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DSP Lock-In Amplifiers 810/830--SRS
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  • 1 mHz to 102.4 kHz frequency range
  • >100 dB dynamic reserve
  • 5 ppm/C stability
  • 0.01 degree phase resolution
  • Time constants from 10 s to 30 ks (up to 24 dB/oct rolloff)
  • Auto-gain, -phase, -reserve and -offset
  • Synthesized reference source
  • GPIB and RS-232 interfaces

Input Channel
The SR810 and SR830 have differential inputs with 6 nV/Hz input noise. The input impedance is 10 MΩ, and minimum full-scale input voltage sensitivity is 2 nV. The inputs can also be configured for current measurements with selectable current gains of 106 and 108 V/A. A line filter (50 Hz or 60 Hz) and a 2 line filter (100 Hz or 120 Hz) are provided to eliminate line related interference. However, unlike conventional lock-in amplifiers, no tracking band-pass filter is needed at the input. This filter is used by conventional lock-ins to increase dynamic reserve. Unfortunately, band pass filters also introduce noise, amplitude and phase error, and drift. The DSP design of these lock-ins has such inherently large dynamic reserve that no band pass filter is needed.

Extended Dynamic Reserve
The dynamic reserve of a lock-in amplifier, at a given fullscale input voltage, is the ratio (in dB) of the largest interfering signal to the full-scale input voltage. The largest interfering signal is defined as the amplitude of the largest signal at any frequency that can be applied to the input before the lock-in cannot measure a signal with its specified accuracy. Conventional lock-in amplifiers use an analog demodulator to mix an input signal with a reference signal. Dynamic reserve is limited to about 60 dB, and these instruments suffer from poor stability, output drift, and excessive gain and phase error. Demodulation in the SR810 and SR830 is accomplished by sampling the input signal with a high-precision A/D converter, and multiplying the digitized input by a synthesized reference signal. This digital demodulation technique results in more than 100 dB of true dynamic reserve (no prefiltering) and is free of the errors associated with analog instruments.

Digital Filtering
The digital signal processor also handles the task of output filtering, allowing time constants from 10 s to 30,000 s with a choice of 6, 12, 18 and 24 dB/oct rolloff. For low frequency measurements (below 200 Hz), synchronous filters can be engaged to notch out multiples of the reference frequency. Since the harmonics of the reference have been eliminated (notably 2F), effective output filtering can be achieved with much shorter time constants.

Digital Phase Shifting
Analog phase shifting circuits have also been replaced with a DSP calculation. Phase is measured with 0.01 resolution, and the X and Y outputs are orthogonal to 0.001.

Frequency Synthesizer
The built-in direct digital synthesis (DDS) source generates a very low distortion (.80 dBc) reference signal. Single frequency sine waves can be generated from 1 mHz to 102 kHz with 4½ digits of resolution. Both frequency and amplitude can be set from the front panel or from a computer. When using an external reference, the synthesized source is phase locked to the reference signal.

Useful Features
Auto-functions allow parameters that are frequently adjusted to automatically be set by the instrument. Gain, phase, offset and dynamic reserve are quickly optimized with a single key press. The offset and expand features are useful when examining small fluctuations in a measurement. The input signal is quickly nulled with the auto-offset function, and resolution is increased by expanding around the relative value by up to 100. Harmonic detection isnt limited to 2F.any harmonic (2F, 3F, ... nF) up to 102 kHz can be measured.

Analog Inputs and Outputs
Both instruments have a user-defined output for measuring X, R, X-noise, Aux 1, Aux 2, or the ratio of the input signal to an external voltage. The SR830 has a second, user-defined output that measures Y, , Y-noise, Aux 3, Aux 4 or ratio. The SR810 and SR830 both have X and Y analog outputs (rear panel) that are updated at 256 kHz. Four auxiliary inputs (16-bit ADCs) are provided for general purpose normalizing the
input to source intensity fluctuations. Four programmable outputs (16-bit DACs) provide voltages from .10.5 V to +10.5 V and are settable via the front panel or computer interfaces.

Internal Memory
The SR810 has an 8,000 point memory buffer for recording the time history of a measurement at rates up to 512 samples/s. The SR830 has two, 16k point buffers to simultaneously record two measurements. Data is transferred from the buffers using the computer interfaces. A trigger input is also provided to externally synchronize data recording.

Easy Operation
The SR810 and SR830 are simple to use. All functions are set from the front-panel keypad, and a spin knob is provided to quickly adjust parameters. Up to nine different instrument configurations can be stored in non-volatile RAM for fast and easy instrument setup. Standard RS-232 and GPIB (IEEE-488.2) interfaces allow communication with computers.


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