SRS Labs SR530, Lock-In Amplifier Signal Channel, Reference Channel, Phase Sensitive Detectors

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The Signal Channel

The instrument has both current and voltage inputs. The current input is a virtual ground, and the 100 MΩ voltage inputs can be used as single- ended or true differential inputs.

There are three signal filters. Each of these filters may be switched 'in' or 'out' by the user. The first filter is a line notch filter. Set to either 50 or 60 Hz, this filter provides 50 dB of rejection at the line frequency. The second filter provides 50 dB of rejection at the first harmonic of the line frequency. The third filter is an auto-tracking bandpass filter with a center frequency tuned by the micro- processor to the frequency of the signal. These three filters eliminate most of the noise from the signal input before the signal is amplified.

A high-gain ac amplifier is used to amplify the signal before entering the phase sensitive detector. The high gain which is available from this programmable amplifier allows the lock-in to operate with a lower gain in its dc amplifier. This arrangement allows high stability operation even when used on the most sensitive ranges.

Reference Channel

The processor controlled reference input discriminator can lock the instrument's PLL to a variety of reference signals. The PLL can lock to sine waves or to logic pulses with virtually no phase error. The PLL outputs are phase shifted and shaped to provide two precision sine waves. The two sine waves have 90° of phase shift between them.

Phase Sensitive Detectors

The Phase Sensitive Detectors are linear multipliers which mix the amplified and filtered signal with the reference sine waves. The difference frequency component of the multipliers' outputs are dc signals that are proportional to the amplitude of the signal. The low-pass filters which follow each multiplier can reject any frequency components which are more than a fraction of a Hertz away from the signal frequency.

DC Amplifiers and System Gain

Dc amplifiers amplify and offset the outputs of the two low pass filters. The total system gain is the product of the ac and dc amplifier gains. The partitioning of the system gain between these ac

and dc amplifiers will affect the stability and dynamic reserve of the instrument. The output is most stable when most of the gain is in the ac amplifier, however, high ac gain reduces the dynamic reserve.

For the most demanding applications, the user may specify how the system gain is partitioned. However, with prefilters that are able to provide up to 100 dB of dynamic reserve, and with chopper stabilized dc amplifiers, most users will not be concerned with just how the system gain is allocated.

A Microprocessor Based Design

The instrument was designed to take full advantage of its microprocessor controller. This approach provides several key advantages...

The instrument may be interfaced to a laboratory computer over the RS232 and IEEE-488 interfaces. In addition to simply reading data from the lock-in, the computer can control all of the instrument settings with simple ASCII commands.

A key feature of the instrument is its four A/D inputs and two D/A outputs. These analog I/O ports may be used to read and supply analog voltages to an experiment or measurement. All of the input and output ports have a full-scale range of ±10.24VDC with 2.5 mV resolution and 0.05% accuracy.

Computer control can eliminate set-up errors, reduce tedium, allow more complete data recording and post measurement analysis. Also, the computer can play an active role in the data acquisition by adjusting gains, etc., in response to changing measurement conditions.

The microprocessor based design eliminates many analog components to improve performance, reliability, and reduce cost. For example, the magnitude and phase outputs are calculated by the microprocessor instead of using an analog vector summer. This eliminates the temperature drifts and inaccuracies associated with nonlinear analog circuits and greatly reduces the number of parts. Each unit is computer calibrated at the factory, and calibration constants are placed in the instrument's read-only memory. The SR530 has only one-fifth of the analog trimming components that are found in older designs.

