Re: [Discuss-gnuradio] Looking for tutorials (was: Hi all)

[Marcus Müller]

Hi Nirmala,

> When I use the signal source block, although its already digital, I
> thought I must specify sampling rate as per Nyquist condition in the
> "samp_rate" variable.

You must specify a sampling rate; but you don't have to adhere to
Nyquist. Just be aware that if you use a signal frequency that isn't in
the first Nyquist range, it will be aliased. 

> For the USRP sink, "center
> frequency parameter", I understood that I can specify the carrier
> frequency that carries the baseband signal.

True!

> But I also observed that if I change the variable samp_rate, it
> changes throughout in all other blocks as well. So it must be a
> global parameter and not necessarily the Nyquist sample rate as I
> thought it to be! Request you to kindly explain.

Well, that's the beauty of using *Variables*. The fact that you
interpret the variable called *samp_rate* as sampling rate is just
interpretation of some block's parameters. Don't mix your specific
usage of variables with signal theory meaning of block parameters!

Best regards,
Marcus

> When I multiply two signals and plot the FFT, the plot mentions dB
> for the frequency components. You already corrected me earlier that
> its not power. Then what does this dB mean?

Decibel. It's decibel relative to a digital mag² = 1 signal.
If you're confused about the difference between dB and dBm, I recommend
revisiting the wikipedia article on decibel.

Best regards,
Marcus


¹ nitpicking: If you sample a 10 kHz signal with a 20 kHz sampling
rate, you might end up getting a constant value; 9.999kHz would be
fine, if observed long enough.
On Mon, 2017-11-06 at 20:39 -0500, Nirmala Soundararajan wrote:
> Hi Marcus,
> 
> Yes. I understand that I need to be more specific in the subject.
> Thanks for correcting. 
> 
> When I use the signal source block, although its already digital, I
> thought I must specify sampling rate as per Nyquist condition in the
> "samp_rate" variable.  (Suppose the tone is 10 kHz, then I must give
> more than 20 kHz as sampling rate). For the USRP sink, "center
> frequency parameter", I understood that I can specify the carrier
> frequency that carries the baseband signal.
> 
> But I also observed that if I change the variable samp_rate, it
> changes throughout in all other blocks as well. So it must be a
> global parameter and not necessarily the Nyquist sample rate as I
> thought it to be! Request you to kindly explain.
> 
> When I multiply two signals and plot the FFT, the plot mentions dB
> for the frequency components. You already corrected me earlier that
> its not power. Then what does this dB mean?
> 
> regards
> 
> Nirmala
> 
> 
> 
> On Mon, Nov 6, 2017 at 3:37 AM, Marcus Müller <address@hidden>
> wrote:
> > Hi Nirmala,
> > could you try to use descriptive subject lines in the future? I
> > took the freedom of changing yours, so that people know what your
> > mail is about, in hopes you get better answers that way!
> > Anyway, Ben is right, you should probably start with the
> > established tutorials at http://tutorials.gnuradio.org.
> > Your task
> > > First I want to send a simple tone within 20 kHz, sample it and
> > > transmit with a high frequency carrier in the ISM band and
> > > receive it. 
> >  does sound very feasible for someone who's read the first few
> > chapters of that, although
> > > First I want to send a simple tone within 20 kHz, sample it
> >  is a bit misleading: your tone is a digital signal, as it gets
> > created within your PC; you can't sample it, it's already digital
> > :) But seriously, you can generate a 20kHz with a single block
> > (signal source), and actually send it by connecting that block to
> > e.g. a USRP Sink (if you have a USRP). But you said you wanted a
> > simulation, so I'm a bit left to wonder what to simulate – you see,
> > GNU Radio (and software defined radio) often, if not usually, deals
> > with equivalent baseband so that we don't actually care about the
> > carrier at all, but model all things that happen in the RF passband
> > as happening in baseband. So, your choice for a first simulation
> > seems a bit unusual, if not even slightly unlucky.
> > 
> > Best regards,
> > Marcus
> > 
> 
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