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<blockquote data-quote="Adam Black" data-source="post: 56174" data-attributes="member: 702">Re: Dual Channel FFTWow. There is a lot of ground covered in this thread. I won&#039;t comment on&nbsp; all of it but there were a few points I wanted to address for&nbsp; clarity.Firstly there seems to be some confusion on the difference between a&nbsp; single channel and dual channel measurement. They are completely&nbsp; different measurements and serve vastly different purposes. Learning the&nbsp; difference between them and what their applications are is time well&nbsp; spent. I don&#039;t mean that in a snarky manner. It&#039;s just that&nbsp; understanding the fundamentals of ones toolkit will go a long way to&nbsp; improving their use and thus the results. In a nutshell the difference is thus. A single channel measurement shows&nbsp; you the spectral content of one signal. In other words, it depicts the&nbsp; amount of energy present for various frequencies. A dual channel&nbsp; measurement shows you the difference between two signals, in&nbsp; frequency, energy and time. In other words, what is in one signal but&nbsp; not the other and by how much. With that distinction made I&#039;ll address&nbsp; at the original question.&nbsp;The answer is, nothing. Using a dual channel measurement doesn&#039;t improve&nbsp; anything in relation to a single channel measurement. They aren&#039;t like&nbsp; quantities.Assuming that you&#039;re using the output as your reference signal and the&nbsp; input as your measurement signal, this is perfectly acceptable for a&nbsp; dual channel measurement. Getting the output signal via a hardware or&nbsp; software loop back is merely a convenience and requires less cabling.&nbsp; Though as Langston pointed out, the hardware path is nice in that the&nbsp; reference signal underwent the same D/A and A/D processing as the&nbsp; measurement signal. Whereas with the software path the reference signal&nbsp; remained in the digital. Which is precisely why the software loop back&nbsp; has additional delay. More on this below.The added delay isn&#039;t propagation delay, but rather the lack of&nbsp; propagation delay for the reference signal. With a software loop back,&nbsp; the reference signal has virtually no propagation. It&#039;s created in the&nbsp; digital and remains in the digital before being fed to the measurement. Whereas, at a minimum, the&nbsp; measurement signal undergoes a D/A and A/D conversion and some amount of&nbsp; additional processing and propagation before it reaches the measurement. This process takes a finite&nbsp; amount of time, considerably more time than it took the reference signal. As we are measuring the difference between the two signals, the difference is manifested as additional delay.Hopefully that clears things up for you.-A</blockquote>

[QUOTE="Adam Black, post: 56174, member: 702"] Re: Dual Channel FFT Wow. There is a lot of ground covered in this thread. I won't comment on all of it but there were a few points I wanted to address for clarity. Firstly there seems to be some confusion on the difference between a single channel and dual channel measurement. They are completely different measurements and serve vastly different purposes. Learning the difference between them and what their applications are is time well spent. I don't mean that in a snarky manner. It's just that understanding the fundamentals of ones toolkit will go a long way to improving their use and thus the results. In a nutshell the difference is thus. A single channel measurement shows you the spectral content of one signal. In other words, it depicts the amount of energy present for various frequencies. A dual channel measurement shows you the [B]difference[/B] between two signals, in frequency, energy and time. In other words, what is in one signal but not the other and by how much. With that distinction made I'll address at the original question. The answer is, nothing. Using a dual channel measurement doesn't improve anything in relation to a single channel measurement. They aren't like quantities. Assuming that you're using the output as your reference signal and the input as your measurement signal, this is perfectly acceptable for a dual channel measurement. Getting the output signal via a hardware or software loop back is merely a convenience and requires less cabling. Though as Langston pointed out, the hardware path is nice in that the reference signal underwent the same D/A and A/D processing as the measurement signal. Whereas with the software path the reference signal remained in the digital. Which is precisely why the software loop back has additional delay. More on this below. The added delay isn't propagation delay, but rather the lack of propagation delay for the reference signal. With a software loop back, the reference signal has virtually no propagation. It's created in the digital and remains in the digital before being fed to the measurement. Whereas, at a minimum, the measurement signal undergoes a D/A and A/D conversion and some amount of additional processing and propagation before it reaches the measurement. This process takes a finite amount of time, considerably more time than it took the reference signal. As we are measuring the difference between the two signals, the difference is manifested as additional delay. Hopefully that clears things up for you. -A [/QUOTE]

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