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* <emb...@madlab.it> 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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