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A good place to start might be to have students study the frequency response of the preamplifier as a function of its gain. For this, the modules would be configured as shown in Figure 1.

Figure 2

The SPLIA1-A has a sophisticated filter that has many applications. It can be used as a low-pass, high-pass, or bandpass filter with variable Q. The most common application of this filter is in bandpass mode, where it reduces the noise into the detector module.

Figure 3a shows the measured frequency response in the bandpass mode with two different values of Q. In Figure 3b, we show the measured phase shifts of the signal through this filter for the two values of Q. These measurements point to important phase stability considerations, which must be accounted for when using high Q filters and lock-in detection.

COMPARING FILTERS

This module also has Chebyshev,Butterworth and Bessel types of filters. Examining the
transient responses of these filters helps students to understand their function. Figure 4 shows the response of the Bessel filter (middle trace) and the Chebyshev filter (lower trace) to the square wave input shown on the upper trace.

Figure 4

LOOKING AT ROLL-OFF EFFECTS

The low-pass amplifier module has both a6db/oct and 12db/oct roll-off, with time constants varying from 0.3 to 10 seconds. The measured response curves for this output amplifier-filter are shown in Figure 5.

Figure 5

Students can study the effects of various time constants on signals that vary with time, such as when sweeping through a magnetic resonance signal. In that case, it is necessary to select a time constant compatible

with the sweep rate, in order to optimize signal-to-noise enhancement, without distorting the signal. Students can also compare the effects of a 6db/oct and a 12db/oct roll-off filter on enhancement for time varying signals.

A unique feature of the SPLIA1-A is the ability to process a real physical signal (or a test signal) in different ways. In particular, it is useful to compare signal-to-noise enhancement using amplitude detection with lock-in or phase sensitive detection. Figure 6 shows the lock-in configuration of the modules while Figure 7 shows the amplitude detection architecture.

Figure 6

Figure 7

Figure 8 is a slow time scan of a test signal which is being detected both by the lock-in (lower trace) and the amplitude detector (upper trace) using the same overall signal bandwidth.

Figure 8

FINDING A KNOWN SIGNAL

TeachSpin's instrument even has a built-in noise source, which can be used to create a test signal with variable signal-to-noise ratios. This signal is created by connecting the reference oscillator through the signal attenuator. This allows students to experiment with signal processing before they use the instrument to process a weak signal from any number of real experiments.

This apparatus offers a wide variety of experiments to help students understand the nature of signal processing and develop a mastery of the lock-in detector. Considering the importance and omnipresence of the modern lock-in amplifier in the research labs of all kinds of experimental sciences, especially physics, it seems clear to us at TeachSpin that the SPLIA1-A belongs in every advanced lab. It is affordable; it is essential; it is ready for your advanced, electronics, or instrument laboratory course.