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Altera_Forum
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14 years ago

system design using cyclone II

Hi, I am trying to implement a hardware system which uses current to compute distances. The incoming current is obtained from a sensor, and will be sampled and input into my FPGA. The ADC is 14...
Altera_Forum's avatar
Altera_Forum
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14 years ago

Hi Dave,

I read through the links you gave me yesterday, especially the user manual of NCO. very helpful. Thx!

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It is the light that is being modulated, so that you get modulated currents at the output of your sensor. The reference of the 154kHz modulation source is important, because the sensor processing needs to be synchronous with that source.

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It's the current from the PSD sensor that 's been modulated. not the light. Now, let's just assume tat we can get a continuous current signal from a certain source. What we are interested is the 154Khz components in the signal.

1. suppose we decide to use a 12bit, 40MSPS ADC to perform the analog-to-digital task (about that, i searched online trying to find information about ADC filter that can suppress frequency components other than 154Khz (for example). but all i found were anti-aliasing filters. it seemed to me like when selecting an ADC, the most important thing was output bits (usually 8bit is more than enough). none of them mentioned the center frequency, like what you said yesterday. could you explain it more to me please?)

2. we need a lock-in amplifier.First I implement a PLL, which produce an output clk of 10Mhz. (btw, can this signal be locked with the 154kHz input???)

3. pass the 10Mhz clk into a NCO, which produces a 154Khz sine wave. (also, there is an optional frequency modulator function in this NCO, I read through the manual sheet, not sure if I goet what they meant. Does it say that through this function, the output sine wave can be locked with an external frequency, say, my original 154Khz signal? if so, why should I still need a PLL? if not, how can I make sure this signal is locked with my 154khz input?)

4. pass the sine wave and the original signal into a mixer.

5. use a LPF that only keeps the DC components.

sorry if I ask too much or some of them dont make sense.... i really want to make sure i understand the whole thing and am able to complete the task. Thanks for your help...

Allison