Digital Filtering

SR810 and SR830 DSP Lock-In Amplifiers

Digital Phase Shifting The digital signal processor also handles the task of output filtering, allowing time constants from 10 Analog phase shifting circuits have also been replaced with aDSPcalculation. Phase is measured with 0.01° resolution, and the X and Youtputs are orthogonal to 0.001°. µ s to 30,000 s witha 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. signal to the full-scale input voltage. The largest interferingsignal 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 tomix 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. 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 isn’t limited to 2F  anyharmonic (2F, 3F, ... nF) up to 102 kHz can be measured. Frequency Synthesizer The built-in direct digital synthesis (DDS) source generates avery low distortion ( Useful Features Analog Inputs and Outputs  like normalizing theinput to source intensity fluctuations. Four programmable outputs (16-bit DACs) provide voltages from − 80dBc) 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. θ , Y-noise, Aux 3, Aux 4 or ratio. The SR810and SR830 both have X and Yanalog outputs (rear panel) that are updated at 256kHz. Four auxiliary inputs (16-bit ADCs) are provided for general purpose use Auto-functions allow parameters that are frequently adjustedto 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 The SR810 and SR830 are simple to use. All functions are setfrom 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. 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, − 10.5 Vto +10.5 Vand are settable via the front panel or computer interfaces. The SR810 has an 8,000 point memory buffer for recordingthe 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. Atrigger input is also providedto externally synchronize data recording. Easy Operation Internal Memory

Ordering Information

SR830 DSPdual phase lock-in $4500amplifier (w/ rack mount)SR810 DSPsingle phase lock-in $3850amplifier (w/ rack mount)SR550Voltage preamplifier $595(100 M Ω, 3.6 nV/ √ Hz)SR552Voltage preamplifier $595(100 k Ω, 1.4 nV/ √ Hz)SR554 Transformer preamplifier $995(0.091 nV/ √ Hz)SR540Optical chopper$1095 SR810 DSPSingle Phase Lock-In Amplifier SR810/830 rear panel

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