Newsletter – Noise Fundamentals - Where Noise IS the Signal
Several experiments require students to open the electronics box, examine the circuit board and both select and install appropriate components needed to make their measurements. As is characteristic of all TeachSpin experiments, the apparatus is robust enough to allow students to make and correct a wide variety of “mistakes” without damaging the instrument. And it is this ability to explore with confidence that gives students not only ownership of a particular measurement, but also a taste of the satisfaction of experimental exploration.
Also in the photograph are two user-supplied components needed to complete the necessary diagnostics and measurement capability: a digital multimeter and an oscilloscope. Not shown are the ‘parts bins’, containing all the electronic parts, tools, and cables useful for configuring this versatile apparatus into the form required for a wide variety of projects.
The Low-Level Electronics contains some utility functions (at the bottom) and two Modules. At the upper left, you see the Thermal Control module, for interfacing to the Probe. In it are a voltage supply (for the heater which raises the probe’s temperature above 77 K) and a current supply (for powering the transdiode temperature transducer in the probe).
At the upper right in the photograph is the Pre-amplifier, the front-end electronics for all noise measurements. Front-panel controls supplement internal user-chosen configurations to make this a very versatile first stage in the measurement of various kinds of noise.
At the bottom, you see provision for Signal Attenuation (used in optional calibration activities) and a utility Series Resistance. Also visible are two low-noise power supplies, which are used for various biasing and energizing tasks. Additional low-noise power supplies energize all the circuits in the low-level electronics – no batteries are required.
The entire front panel of the low-level electronics is readily removed from its metal case, allowing easy access to the interior for user re-configuration. In this view, the Pre-amp section is at the lower right, and the screw-terminal blocks can be seen. These allow the re-wiring (without soldering!) of the front end. Students can create inverting and non-inverting amplifiers, or a current-to-voltage converter. Even the front-end operational-amplifier chip is easily exchanged or replaced.
In this view, the Temperature Control electronics are at lower right. They too allow reconfiguration via terminal blocks. In particular, users have full control over the leads headed down into the thermal probe.
At the upper right is a heavily-shielded aluminum ‘bathtub’ in which the input power for the whole box is conditioned for low noise. Across the top is the board supporting the utility functions within the low-level electronics. Here too, terminal blocks allow maximum flexibility of use of present (and future!) front-panel modules.
The High-Level Electronics are also organized into modular form, to be flexibly interconnected by the user with the cables supplied. Here you can see the two frequency-filtering modules (which specify the bandwidth of noise to be quantified), a variable-gain main amplifier module, and the multiplier module (typically used to execute the squaring function). Here also is the output section which performs the time average, runs the display meter, and sends a voltage to a user-supplied digital multimeter or other d.c. voltage-measuring device.
Not shown in this view is a rear-panel Noise Calibrator, a source of fixed-power, fixed-bandwidth, and accurately-quantifiable pseudo-noise. This provides an optional reality check for students learning to quantify noise.
Power to the entire chain of electronics is supplied by a universal power supply, mounted remotely in the supply cord to the high-level electronics. All the issues of power conditioning and ground-referencing have been handled, so our system is ready to use, without tears.