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

[Marcus Müller]

Hi Abhinav,

On 28.04.2016 20:20, abhinav narain wrote:

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/

Interesting! Why is there a time axis? I must admit those graphs look like they took some effort to create; really, a clean drawing on paper would have been as good for me, but thanks.

 

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.

No, I didn't mean that.
I mean: Is the signal you *want* to observe a single voltage signal varying over time, or is it an I & Q signal pair?

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.

I'm not convinced you actually are in both cases! That's why I'm asking :)

My question was related

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

in GRC)

OK, we need to talk about sampling:
Sampling takes place where your analog signal becomes a series of digital numbers, in the ADC of the N210. That is before you digitally tune to e.g. 125 kHz. The LF and Basic daughterboards don't have tuners!

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

500kHz, with center frequency 0Hz  using LFRX.

That really really depends! It depends on whether you want to observe a single real signal or an I&Q signal pair. So: what do you want to observe?

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).

You should really really understand these books. Knowing why there is no "additional information" in the negative frequencies is very essential to understanding complex baseband. In fact, it's so essential, it's the core of the first lecture about communication systems that every EE student here has to take.
The spectrum being symmetrical to the f=0 axis is not an artifact – it's an effect of how "spectrum" is defined, and again, it's a pretty important concept.

So here's the problem I'm having with your original question:
You said

I want to sample first 250KHz on the channel.

But: What *are* the first 250 kHz?
If you're just sampling a real signal with a single ADC, then obviously, you get a real-valued stream of samples, and to cover the bandwidth of 0 Hz – 250 kHz you will need to sample at  500 kHz.
If we're talking about complex baseband, I'd define (and that's purely something that I like, not something that is uniformly defined somewhere) the "first 250 kHz" to be the spectrum from -125 kHz – +125 kHz, because those 250 kHz correspond to the 250 kHz bandpass signal "closest" to the center frequency of your down-mixer. To get those 250 kHz, your ADC will need to run at 250 kHz, but it will need to produce one I and one Q sample simultaneously.

You can configure the N210 carrying a Basic or LF board into either mode – real sampling of up to two channels, and complex sampling of an I&Q pair that you feed into the two SMA connectors on the daughterboard.
We really need to understand what you *want* to be doing!
 

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

knowleagable.

That's a good approach!

So, I really don't know what literature you've got access to – if you have access to the scriptum for a communication theory introduction, that might be a good start.

Best regards,
Marcus