A Novel Method for Bandwidth Measurement of Oscilloscopes in Automatic Testing/Calibration

摘要

Bandwidth determination is one of the major tests in oscilloscope performance verification/calibration along with other vertical amplifier tests like DC gain determination, triggering sensitivity etc. Usually oscilloscope bandwidth is defined as the frequency where the gain falls from the DC or very low AC frequency value by -3dB. Traditionally while testing one feeds a specified frequency (e.g. 50 kHz) sine wave with known amplitude (say 1 Vp-p) and reads the amplitude value from the scope. This value is taken as the reference level. The frequency of sinusoid signal is then varied in steps and the corresponding amplitude levels are noted down till it falls by -3dB from the reference level. The frequency value at this point gives the bandwidth of the scope. Though this method is well suited for manual testing, it is not optimized for automatic test procedures. The reason is that the instrument controller is required to command the signal source to feed the required signal frequency and levels, read back the level reported by the equipment under test and compare this with the reference point level. In this way lot of intermediate frequency points are used in the measurement. Usually before measurements are made from the scope enough settling time has to be provided after applying the signal, otherwise the readings will be in error. All these delays add up and test completion takes a long time in traditional method if implemented for automation. In this work a new method for bandwidth determination which can cut down the number of test points drastically by adopting a numerical algorithm (bisection method) is proposed. The traditional way of bandwidth determination and new method are compared for accuracy of the test results, iteration count and test completion time. Results are presented which proves that the new method of bandwidth determination is better than the traditional method based algorithm for oscilloscope testing automation by showing fast convergence to the true bandwidth.