SR850 Basics

REFERENCE CHANNEL

A lock-in amplifier requires a reference oscillator phase-locked to the signal frequency. In general, this is accomplished by phase-locking an internal oscillator to an externally provided reference signal. This reference signal usually comes from the signal source which is providing the excitation to the experiment.

Reference Input

The SR850 reference input can accept an analog signal (like a sine wave) or a TTL logic signal. The first case is called External Sine. The input is AC coupled (above 1 Hz) and the input impedance is 1 MΩ. A sine wave input greater than 200 mV pk will trigger the input discriminator. Positive zero crossings are detected and considered to be the zero for the reference phase shift.

TTL reference signals can be used at all frequen- cies up to 102 kHz. For frequencies below 1 Hz, a TTL reference signal is required. Many func- tion generators provide a TTL SYNC output which can be used as the reference. This is convenient since the generator's sine output might be smaller than 200 mV or be varied in amplitude. The SYNC signal will provide a stable reference regardless of the sine amplitude.

When using a TTL reference, the reference input can be set to External Rising (detect rising edges) or External Falling (detect falling edges). In each case, the internal oscillator is locked (at zero phase) to the detected edge.

Internal Oscillator

The internal oscillator in the SR850 is basically a 100 kHz function generator with sine and TTL sync outputs. The oscillator can be phase-locked to the external reference.

The oscillator generates a digitally synthesized sine wave. The digital signal processor, or DSP, sends computed sine values to a 16 bit digital-to- analog converter every 4 µs (256 kHz). An anti- aliasing filter converts this sampled signal into a low distortion sine wave. The internal oscillator sine wave is output at the SINE OUT BNC on the front panel. The amplitude of this output may be set from 4 mV to 5 V.

When an external reference is used, this internal

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oscillator sine wave is phase-locked to the refer- ence. The rising zero crossing is locked to the detected reference zero crossing or edge. In this mode, the SINE OUT provides a sine wave phase- locked to the external reference. At low frequen- cies (below 10 Hz), the phase locking is accom- plished digitally by the DSP. At higher frequencies, a discrete phase comparator is used.

The internal oscillator may be used without an external reference. In the Internal Reference mode, the SINE OUT provides the excitation for the experiment. The phase-locked-loop is not used in this mode since the lock-in reference is provid- ing the excitation signal.

The TTL OUT on the rear panel provides a TTL sync output. The internal oscillator's rising zero crossings are detected and translated to TTL levels. This output is a square wave.

Reference Oscillators and Phase

The internal oscillator sine wave is not the refer- ence signal to the phase sensitive detectors. The DSP computes a second sine wave, phase shifted by θref from the internal oscillator (and thus from an external reference), as the reference input to the X phase sensitive detector. This waveform is sin(ωrt + θref). The reference phase shift is adjust- able in .001° increments.

The input to the Y PSD is a third sine wave, com- puted by the DSP, shifted by 90° from the second sine wave. This waveform is sin(ωrt + θref + 90°).

Both reference sine waves are calculated to 20 bits of accuracy and a new point is calculated

every 4 µs (256 kHz). The phase shifts (θref and the 90° shift) are also exact numbers and accurate

to better than .001°. Neither waveform is actually output in analog form since the phase sensitive detectors are actually multiply instructions inside the DSP.

Phase Jitter

When an external reference is used, the phase- locked loop adds a little phase jitter. The internal oscillator is supposed to be locked with zero phase shift relative the external reference. Phase jitter means that the average phase shift is zero but the instantaneous phase shift has a few milli-

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SRS Labs SR850 Reference Channel, Reference Input, Internal Oscillator, Reference Oscillators and Phase, Phase Jitter
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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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