Odd behaviur of "Fractional Interpolator"

I’m using gnuradio-3.3.0 with GRC.
I’ve builded a graph with a Complex cosine (8Ms/s of sample rate and 1
MHz of frequency), the fractional interpolator with 12/8 value for
interpolation and a FFT sink with 12MS/s of sample rate. (throttle block
included).
I’m expecting to see a 1MHz cosine with the FFT, but i see a 2.25MHz
cosine. If i put 8/12 (the inverse) i see the (correct) 1MHz cosine.
Why?
If i use the Rational resampler, with interpolation 12 and decimation 8,
i see the correct 1MHz cosine. What’s wrong with fractional interpolator
block?

On 18/10/11 01:38 PM, Mattia R. wrote:

interpolator block?


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Because the fractional interpolator takes the inverse as the desired
fraction–think of it as a decimation
ratio.

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