Re: [Discuss-gnuradio] Signal Source Coherence

[Marcus Müller]

Hi Liwei,

they *should* be coherent, because GNU Radio has no relationship to 
"real wall-clock time" and just outputs mathematically calculated 
numbers, just like matlab does.
Depending on the number of samples we talk about when you say "over 
seconds", this might simply be a matter of numerical accuracy -- GNU 
Radio internally uses (single precision) floating point numbers, and 
thus the accuracy of the generated frequency might be limited, and so is 
the accuracy of the phase comparison.

Having a few numbers would be nice here -- how large is your drift in 
2pi/sample?

Greetings,
Marcus


On 11/11/2014 06:34 PM, Liwei wrote:
> Hello list,
>      Please refer to the attached flowgraph. In it I use two methods to
> generate 3 tones of equal amplitude.
>      The first method simply uses 3 signal sources to generate the
> required tones directly and sums them together. The second method
> multiplies two signal sources to generate the upper and lower tones,
> and sums that result with the higher signal source. Both methods
> generate the same result in the magnitude spectrum.
>      However, using an external Matlab script (not written by me, so I
> can't share) to up-convert the lower tone and down-convert the upper
> tone to the same frequency as the middle tone (so all three tones end
> up being the same frequency, using the middle tone as reference), and
> measuring the phase difference between all tones, I get unexpected
> results.
>      With the second method, the phase relationships are constant, as
> expected. However, with the first method, the phase relationships
> drift (linearly I think) over seconds.
>      I did some equation manipulation, but it does seem like, for the
> first method, even with arbitrary phase offsets in each signal source,
> the final phase relationships between the 3 tones should still be
> constant, and not change over time.
>      A quick search did not come up with relevant answers, and maybe
> this is a purely academic error on my part, but the remaining
> possibility that I can think of is that the signal sources in GNU
> Radio are somehow not coherent?
>
>
> Regards,
> Liwei
>
>
>
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