[Discuss-gnuradio] Odd behaviur of "Fractional Interpolator"

[Mattia Rizzi]

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?