SRS SR865A DSP Lock-In Amplifier, 4MHz

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The new 4MHz SR865A Lock-in Amplifier is the latest in a line of innovative lock-ins from SRS. With unparalleled analogue performance, sophisticated new digital signal processing features, a modern, intuitive user interface, and a wide range of computer connectivity options, the SR865A is the ideal choice for any synchronous detection application.

With over 30 years design experience, SRS optimize each detail from a toroidal transformer that eliminates switch-mode noise, iOS mobile connectivity, to advanced DSP filters that eliminate more noise while speeding up your experiment, the SR865A is truly the ultimate lock-in amplifier.

Signal Inputs
Lock-in performance starts at the front end. The SR865A offers both state-of-the art voltage and current input amplifiers. The voltage input is a switchable single-ended/differential JFET-pair amplifier with 2.5nV/√Hz of noise at 1 kHz and under 10nV/√Hz of noise at 10Hz. The voltage input has a 10MΩ input impedance and can be AC or DC coupled. Input connector shields can be connected to the instrument ground through a user selectable 10Ω (Ground) or 10kΩ (Float) resistor.

The SR865A’s built-in current amplifier represents a significant improvement over previous designs. The current input range is selectable from 1μA or 10nA. The 1μA range has 400 kHz of bandwidth and 130 fA/√Hz of noise, while the 10nA range offers 2 kHz of bandwidth and 13 fA/√Hz of noise.

While the built in voltage and current amplifiers are suitable for most applications, the SR865A is also compatible with the complete range of specialized pre-amplifiers offered by SRS.

Sensitivity and Input Range
As with previous instruments, the Sensitivity setting of the SR865A is the voltage (or current) which produces a full scale output. But unlike previous designs, the input range of the SR865A can be set from the front panel without having to consult a confusing “dynamic reserve” equation. Simply choose the sensitivity required by your experiment and then select the smallest input range that doesn’t overload.

The SR865A’s effective dynamic reserve is simply the ratio of these two settings. E.g. a 10nV sensitivity setting and a 300mV input range the effective dynamic reserve of the SR865A is 3 × 107, or nearly 150 dB.

Output Time Constants and Filtering
The SR865A offers traditional RC-response output time constants from 1µs to 30ks with rolloffs of 6, 12, 18, and 24dB/oct. In addition, advanced digital filters can significantly reduce measurement time. Synchronous filtering may be selected at reference frequencies below 4 kHz.

Reference Channel
The SR865A has a specified reference frequency range of 1mHz to 4MHz. Detection can be done at the fundamental of the reference frequency, or at up to the 99th harmonic. Several reference modes are available: Internal Mode, External mode, Dual Mode or Chop mode.

Sine Output
The SR865A offers a precision sine wave output which can be set with 6 digits of frequency resolution and from 1nV to 2V. The SR865A output is unique in that it can be configured as a single-ended or differential (balanced) signal. A DC offset of up to ±5 V can be applied to the sine output.

Timebase
Rear-panel 10 MHz inputs and outputs are provided allowing the SR865A to be locked to an external frequency reference (such as the FS725 10 MHz Rubidium Frequency Standard).

FFT Displays
Lock-in amplifiers are traditionally time-domain instruments but sometimes it’s easier to understand in the frequency domain. An FFT display shows the spectrum of the input signal, the post-mixer signal, or the spectrum of the signal after the time-constant filters.

Front-Panel Touchscreen Display
The centre of the SR865A’s front panel is a colour 640 × 480 touchscreen which can display up to 4 channels of data. Each data channel can be configured to display X, Y, R, Θ, Aux In (1-4), Aux Out (1-2), X noise, Y noise, Sine Amplitude, Sine Out DC Level, Reference Phase, or Reference Frequency. The screen can be set up to show the data channels as large numbers or as a “strip-chart” display showing a complete history of each channel with selectable time scales from 0.5 s/div to 2 days/div. The touchscreen continually displays key lock-in setup parameters such as phase, reference frequency, and sine amplitude.

All commonly used controls are controllable with dedicated front panel knobs or buttons. Infrequently accessed configuration settings, such as the TCP/IP settings and other communication settings, are accessed through menus shown on the touchscreen.

Computer Connectivity
GPIB, RS-232, USB (Test and Measurement Class) and Ethernet (VXI-11 and telnet) are fitted as standard. The SR865A hosts its own webserver allowing the instrument to be monitored and controlled remotely with just a browser. The SR865A can be controlled remotely from an iPhone. The front USB port allows data and screen-shots to a USB flash drive.