[LAD] Latency measurements: External round-trip vs. absolute in/out values

[Tim]

On 4/7/19 6:48 AM, Fons Adriaensen wrote:
> On Sat, Apr 06, 2019 at 12:32:29PM -0400, Tim wrote:
> 
>> So the software would be able to deal with sines better,
>>   and measure phase differences rather than a pulse edge.
> 
> Sure, the pulse will be 'smeared out' a bit, and its actual
> position may seem to be ambiguous. But that's no problem.
> 
> What you get in the sample stream is the impulse response
> of the A/D converter. Assuming that this is a linear and
> time-invariant system, the IR contains all information.
> 
> If the pulse is smeared out, one way to find the delay is
> 
> 1. Compute the FFT of the part of the sample stream that
>     contains the smeared-out pulse.
> 
> 2. Compute the phase of each frequency bin. If the delay
>     is independent of frequency (it should be if the A/D
>     is any good), then the phase will be a linear function
>     of frequency. The slope of that function indicates
>     the delay (relative to the start of the FFT input).
> 
> The only advantage of using sine waves is that your test
> signal has much higher energy than a single short pulse,
> so you get a better S/N ratio. But for this test, the
> S/N will be very good anyway.
> 
> 
> Ciao,
> 

Awesome, thank you!
Tim.