SR510 Block Diagram

Several new concepts are used to simplify the design of SR510 lock-in amplifier. In addition to implementing recent advances in linear integrated circuit technology, the instrument was designed to take full advantage of its microprocessor controller to improve performance and to reduce cost.

As an example of the new techniques used in the SR510, consider the harmonic rejection problem. Previously, lock-in amplifiers used a PLL with a square wave output. The Fourier components of the square wave created a serious problem -- the lock-in would respond to signal and noise at f, 3f, 5f,.ad infinitum. Quite often, one component of this picket fence of frequencies would land on

some noise source, giving a spurious result. To overcome this difficulty designers employed tuned amplifiers or heterodyning techniques. All of these 'fix-ups' had drawbacks, including phase and amplitude errors, susceptibility to drift, and card- swapping to change frequencies.

In contrast, the SR510 detects the signal by mixing a reference sine wave in a precision analog multiplier. Because of the low harmonic content of this sine wave, the instrument is insensitive to harmonics. This approach has eliminated the difficulty, performance compromises, and cost of the older techniques.

28