The principle of random sampling
According to the Nyquist sampling theorem, when the signal frequency is much higher than the sampling frequency of A/D, the signal waveform cannot be reconstructed.Therefore, random sampling can be used for high-speed signals. Random sampling is determined by measuring each time A/D sampling sequence as the starting point and A fixed reference point (signal trigger point) lag Δ t, due to the randomness of Δ t, so through n times of signal sampling, if n is large enough, by random sampling sequence of superposition can be signal waveform
System circuit design scheme
Is the circuit diagram of random sampling system.After the measured signal is attenuated and amplified by the analog channel, it is sampled and quantified by the fast analog-to-digital converter (FADC), and the sampled data is sent to FPGA and cached in FIFO.Signal after the trigger circuit and trigger signal and the random sampling time measurement circuit trigger point and then the time interval between the first sampling pulse Δ t.The slow adc SADC Δ t into digital quantity, sent to FPGA.
FPGA is the control core of the system. It receives various control commands sent by ARM through SPI serial bus to control system work and complete digital fluorescence processing of waveform data.
Random sampling, the FPGA internal control commands from the control module based on ARM external random sampling time work to control the measurement circuit, waveform reconstruction module according to read the value of the corresponding calculated between 0 ~ 1 M - a set of values, and according to the values for each group I sampled data sorting, calculate the corresponding RAM memory address, and FIFO cache each sampled data according to the corresponding address in RAM.
When the group value I is spread over all values between 0 and m-1, a complete waveform has been reconstructed. The waveform reconstruction module will send the set of data in RAM to the digital fluorescence processing module to convert it into the image data displaying waveform. The digital fluorescence processing module will send the waveform image data to ARM for display on a regular basis.
The random sampling time measurement circuit is the key to accurate measurement of Δ t and the restructuring of the waveform data.