bandpass filter adds a few nanovolts of noise to the front end of the instrument when it is in use.

Line Notch Filters should be used in most measurement situations. The filters will reject about 50 dB of line frequency noise (about a factor of 300). If your reference frequency is one octave away, then these filters will introduce a 10° phase shift error, and a few percent amplitude error. Their effect on your signal is negligible if your reference frequency is more than two octaves away.

The frequency range of the SR510 lock-in amplifier extends from 0.5Hz to 100KHz. No additional cards are required for the instrument to cover its full frequency range. The SR510 can be used to detect a signal at the reference frequency or at twice the reference frequency to allow for convenient measurement of the harmonic of the signal.

Noise measurement is a feature which allows direct measurement of the noise density of the signal at the reference frequency. This is a useful feature to assess at what frequency you should run your experiment.

Output Filters can have one pole (6 dB per octave) or two poles (12 dB/octave). A two-pole filter provides a signal to noise improvement over a single-pole filter due to its steeper roll off and reduced noise bandwidth. Single-pole filters are preferred when the lock-in is used in a servo system to avoid oscillation.

In many servo applications, no output filtering is needed. In this case, the SR510 may be modified to reduce the output time constant to about 20 ∝ S. Contact the factory for details.

Ratio Capability allows the lock-in's output to be divided by an external voltage input. This feature is important in servo applications to maintain a constant loop gain, and in experiments to normalize a signal to the excitation level.

Computer Interface allows a computer to control and to record data from the instrument. This is the single most important feature for extending the lock-in's capabilities and it's useful lifetime. Measurements which are impractical without a computer become simple when a computer is used to coordinate various parts of the experiment.

The Internal Oscillator provides a reference source for the lock-in. This allows the lock-in's frequency to be set without an additional signal generator. It also provides a sine wave to be used as the signal stimulus in an experiment. The frequency may be set via the computer interface as well as manually.

27