Noise cancellation: The lock in amplifier

Noise cancellation in systems with low amplitude input signals embedded in large noise levels is always a problem. Few techniques are available to extract these low level signals from noise but tend to be expensive in many ways. Lock-in amplifiers can be used to measure very small AC signals(a few nanovolts).

The basic technique used by lock in amplifiers is known as phase-sensitive detection to single out the component of the signal at a particular reference frequency and phase. i.e. it is a very narrow band filter equivalent and noise signals are attenuated. For example. The signal is a 10 nV sine wave at 10 kHz. Amplification is used to bring the signal above the noise. A good low-noise amplifier will have about 5 nV/√Hz of input noise. If the amplifier bandwidth is 100 kHz and the gain is 1000, the output will be 10 µV of signal (10 nV × 1000) and the noise will be 1.6mv. This means it will be very difficult if not impossible to measure the signal of interest.

If the amplifier is followed by a narrow band filter, with a Q=100 centered at 10 kHz, any signal in a 100 Hz bandwidth will be detected (10kHz/Q). The noise in the filter pass band will be 50 µV (5 nV/√Hz ×√100 Hz × 1000), and the signal will still be 10 µV. However, the output noise is still much larger than the signal, and a measurement can not be made.

If the amplifier is followed by a phase-sensitive detector, then the PSD can detect with an extremely narrow bandwidth of 0.01 Hz! In this case the noise in the detection bandwidth drops to 0.5 µV (5 nV/√Hz ×√.01 Hz × 1000), but the signal stays at 10 µV. The S/N is now 20, and the signal can be measured.

Creative circuit design can be used to measure small signals in other parts of the signal spectrum also. The interested reader is referred to Signal Processing Group Inc ( website http://www.signalpro.biz) for more information. Please contact the SPG techteam through the "contact" menu item if needed.

Interestingly enough a monolithic version of the lock in amplifier is available with a 100dB range at a reasonable cost.