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Papers and App Notes

The following documents from the SR Library may be of interest.
Some of the papers and code review topics of general interest, while others discuss specific SR equipment.

Scaling and Biasing Analog Signals PDF icon

This paper describes techniques for interfacing electronic equipment with different input and output voltage ranges together. Theory and circuits for changing the range and offset of analog signals is reviewed, including passive 2 and 3 resistor designs as well as basic op amp techniques.

These techniques are useful in real life for interfacing sensors with one voltage range to data acquisition equipment with a different range. While not particularly difficult, the theory for many of the circuits is interesting. Subjects like scaling and projective transforms even prove to be useful.

Using a Meinberg DCF Radio Clock w/ USBxCH PDF icon

Written by Giovanni Rapagnani of the Royal Observatory of Belgium, this paper describes the steps required to configure and connect a Meinberg C600RS DCF77 radio clock so it can provide accurate time stamp information to the SR USBxCH data acqusition systems.

The DCF77 long-wave (77.5 kHz) radio signal with timing info emitted from an antenna in Frankfurt, Germany is received by the Meinberg radio clock. The benefit of using this radio signal is it does not require sky view and works even when installed in the basement of a building. The DCF77 signal is only available at locations within 1900 km of Frankfurt.
Meinberg DCF77 radio clock

Analog Response of Sigma Delta Converters PDF icon

Written by Jeff Thorson of Henry-Thorson consulting, this paper describes the analog filter response of the Burr-Brown ADS1210 chip used on the SR PARxCH data acquisition systems. The ADS1210 is a second-order sigma-delta A/D converter incorporating a three-stage low-pass filter which defines the analog system response.

The modulator, low-pass digital filter, and decimation stages of sigma-delta converters are discussed. Their theoretical response is also compared experimentally with measured responses to square-wave and triangle-wave analog input signals. The concepts described here are applicable to all sigma-delta converters including the ADS1255 used on the SR USBxCH.

Precision sine waves with the DSP32C PDF icon

A precision sine wave generator program suitable for use with D/A converters hooked up to a DSP32C digital signal processor is reviewed. While simple in principle, precision sine wave generation requires careful programming.

Low precision methods for generating a sine wave include table look up, interpolation, and incremental methods based on the addition formulas for sine and cosine. But these result in tables of considerable size, interpolation noise, or accumulated incremental error. Better solutions are discussed.

FFT routines in C

This zip file contains a sequence of programs developing code for the Fast Fourier Transform ( FFT ). The programs proceed from a simple recursive FFT function, onward to a production level radix 2 algorithm. All examples are in standard C source code.

Customers are welcome to use these routines for custom programing in their applications. Executables are for standard x86/DOS architectures. Recompiling for other platforms is straighforward. The final radix 2 FFT does not use any C runtime library functions, but does expect floating point arithmetic to be available.

See the readme.txt file for a description of the algorithms covered.
FFT algorithm pseudo code Copyright © Symmetric Research