A
Anonymous
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I don't have a PI board yet, but I have my first results from the digital PI project by connecting the PI processor to my VLF board. To keep track of which board is which, I will call the new processor board the PI DSP (Digital Signal Processor).
I connected the logical coil drive signal from the VLF board to an external interrupt line on the PI DSP. And I connected the output of the VLF amplifier to the analog input of the PI DSP. The VLF frequency is approximately 6600 Hz (158 usec period). You can think of this as a PRF of 6600.
On the falling edge of the coil drive signal an interrupt occurs. A few microseconds after the interrupt I read the A-D and add the sample into an integrator. I also start a timer that will go off 180 degrees later (79 usec). When the timer goes off I read the A-D again and subtract this sample from the integrator. This gives one differential sample per cycle for the integrator.
I count 220 cycles and then send the output of the integrator to a D-A converter. So I can see the output of the integrator on a scope. This gives samples at about 30 Hz. Later I will be filtering these samples.
The integrator is a low pass filter with a time constant of 256 cycles which is about 40 msec.
The output from the PI DSP looks about as good as the output from the VLF board which has a lot more filtering. The PI DSP has a couple of advantages over the other board. It has a 12 bit A-D vs. 10 bit. And because of the faster A-D it can get one differential sample per cycle while the VLF board only gets one differential sample every three cycles. So the PI DSP is getting more samples with more bits per sample.
This is not much of a test though because the VLF amp has a much narrower bandpass than a PI amp. I will be seeing a lot more noise at the A-D when I put this on a PI board.
Robert
I connected the logical coil drive signal from the VLF board to an external interrupt line on the PI DSP. And I connected the output of the VLF amplifier to the analog input of the PI DSP. The VLF frequency is approximately 6600 Hz (158 usec period). You can think of this as a PRF of 6600.
On the falling edge of the coil drive signal an interrupt occurs. A few microseconds after the interrupt I read the A-D and add the sample into an integrator. I also start a timer that will go off 180 degrees later (79 usec). When the timer goes off I read the A-D again and subtract this sample from the integrator. This gives one differential sample per cycle for the integrator.
I count 220 cycles and then send the output of the integrator to a D-A converter. So I can see the output of the integrator on a scope. This gives samples at about 30 Hz. Later I will be filtering these samples.
The integrator is a low pass filter with a time constant of 256 cycles which is about 40 msec.
The output from the PI DSP looks about as good as the output from the VLF board which has a lot more filtering. The PI DSP has a couple of advantages over the other board. It has a 12 bit A-D vs. 10 bit. And because of the faster A-D it can get one differential sample per cycle while the VLF board only gets one differential sample every three cycles. So the PI DSP is getting more samples with more bits per sample.
This is not much of a test though because the VLF amp has a much narrower bandpass than a PI amp. I will be seeing a lot more noise at the A-D when I put this on a PI board.
Robert