Re: [Discuss-gnuradio] UHD Warning on ODD Interpolation

[embyte]

Dear Marcus,

> How wide is your signal?

My signal is 25 KHz wide, centered at 25 KHz. (I mix the baseband signal 
with a sin function at 25 KHz)

> Then find the nearest sample rate > your_bandwidth that satisfies the
> desired outcome that the computed
>    interpolation rate  (64.0e6 / desired_rate) is an even number.
> If the signal has artefacts that "spill" outside the required sample
> rate, then, yet, filter it first.

While experimenting, I discovered that with 870 KHz sampling rate works, 
i.e. without the half-band filter problem. However, I get the baseband 
signal mirrored at 50 KHz lower than the center frequency configured in 
the USRP. It seems like related to the sampling theorem (I mean 
aliasing, 2*Fmax=Fs)

I can't figure out how the USRP works internally, and why moves the 
baseband signal to the transmitting frequency (Yes, I'm not an RF/DSP 
expert, just intermediate :)
Looking on-line,I found out that the function block diagram is something 
like: CIC (Intepolation) -> half-band filter -> DAC -> TX. Can you 
confirm it?

I appreciate.


> on Aug 01, 2013, *embyte* <address@hidden> wrote:
>
>     Hey Marcus, thanks for answering.
>
>      > (1) 720Khz is not an exact integer fraction of 64MHz (the
>     sampling rate
>      > of the device)--so it rounded
>      >
>      > (2) It's warning you that you'll get a "humped" frequency
>     response with
>      > more rolloff than you'd like, due to the rounded-down sample rate
>      > requiring an interpolation rate that is not divisible by two.
>
>     If I understood correctly, this it means that the transmitted signal
>     gets filtered more on the edges than expected?
>
>     Note that by setting a sampling rate of 720KHz, the transmission and
>     receiving work fine.
>
>     That is, can I fix the sampling rate warning and preserve the
>     transmitted signal by modifying the sampling rate (e.g. to 480KHz) and
>     by adding a band-pass filter before the USRP sink block?
>
>     Thanks for your insights. I appreciate.
>
>
>
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