NOISE MEASUREMENTS

Lock-in amplifiers can be used to measure noise. Noise measurements are generally used to char- acterize components and detectors.

The SR850 measures input signal noise AT the reference frequency. Many noise sources have a frequency dependence which the lock-in can measure.

How does a lock-in measure noise?

Remember that the lock-in detects signals close to the reference frequency. How close? Input signals within the detection bandwidth set by the low pass filter time constant and roll-off appear at the output

at a frequency f=fsig-fref. Input noise near fref appears as noise at the output with a bandwidth of

DC to the detection bandwidth.

The noise is simply the standard deviation (root of the mean of the squared deviations)of the meas- ured X, Y or R . The SR850 can measure this noise exactly by recording the output quantity on a chart display and then calculating the standard deviation using the trace math functions. The noise, in Volts/√Hz, is simply the standard devia- tion divided by the square root of the equivalent noise bandwidth of the time constant.

For Gaussian noise, the equivalent noise band- width (ENBW) of a low pass filter is the bandwidth of the perfect rectangular filter which passes the same amount of noise as the real filter. The ENBW is displayed along with the time constant in the GAIN/TC menu.

Noise estimation

The above technique, while mathematically sound, can not provide a real time output or an analog output proportional to the measured noise. For these measurements, the SR850 can estimate the X, Y or R noise directly.

To display or record the noise of X, for example, simply define a trace as Xn (in the Trace/Scan menu). The quantity Xn is computed in real time and is an estimate of the noise of X. The quantities Yn and Rn are estimations of the Y noise and R noise.

The quantity Xn is computed from the measured values of X using the following algorithm. The

SR850 Basics

moving average of X is computed. This is the mean value of X over some past history. The present mean value of X is subtracted from the present value of X to find the deviation of X from the mean. Finally, the moving average of the abso- lute value of the deviations is calculated. This cal- culation is called the mean average deviation or MAD. This is not the same as an RMS calculation. However, if the noise is Gaussian in nature, then the RMS noise and the MAD noise are related by a constant factor.

The SR850 uses the MAD method to estimate the RMS noise quantities Xn, Yn and Rn. The advan- tage of this technique is its numerical simplicity and speed.

The noise calculations for X, Y and R occur at 512 Hz. At each sample, the mean and moving average of the absolute value of the deviations is calculated. The averaging time (for the mean and average deviation) depends upon the time con- stant. The averaging time is selected by the SR850 and ranges from 10 to 80 times the time constant. Shorter averaging times yield a very poor estimate of the noise (the mean varies rapidly and the deviations are not averaged well). Longer averaging times, while yielding better results, take a long time to settle to a steady answer.

To change the settling time, change the time con- stant. Remember, shorter settling times use small- er time constants (higher noise bandwidths) and yield noisier noise estimates.

The quantities Xn, Yn and Rn are displayed in units of Volts/√Hz. The ENBW of the time constant is already factored into the calculation. Thus, the mean value of Xn should not depend upon the time constant.

The SR850 performs the noise calculations all of the time, whether or not Xn, Yn or Rn are being recorded or displayed. Thus, as soon as Xn is dis- played, the value shown is up to date and no set- tling time is required. If the sensitivity is changed, then the noise estimate will need to settle to the correct value.

For most applications, noise estimation and stan- dard deviation calculations yield the same answer.

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SRS Labs SR850 manual Noise Measurements, How does a lock-in measure noise?, Noise estimation
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SR850 specifications

The SRS Labs SR850 is a high-performance audio processor designed to enhance the listening experience across a variety of applications. With its advanced technologies, the SR850 delivers superior sound quality that is particularly noticeable in environments where audio clarity and fidelity are paramount. This device caters to audio professionals, audiophiles, and casual listeners who demand exceptional performance from their audio systems.

One of the standout features of the SR850 is its proprietary SRS 3D Audio technology. This innovative feature creates a more immersive soundstage, allowing users to experience audio as if they were in a live performance setting. By adding depth and dimension, it elevates the listening experience, making music, movies, and games far more engaging.

Additionally, the SR850 incorporates SRS dialog clarity technology, which enhances speech intelligibility in dialogues and vocals. This is particularly beneficial in film and television viewing, where dialogue can sometimes be drowned out by background music or sound effects. With dialog clarity, users can enjoy crystal-clear conversation tracks without adjusting volume levels continuously.

The device also supports various audio formats, ensuring compatibility with a broad range of media. It boasts inputs for both digital and analog sources, allowing connections to various devices such as TVs, laptops, gaming consoles, and more. The versatility of inputs makes the SR850 a convenient addition to any home entertainment system.

Another impressive characteristic of the SR850 is its compact and user-friendly design. The intuitive interface makes it easy for users to adjust settings to their preference, whether they are a novice or an audio expert. The device is equipped with programmable presets, enabling users to store their favorite sound profiles, making switching between settings quick and hassle-free.

Furthermore, the SR850 features advanced EQ capabilities, granting users the ability to tailor the audio output to their specific tastes. This personalized approach to sound adjustment enhances the overall experience, allowing users to get the most out of their audio content.

In summary, the SRS Labs SR850 is a sophisticated audio processor packed with features designed to significantly uplift sound quality and listening pleasure. With its cutting-edge technologies such as SRS 3D Audio and dialog clarity, an array of connectivity options, and customizable EQ settings, the SR850 stands out as an outstanding option for anyone looking to enhance their audio experience. Whether for professional use in sound engineering or personal enjoyment in home entertainment, the SR850 proves to be a valuable asset.
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