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SRAS: Signal conditioning and DAQ
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SRAS signal chain
SRAS data acquisition uses an oscilloscope to digitise data
Input has 8 bits over full scale with options for sensitivity i.e. 1mV / division all the way up to 5v/div. There are 8 divisions for full scale.
Recall, we worked out roughly what the signal level would be after the photodiode circuit. ~0.2mV*Circuit rf Gain which is typically 10
So from the circuit we have a max signal level of ~2mV
We have 2^8 levels over 8mV, giving LSB of 31uV. Our signal is then represented by only 64 levels or 6 bits.
We need some additional gain to get into a good working range for the scope and to have plenty of bits to represent our signal and to overcome any noise in the electronics chain.
SRAS signal chain
2.0mV peak signal at output of PD circuit
Total gain is 37dB which is voltage gain of 10^(37/20) = 70
140mV peak signal at input of scope
Bandwidth restricted from 50kHz-400MHz to 100-200MHz
PD Circuit
Attenuator
50kHz-400MHz
50kHz-1GHz
50kHz-1GHz
SRAS signal chain
We have a large coherent noise source on our data – we capture this before the experiment with a lot of averages and subtract this from our data.
SRAS signal chain
Actual signal in this case is a bit smaller than what we calculated ~+/-0.7mV
Signal is hard to see against the noise, even after coherent noise removed.
SRAS signal chain
Adding the 37dB of gain improves things a lot, the signal is now obvious, the background noise is having less impact. The signal is ~+/-200mV so the gain at this frequency slightly higher than the nominal rated value.
SRAS signal chain
If we add a high pass filter inline and use the inbuilt low pass 200MHz filter on the scope input, then we get a much cleaner waveform.
Remember this is single shot with no averaging. The ac signal is tiny compared to the dc but with careful sensor and signal conditioning choices we can get a good signal.
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