there is an optical chopper that "feeds" the lock in (reference signal). My input is a photodiode (in current mode). The problem is,the unlock (unlk) light is always on in the...
  4. N

    SR830 lock-in amp measurement problem

    Hello I am currently have difficulties trying to measure dc-modulated voltages while applying a dc current, via Sr830 Lock-in Amplifier (LIA). Basically, I apply an 2 GHz ac current (amplitude modulated@10kHz and referenced to the LIA for lock-in measurement) and a dc current to my DUT...
  5. Hugo_Researcher

    Research on Lock-in Amplifiers

    Dear Physics Forums, For a research we are conducting, we're seeking some input from users of a Lock-in Amplifier. If you use a Lock-in Amplifier, could you provide me with a few answers to the following questions? What's your overall experience with lock-in amplifiers? Have you encountered...
  6. B

    Simple photodiode connected to lock-in amplifier

    Hello. I am currently modifying a semiconductor experiment. A laser beam is modulated with an acousto-optical modulator and illuminate the semiconductor sample. The sample is probed by microwaves. The reflected microwave signal go through a demodulator, and then in a lock-in amplifier. Then I...
  7. alanwake90

    Questions about Lock-in detection in optical reflectivity

    Hi guys~ I have a question about lock-in detection method that commonly used in optical spectroscopy experiments, for example, the optical pump-probe reflectivity exeperiments on some semiconductor surfaces, or kinda like that. Usually, the sinario is: you use laser pulses, divided as pump and...
  8. M

    Phase change in lock-in amplifier

    I just started to use a lock-in amplifier, and I am trying to following the procedures by adjusting zero phase. But my input phase changes with time and sensitivity, and I couldn't lock in onto it. The question is, is this normal that phase changes all the time during the experiment or if there...
  9. F

    Sensitivity of lock-in amplifier

    Hey I have a quick question whose answer is well-hidden on google since the key word of the search attempts is sensitivity, which of course returns results about phase-sensitive detection, and seemingly phase-sensitive detection only... My question is what the exact definition of the...
  10. A

    Designing a Lock-in amplifier from zero

    Hello everyone I have completed my freshman year in EE. For the summer, my professor has asked me to design a lock-in amplifier. First he has asked me to read about AD630. Now after reading the datasheet at (http://www.analog.com/static/imported-files/data_sheets/AD630.pdf), I was a bit...
  11. Topher925

    Lock-in amplifier driving me crazy

    I have a project where I'm measuring the phase of a signal compared to a signal reference using an SR830 lock-in amplifier. My problem is that the amplitude of the input signal periodically changes over time and this is causing a false measurement in the phase of the input signal. I can't figure...
  12. N

    Lock-in Amplifier & Square Wave: How to Get DC-term?

    Hi Usually when a lock-in amplifier is introduced to students, it is done when the input signal is a harmonic function and the reference is one as well. This is a nice concept, and it is easy to see how a DC-term propotional to the (desired) incident amplitude can be obtained. My question...
  13. Z

    What is a lock-in amplifier with a forcing grid ?

    What is a "lock-in amplifier with a forcing grid"? The context of the title is in an undergrad paper, concerning plasma waves in an glow discharge column. The paragraph which mentioned this is in the section of the paper, Suggestions for Further Research: The most important step remaining...
  14. C

    Current measurement with Lock-In Amplifier SR830

    I have two current signals from a two color Si-photodiode/PbS-photodetector. I want to measure the signal with the Stanford Research Lock-In Amplifier 830. I read in the manual that I can measure currents with the SR830. Does anyone have experience with this? Do I get better signals if I...
  15. E

    How can I accurately measure high resistances using a lock-in amplifier?

    Dear all, I measured some very nice data, but it's difficult to analyze, but I try to get out of it what I can. I have diverging resistance and did measure with standard lock-teqnique: apply 0.45V over preresistor and send current over sample and measure the two-probe resistance over the same...
  16. M

    Lock-in Amplifier with Labview

    Hi to all, For a project I need to measure output (as Volts) of a circuit. I want to do this with a SRS830 lock-in by using Labview application. Google search gave me nothing. If anyone knows how to set this up, please help. Thanks