SRS Labs SR530, Lock-In Amplifier Understanding the Specifications, Shielding and Ground Loops

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Vpsd1 = Vs cos(wrt) cos(wst+Ø)

=1/2 Vs cos[(wr + ws)t+Ø] + 1/2 Vs cos[(wr - ws)t+Ø]

Vpsd2 = Vs sin(wrt) cos(wst+Ø)

=1/2 Vs sin[(wr + ws)t+Ø] + 1/2 Vs sin[(wr - ws)t+Ø]

The sum frequency component of each PSD is attenuated by a low pass filter, and only those difference frequency components within the low pass filter's narrow bandwidth will pass through to the dc amplifiers. Since the low pass filter can have time constants up to 100 seconds, the lock-in can reject noise which is more than .0025 Hz away from the reference frequency input.

For signals which are in phase with the reference (¯=0¡), the output of PSD1 will be a maximum and the output of PSD2 will be zero. If the phase is non-zero, Vpsd1 ~ cos(Ø) and Vpsd2 ~ sin(°). The magnitude output is given by,

R = {(Vpsd1)2 + (Vpsd2)2}1/2 ~ Vs

and is independent of the phase Ø. The phase output is defined as

Ø= - tan-1(Vpsd2/ Vpsd1)

Thus, a dual-phase lock-in can measure the amplitude of the signal, independent of the phase, as well as measure an unknown phase shift between the signal and the reference.

Understanding the Specifications

The table below lists some specifications for the SR530 lock-in amplifier. Also listed are the error contributions due to each of these items. The specifications will allow a measurement with a 2% accuracy to be made in one minute.

We have chosen a reference frequency of 5 kHz so as to be in a relatively quiet part of the noise spectrum. This frequency is high enough to avoid low frequency '1/f' noise as well as line noise. The frequency is low enough to avoid phase shifts and amplitude errors due to the RC time constant of

the source impedance and the cable capacitance.

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The full-scale sensitivity of 100 nV matches the expected signal from our sample. The sensitivity is calibrated to 1%. The instrument's output stability also affects the measurement accuracy. For the required dynamic reserve, the output stability is 0.1%/°C. For a 10°C temperature change we can expect a 1% error.

A front-end noise of 7 nV/√Hz will manifest itself as a 1.2 nVrms noise after a 10 second low-pass filter since the equivalent noise bandwidth of a single pole filter is 1/4RC. The output will converge exponentially to the final value with a 10- second time constant. If we wait 50 seconds, the output will have come to within 0.7% of its final value.

The dynamic reserve of 60 dB is required by our expectation that the noise will be a thousand times larger than the signal. Additional dynamic reserve is available by using the bandpass and notch filters.

A phase-shift error of the PLL tracking circuits will cause a measurement error equal to the cosine of the phase shift error. The SR530's 1° phase accuracy will not make a significant contribution to the measurement error.

Specifications for the Example Measurement

Specification

Value

Error

Full Scale Sensitivity

100 nV

 

Dynamic Reserve

60 dB

 

Reference Frequency

5 kHz

 

Gain Accuracy

1%

1%

Output Stability

0.1%/°C

1%

Front-End Noise

< 7 nV/√Hz 1.2%

Output Time Constant

> 10 S

0.7%

Total RMS Error

 

2%

Shielding and Ground Loops

In order to achieve the 2% accuracy given in this measurement example, we will have to be careful to minimize the various noise sources which can be found in the laboratory. (See Appendix A for a brief discussion on noise sources and shielding) While intrinsic noise (Johnson noise, 1/f noise and alike) is not a problem in this measurement, other noise sources could be a problem. These noise sources can be reduced by proper shielding. There are two methods for connecting the lock-in to the experiment: the first method is more convenient, but the second eliminates spurious pick-up more effectively.

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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 ChannelExpand Channel Rcosø OutputChannel 2 Display Auto Phase Rsinø Output Reference InputTrigger Level Phase Controls Reference ModeReference Display Time ConstantPower DefaultsLocal 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 Errors Status ByteBit Trouble-Shooting Interface Problems ResetCommon Hardware Problems include Common Software Problems includeSR530 with the Gpib Interface SR530 with the RS232 InterfaceSerial Polls and Service Requests Gpib with RS232 Echo ModeSR530 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 ControlPower Supplies RS232 InterfaceGpib Interface Multiplier Adjustments Amplifier and Filter 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 CX1CY1 FU1MPSA18 SR530 Component Parts List SR530 Component Parts List SR530 Component Parts List SR530 Component Parts List SR530 Component Parts List SPSTX8 4PDTSR513 Assy SR530 Component Parts List Static RAM, I.C Z80A-CPUTranscover TIE AnchorMica #4 FlatFront Panel Board Parts List RED LD1 LD2LD3 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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