Re: [Discuss-gnuradio] Question about sampling rate and center freq

[Marcus Müller]

Hi Abhinav,

thanks for the sketch; for the next time, screenshots are best done by using GRC's "Screen Capture" functionality, or at least with your operating system's screenshot function. Can't be that hard.

Now, I must admit that I really don't undestand the axis in your sketches. These sketches are horribly unlabeled! What's the vertical, what's the horizontal axis? What is the rough line? Why did you chose to photograph a reflective surface agains daylight?

And, as I mentioned in IRC when I asked for them, it is important to define wheter you're doing I&Q or real sampling. It's no use discussing any of your question until we know whether we're looking at a complex equivalent baseband or at a real signal.

Best regards,
Marcus

On 27.04.2016 19:13, abhinav narain wrote:

Hi all,

I want to sample first 250KHz on the channel.

I am using the (sampling_with_lowpass.JPG) setup to extract quadrature samples.

I have  LFRX card where i just use the RX-B pin to extract complex samples from the channel.

My question is (question.JPG) how are the two setups different, assuming the connections are correctly made.

a) I set the sample rate= 250KHz, and center freq=125KHz.

b) I set the sample rate=500KHz, and center freq= 0 KHz (I hope the card is fine with setting center freq=0)

Q1 ) I understand that the sampling rate will be different, but I should be getting the samples from same bandwidth ? 0-250KHz

Q2 ) in b), how do i interpret the negative frequencies in complex FFT.

( I understand these are frequency in opposite direction acc to books, but not sure if they make sense here, and hence I am doing way a) )

Apologies, I am CS student so I might not be very clear with my EE fundamentals.

Thanks,

Abhinav

LFTX setup- http://postimg.org/image/9s6hy4rqp/

sampling_with_lowpass - http://postimg.org/image/qkpeeqgz5/

question - http://postimg.org/image/3mjsmwcc1/

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