A
Anonymous
Guest
To get ready for a digital PI project I have been doing some tests on my VLF test bed. It reads signals below 1 uV at the coil, but the difference is that it uses a narrower bandwidth amplifier. So there is less noise going into the A-D than there would be in a PI.
I increased the bandwidth of the amplifier to 6 kHz for a test, and the noise after integration increased by about a factor of 10 compared to what I normally see. This causes me to worry about the quality of the signal I am going to get if I just directly sample the output of the amplifier.
The first graph below shows the frequency response for a sampling rate of 4400 Hz with every other sample being subtracted and an amplifier bandwidth of 14 kHz. This is similar to a 2200 Hz pulse repetition frequency and differential sampling. The first peak in the response at 2200 Hz is the one that collects useful information. All the other peaks to the right pickup noise. These other peaks cannot be removed after sampling. They can be reduced by using a sharper roll off in the amplifier, or by taking more samples.
The most straight forward way to reduce those unwanted peaks is to increase the PRF. The second graph below shows the frequency response for a 7 kHz PRF and a 14 kHz amplifier. With fewer peaks to the right there is less opportunity to pick up noise.
In a sampled system there is normally a low pass filter before the sampling to cut off the peaks on the right. But that is a problem in a PI where there is a very large voltage spike just before the signal of interest.
Robert
I increased the bandwidth of the amplifier to 6 kHz for a test, and the noise after integration increased by about a factor of 10 compared to what I normally see. This causes me to worry about the quality of the signal I am going to get if I just directly sample the output of the amplifier.
The first graph below shows the frequency response for a sampling rate of 4400 Hz with every other sample being subtracted and an amplifier bandwidth of 14 kHz. This is similar to a 2200 Hz pulse repetition frequency and differential sampling. The first peak in the response at 2200 Hz is the one that collects useful information. All the other peaks to the right pickup noise. These other peaks cannot be removed after sampling. They can be reduced by using a sharper roll off in the amplifier, or by taking more samples.
The most straight forward way to reduce those unwanted peaks is to increase the PRF. The second graph below shows the frequency response for a 7 kHz PRF and a 14 kHz amplifier. With fewer peaks to the right there is less opportunity to pick up noise.
In a sampled system there is normally a low pass filter before the sampling to cut off the peaks on the right. But that is a problem in a PI where there is a very large voltage spike just before the signal of interest.
Robert