SRS Labs Lock-In Amplifier Manuel d'utilisateur télécharger pdf (Page 36)

SR530 Block Diagram

Several new concepts are used to simplify the

design of SR530 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

SR530, 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 SR530 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.

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SRS Labs Lock-In Amplifier Manuel d'utilisateur Page 36