SRS Labs Lock-In Amplifier, SR530 manual Circuit Description

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Circuit Description

Introduction

The SR530 Lock-in amplifier is an integrated instrument combining state of the art analog design with advanced microprocessor based control and interfaces. This discussion is intended to aid the advanced user in gaining a better understanding of the instrument.

The SR530 has eight main circuit areas: the signal amplifier, the reference oscillator, the demodulator, the analog output and controls, the front panel, the microprocessor, the computer interfaces, and the power supplies. With the exception of the front panel, the quadrature oscillator and demodulator, and a few pieces of hardware, the entire lock-in is built on a single printed circuit board. Each section is isolated from the others as much as possible to prevent spurious signal pickup. To aid in the location of individual components, the first digit (or first two digits of a four digit part number) of each part number generally refers to the schematic sheet number on which it occurs. To help find the part on the circuit board, the parts list includes a location on the circuit board for each component. Parts with a four-digit part number beginning with 10,11, or 12 are found on the quadrature detector plug-in board located in the center of the main circuit board. Part numbers beginning with 6 refer to parts on the front panel.

Signal Amplifier

Assuming the input selector switch is set to a voltage input, the signal is coupled in through capacitors C101 and C102. The input impedance is set by the 100 MΩ resistors R101 and R102 over the operating frequency range. Note that R103 isolates the signal shields from the instrument ground forcing the return signal current back along the cable shields. The signal is then applied differentially to the gates of Q101. Q101 is a low noise dual JFET. The drain current through R109 is kept constant by 2/2 U101. The other half of U101 maintains a virtual null between the drains of the two transistors and thus an identical current flows through R110. Any input that would cause a differential between the two drains is amplified by 1/2 U101 and fed back via R112 in such a way as to reduce that differential. Since the two transistors are at equal and constant currents, their gate-source potentials are constant. Thus, the fed back signal which appears at the source of the right hand transistor exactly matches the input . Likewise, this signal will match the input

to the left hand transistor but with the opposite sign. Resistors R112 and R110 attenuate the fed back signal from the output of U101 resulting in a differential input, single ended output, fixed gain of 10 amplifier. P101 adjusts the current balance between the two transistors and therefore their gain match and common mode rejection.

The output of the pre-amp is scaled by resistors R119-R122 and analog switch U103 which make up a 1-2-5-10 attenuator. The signal is then amplified by 2/2 U102. Input overload is sensed through diodes D101-D104.

Current Amplifier

When the input selector is set to current, the input to the pre-amp comes from the output of the current to voltage converter, 1/2 U102.

U102 is a low voltage-noise bipolar op amp. Q102 serves as an input buffer to provide low current-noise to the input. The op amp always maintains a null at the gates of Q102 thus providing an input impedance of 1KΩ (R128). The input current is converted to a voltage by R135 and the op amp. Q103 bootstraps out the summing junction capacitance of Q102.

Notch Filters

U107 is a high Q, line frequency, notch filter which can be switched in and out by analog switch 1/4 U106. The frequency and depth of the filter can be adjusted with P102 and P103. Resistors R146-R149 and switch U108 make up a selectable attenuator. U118 is a line frequency 2nd harmonic notch filter selected by 2/4 U106. P104 and P105 adjust the frequency and depth. The second notch filter has a gain of 3 and its output is scaled by U110 and resistors R156-R159. The signal then takes two paths; to inverting amplifier U111 and to the input of the tracking bandpass filter. U111 has the same gain as the bandpass filter. The output of either U111 or the bandpass filter is selected by 3/4 U112 and 4/4 U106 and amplified by U113. U114 and U115 provide a last stage of gain and scaling and the final output is ac coupled and buffered by 4/4 U208.

Bandpass Filter

The bandpass filter is a three op amp state- variable active filter. 3/4 of U201 make up the three op amps of the standard filter. U203,

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Contents Model SR530 Page Table of Contents Appendix C Gpib Operating NON-OPERATINGPage SR530 Specification Summary Demodulator GpibFront Panel Summary Enbw Abridged Command List Configuration Switches Status Byte DefinitionSR510 Guide to Operation Front Panel Signal InputsSignal Filters SensitivityDisplay Select Dynamic ReserveStatus Channel 1 DisplayRel Channel OutputOutput Channel Offset ChannelChannel 2 Display Expand ChannelRcosø Output Auto Phase Trigger Level Rsinø OutputReference Input Reference Display Reference ModePhase Controls Time ConstantLocal and Remote PowerDefaults SR530 Guide to Operation Rear Panel Page Communicating with the SR530 Command SyntaxSR530 Guide to Programming Front Panel Status LEDsRS232 Echo and No Echo Operation Try-Out with an Ascii TerminalSR530 Command List LOW Norm HighN1,n2,n3,n4 Page Bit ErrorsStatus Byte Common Hardware Problems include ResetTrouble-Shooting Interface Problems Common Software Problems includeSR530 with the RS232 Interface SR530 with the Gpib InterfaceSR530 with Both Interfaces Serial Polls and Service RequestsGpib with RS232 Echo Mode Lock-in Technique Measurement ExampleUnderstanding the Specifications Shielding and Ground LoopsPage Page SR530 Block Diagram Reference Channel Signal ChannelPhase Sensitive Detectors DC Amplifiers and System GainCircuit Description Reference Oscillator Demodulator and Low Pass AmplifierFront Panel Analog Output and ControlExpand Microprocessor ControlGpib Interface Power SuppliesRS232 Interface Calibration and Repair Multiplier AdjustmentsAmplifier and Filter Adjustments Notch Filters Replacing the Front-End TransistorsAppendix a Noise Sources and Cures Non-Essential Noise SourcesPage Page Baud Rate Case 1 The Simplest ConfigurationAppendix B Introduction to the RS232 Case 2 RS232 with Control LinesVoltage Levels Stop BitsParity Final TipAppendix C Introduction to the Gpib Bus DescriptionAppendix D Program Examples Program Example IBM PC, Basic, via RS232Program Example IBM PC, Microsoft Fortran v3.3, via RS232 Page Program Example IBM PC, Microsoft C v3.0, via RS232 #include stdio.hPage Program Example 4 IBM PC,Microsoft Basic, via Gpib ′INCREMENT X6 Output by 2.5 MV Program Example HP85 via Gpib Documentation SW1 Oscillator Board Parts ListPC1 DpdtBR2 Main Board Parts ListBR1 BT1SR530 Component Parts List SR530 Component Parts List Gpib Shielded 22U MINPIN D CX1MPSA18 CY1FU1 SR530 Component Parts List SR530 Component Parts List SR530 Component Parts List SR530 Component Parts List SR530 Component Parts List SR513 Assy SPSTX84PDT SR530 Component Parts List Z80A-CPU Static RAM, I.CMica TIE AnchorTranscover #4 FlatFront Panel Board Parts List RED LD3 LD1LD2 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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