A modular, low-cost, digital signal processor-based lock-in card for measuring optical attenuation

A modular, low-cost, digital signal processor (DSP)-based lock-in card is d escribed for measuring optical attenuation. By transferring the lock-in ope ration from the analog domain to the digital domain, the nonlinearities gai n, and offset errors and drifts are virtually eliminated. The dual phase lo ck-in operation has been implemented on the low-cost DSP56002 evaluation mo dule (DSP56002EVM) of Motorola that is widely used in audio signal processi ng. This evaluation board contains a 24 bit DSP56002 DSP and a stereo CD-qu ality audio codec that makes the board ideal for implementing signal proces sing algorithms. Due to the maximum sampling rate of the codec embedded on the DSP56002EVM, the frequencies of the processed signals must be below 20 kHz. This specification is enough for the most common applications in the f ield of optics, where low or very low frequency (<1 kHz) references are fre quent. The software algorithm implementing the lock-in amplifier can be par ticularized by the user on the basis of the needed performances. The effect s of finite word length in the digital filter implementation are analyzed. This analysis reveals that a 24 bit word length is not enough to ensure the filter stability and the required frequency response. To overcome this pro blem, the double precision multiply mode must be used. When the DSP56002 en ters this mode, double precision 48 bit by 48 bit multiplication can be per formed. The lock-in performance has been tested. The measured amplitude var iations of the reference sine signal are about 0.003%, which do not affect the signal measurement. The lock-in behaves like a band-pass filter centere d on the reference frequency whose bandwidth is related to the low-pass fil ter cutoff frequency. The measured frequency response shows that the lock-i n performs as theoretically predicted. The DSP56002EVM can be used as a loc k-in for electrical signals in stand-alone operation. Besides, we have desi gned a card that interconnects to the DSP56002EVM and allows the ensemble t o act as an optical attenuation detector that measures optical losses over 70 dB. This range is similar to that achievable by commercially available o ptical loss testers and makes it suitable for optical return loss measureme nts of all kinds of commercially available optical connectors. (C) 2001 Ame rican Institute of Physics.