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

[abhinav narain]

Hi Marcus,

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.

Not at all!

Sorry, I was taking pictures of hardware and hence just took one for

the GRC too.  As I showed in previous picture, I am only using RX-B. I

am not sure how to connections will be for dual mode, but currently I

am just using one. I get complex values in the GRC and I can plot

complex FFT of them.

My sample rate is 400 KHz and my center frequency is 200 KHz.

GRC 1] : http://postimg.org/image/6653d0rm9/

What is the rough line? Why did you chose to photograph a reflective surface agains daylight?

I have a better picture[2] now : http://postimg.org/image/hptdrl2vl/ 

 

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.

I am dong I&Q sampling as shown in [1]. By real sampling, I suppose

you meant that I am taking  magnitude before feeding to the Spectrogram.

 

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.

 

I am doing complex baseband as in the picture. My question was related

to the sampling. I thought doing sampling (complex baseband as shown

in GRC) in 250kHz at center frequency 125kHz is equavelent to doing

500kHz, with center frequency 0Hz  using LFRX. In 0 Hz case, I wasn't

able to make of the negative frequency (apart from the artifact that

they are in opposite direction, I read in books).

I think I am not clear in my understanding, so I wanted to ask someone

knowleagable.

Thank you for your time,

Abhinav

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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