Digital sounder module and method for detecting
The invention is a digital sounder module and its method for detection. The digital sounder module includes a sonar carrier wave producing means, a band-pass filter, a pre-amplifier for providing a high sensitivity and extending a wide dynamic range, and an analog-to-digital converter for providing a digital implementation of a superheterodyne detector and producing an intermediate frequency. The digital sounder module also includes a programmable logic device for controlling a gain of the pre-amplifier and for digitally filtering the intermediate frequency and a microprocessor. The method implemented by the controlled per-amplifier gain processes the return echo signal by controlled ramp up of the gain over time to compensate for the change in signal strength in proportion to the length of the return path.
1. A digital sounder module comprising:
a sonar transducer for transmitting an acoustic sonar wave at a selected frequency and pulse rate and for receiving echos of said transmitted acoustic sonar wave reflected from objects and for converting said received acoustic sonar wave into a first electrical signal;
a band-pass filter for providing a first wide bandpass filtration on said first signal to produce a second signal;
an amplifier for controlled amplification of said second signal to produce a third signal, said third signal being an amplified carrier wave;
an analog to digital converter for receiving said amplified carrier wave, a rate at which said amplified carrier wave is converted to a digital signal by said analog to digital converter provides a harmonic sub-sampling, and said analog to digital converter transforms said amplified carrier wave into a digital intermediate frequency;
a programmable logic device, said programmable logic device implements a non-coherent windowed in-phase/quadrature (I/Q) detector and performs a Fourier transform of the intermediate frequency sonar return signal, said Fourier transform converts the intermediate frequency sonar return signal into a complex return signal which includes a real value that is an in-phase detected signal and an imaginary value that is a quadrature detected signal, said programmable logic device then multiplies the complex return signal by a programable Kaiser window filter, said programmable Kaiser window filter sets said bandwidth and said frequency, said programmable logic device provides
(i) a variable gain control signal to said amplifier for normalization of the amplitude of reflected echos within said second signal by increasing gain in proportion to the distance of said echo reflection source from said sonar transducer,
(ii) a cosφ signal to said analog to digital converter, said programmable logic device sets said harmonic sub-sampling rate of said analog to digital converter such that said reflected transmitted acoustic sonar wave is above the Nyquist frequency, and
(iii) a piecewise linear approximation of a log2 function, wherein the log value is linear approximated between all integer log2 values;
a programmable Kaiser window filter to transmit a windowed return signal to a power measurement function, said power measurement function transmits a power signal to a signal compression function, said signal compression function transmits a signal to a microprocessor; and
a microprocessor, said microprocessor provides an output for a display of said reflected echos.
2. The digital sounder module of claim 1 wherein said analog to digital converter is a 12 bit analog to digital converter.
3. The digital sounder module of claim 1 wherein said harmonic sub-sampling is performed by under-sampling said sonar carrier waves containing frequencies above the Nyquist sampling rate and directly sampling said sonar carrier waves containing frequencies below the Nyquist rate.
4. The digital sounder module of claim 1 wherein said programmable logic device performs an algorithm for approximating a phase shift, where said approximation is performed by approximating either said in-phase or said quadrature component of the intermediate frequency sonar return signal as a 90 degree phase shift of the other, and said approximation divides the complexity of the calculations by half.
5. The digital sounder module of claim 4 wherein said programmable logic device approximates said phase shift by taking a Fourier transform of the Σw[n]e (jnθ)/k signal from said Kaiser window filter, where sign(x) equals 1 if x>0, 0 if x=0, and −1 if x<0.
6. The digital sounder module of claim 4 wherein said programmable logic device approximates a Euclidian distance by taking a Fourier transform of the Σw[n]e (jnθ)/k signal from said Kaiser window filter, where sign(x) equals 1 if x>0, 0 if x=0, and −1 if x<, and then adding the length of the longest in-phase detected signal to half of the length of the shortest in-phase detected signal to produce said approximation of said second Euclidian distance.