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

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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