FAQ
82 Acqiris SA240P User's Manual
Chapter 9
FAQ
9.1 Q. What is coherent sampling? 82
9.2 Q. How to manage the internal temperature? 82
9.3 Q. What happens if the host processor goes in hibernation mode? 83
9.1 Q. What is coherent sampling?
A. Coherent Sampling refers to the relationship between the input frequency F
in
, sampling
frequency F
s
, number of cycles in the sampled set and the number of samples. With coherent
sampling one is assured that the signal power in an FFT is contained within one FFT bin (assuming
a single input tone).
The condition for coherent sampling is given by:
F
in
/F
s
=Ncycles/Nsamples
For example if we have Nsamples = 2
11
, and Fs = 100e6, and we expect and input frequency close to
Fs/2, let's say Fin = 44 MHz, then Ncycles = 901.12 which is close to an integer. We could therefore
round down to Ncycles = 901 and we would get Fin = 43.994140625 MHz, which is an input frequency
that satisfies coherent sampling.
The integer number should be chosen carefully. We have three possible types of integers, even,
odd, and prime. Even is not a good idea since we would hit the same code every Msamples, where M
can be much less than N. Odd is a better idea since it takes longer to hit the same code. According
to some sources a prime number of cycles is the best (with the exception of the prime 2) because it
takes a long time before the same code repeats.
——————————————————————
9.2 Q. How to manage the internal temperature?
A. The operating temperature of the SA240P as specified in the SA240P datasheet, is the
workstation internal ambient temperature at intake of the ADC card's fan.
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