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Contents Model SR530 Page Table of Contents Appendix C Gpib NON-OPERATING OperatingPage SR530 Specification Summary Gpib DemodulatorFront Panel Summary Enbw Abridged Command List Status Byte Definition Configuration SwitchesSignal Filters Signal InputsSR510 Guide to Operation Front Panel SensitivityStatus Dynamic ReserveDisplay Select Channel 1 DisplayOutput Channel OutputRel Channel Offset ChannelRcosø Output Expand ChannelChannel 2 Display Auto Phase Reference Input Rsinø OutputTrigger Level Phase Controls Reference ModeReference Display Time ConstantDefaults PowerLocal and Remote SR530 Guide to Operation Rear Panel Page SR530 Guide to Programming Command SyntaxCommunicating with the SR530 Front Panel Status LEDsTry-Out with an Ascii Terminal RS232 Echo and No Echo OperationLOW Norm High SR530 Command ListN1,n2,n3,n4 Page Status Byte ErrorsBit Trouble-Shooting Interface Problems ResetCommon Hardware Problems include Common Software Problems includeSR530 with the Gpib Interface SR530 with the RS232 InterfaceGpib with RS232 Echo Mode Serial Polls and Service RequestsSR530 with Both Interfaces Measurement Example Lock-in TechniqueShielding and Ground Loops Understanding the SpecificationsPage Page SR530 Block Diagram Phase Sensitive Detectors Signal ChannelReference Channel DC Amplifiers and System GainCircuit Description Demodulator and Low Pass Amplifier Reference OscillatorExpand Analog Output and ControlFront Panel Microprocessor ControlRS232 Interface Power SuppliesGpib Interface Amplifier and Filter Adjustments Multiplier AdjustmentsCalibration and Repair Replacing the Front-End Transistors Notch FiltersNon-Essential Noise Sources Appendix a Noise Sources and CuresPage Page Appendix B Introduction to the RS232 Case 1 The Simplest ConfigurationBaud Rate Case 2 RS232 with Control LinesParity Stop BitsVoltage Levels Final TipBus Description Appendix C Introduction to the GpibProgram Example IBM PC, Basic, via RS232 Appendix D Program ExamplesProgram Example IBM PC, Microsoft Fortran v3.3, via RS232 Page #include stdio.h Program Example IBM PC, Microsoft C v3.0, via RS232Page Program Example 4 IBM PC,Microsoft Basic, via Gpib ′INCREMENT X6 Output by 2.5 MV Program Example HP85 via Gpib Documentation PC1 Oscillator Board Parts ListSW1 DpdtBR1 Main Board Parts ListBR2 BT1SR530 Component Parts List SR530 Component Parts List PIN D 22U MINGpib Shielded CX1FU1 CY1MPSA18 SR530 Component Parts List SR530 Component Parts List SR530 Component Parts List SR530 Component Parts List SR530 Component Parts List 4PDT SPSTX8SR513 Assy SR530 Component Parts List Static RAM, I.C Z80A-CPUTranscover TIE AnchorMica #4 FlatFront Panel Board Parts List RED LD2 LD1LD3 Quad Board Parts List SR530 Component Parts List PC1 SR530 Component Parts List Miscellaneous Parts List SR530 Component Parts List

SR530, Lock-In Amplifier specifications

The SRS Labs Lock-In Amplifier, model SR530, is a powerful tool designed for high-precision measurements in the realm of scientific research and industrial applications. This state-of-the-art instrument excels in extracting small signals from noisy environments, making it an invaluable asset for experiments in fields such as physics, engineering, and materials science.

One of the main features of the SR530 is its ability to perform synchronous detection, which is key to improving signal-to-noise ratios. By utilizing a reference signal, the device correlates the incoming signal with the reference to effectively filter out noise, allowing for the accurate measurement of weak signals that might otherwise be obscured. This process of phase-sensitive detection is fundamental to the operation of the Lock-In Amplifier.

The SR530 offers a wide frequency range, covering from 0.1 Hz to 100 kHz. This broad frequency response allows it to handle a diverse array of signals, making it suitable for various applications including optical detection, capacitance measurements, and in many cases, voltammetry. The device is also equipped with multiple inputs and outputs, facilitating the integration with other laboratory equipment and enabling complex experimental setups.

Precision is further enhanced with its adjustable time constant, which allows users to optimize the response time based on experimental needs. The user can choose time constants from 10 microseconds to 10 seconds, accommodating fast dynamic measurements as well as those requiring stability over longer durations.

Another remarkable characteristic of the SR530 is its digital processing capabilities. The device features a highly accurate digital voltage measurement system, minimizing drift and ensuring long-term stability. Additionally, the use of microprocessors enhances data handling and allows for features such as programmable settings, facilitating automated measurements.

Moreover, the SR530 includes a range of output options, including analog outputs, which can be used for direct signal processing, as well as digital interfaces for integration with computers. This ensures that users can not only capture high-fidelity data but also analyze and display it efficiently.

In conclusion, the SRS Labs SR530 Lock-In Amplifier stands out due to its sophisticated technology, versatile features, and robust performance. Its precision, flexibility, and ease of use make it an ideal choice for researchers and engineers looking to unlock the potential of weak signal measurement in complex environments.
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