Sine Wave Mixing
The core of the SR510/SR530 is a precision analog sine-wave multiplier. Lock-ins use a multiplier (demodulator) to translate the signal input (at reference frequency) down to DC where it can be filtered and amplified. Many lock-ins use square wave multipliers which introduce spurious harmonic responses. The SR510/SR530 use clean sine-wave multipliers which are inherently free of unwanted harmonics.
Signal Input
The SR510/SR530 have differential inputs with 7 nV/¡îHz of input noise and 100 M¥Ø input impedance. The input can be configured as a voltage input, or as a current input with 106 V/A gain and an input impedance of 1 k¥Ø to virtual ground. Full-scale sensitivities from 500 mV down to 100 nV are available.
Three input prefilters can be selected. The first is a line notch filter providing 50 dB of rejection at the line frequency. The second filter similarly provides 50 dB of rejection at the second harmonic of the line frequency. The third filter is a band pass filter which automatically tracks the reference frequency. These three filters can eliminate much of the noise in the signal before it is amplified.
Reference Input
The reference input can be set to lock to sine waves or to either edge of a pulsed reference. The reference frequency range is 0.5 Hz to 100 kHz, and detection at both the fundam. A built-in ratio feature allows the SR510/SR530to calculate the ratio of its output to a signal at one of the A/D ports. This feature is important in servo applications to maintain a constant loop gain, or in experiments that normalize a signal to an intensity level.
Available Preamplifiers
Although the SR510 and SR530 are completely self contained and require no preamplification, sometimes an external preamplifier can be useful. Remote preamplifiers provide gain where it's most important¢¯ right at the detector, before the signal-to-noise ratio is permanently degraded by cable noise and pickup. The SR550 FET-input preamplifier, the SR552 bipolar-input preamplifier, and the SR554 transformer-input preamplifier are ideally suited for use with the SR510/SR530 lock-ins. These preamplifiers are especially useful when measuring extremely low-level signals.
Computer Interfaces
An RS-232 computer interface is standard on both the SR510 and SR530. An optional GPIB interface is also available. All features of the instruments can be queried and set via the computer interfaces.