Re: [Discuss-gnuradio] USRP sampling rate

[Jeff Long]

Note that I kept everything in terms of "real" samples when talking 
about sampling rates. Also, I should have made clearer the difference 
between rates on receive and transmit. Transmit was not addressed.

For transmit, you know the clock, so you can use a sample rate of >2B 
for real or >1B for IQ.

For receive, you don't know the clock, so you need some some factor 
higher (oversampling). I showed >4B for real or >2B for IQ, which worked 
with the simple example. The actual oversampling required depends on 
lots of things. For an analog signal, sampling at just over the Nyquist 
rate is good enough. But, there was a recent thread showing that 16x was 
the ideal rate for a certain CPFSK modulation.

I'd also welcome feedback on how to explain this all better, in 
non-mathematical terms.

- Jeff

On 09/16/2015 09:45 AM, Tom Rondeau wrote:
> You might also want to check out Jeff Long's presentation on sampling
> rate at the GRCon15 Intro Day:
>
> http://www.trondeau.com/grcon15-presentations#monday_Long_Sampling
>
> Video: https://www.youtube.com/watch?v=AmY7fCGTKs4
>
> Tom
>
>
> On Wed, Sep 16, 2015 at 5:57 AM, Marcus Müller <address@hidden
> <mailto:address@hidden>> wrote:
>
>     Hi Gerome,
>
>     I hope the subject I've added suits the topic of your mail well; if
>     not, feel free to change it when replying)! It's kind of important
>     to have a proper subject line, as it'll allow people to directly
>     address your mails.
>
> >     What does the sampling rate in this usrp block does?
>     It sets the sampling rate of the stream of samples coming from the
>     USRP.
>
> >      Is this the one that samples the IF from my TVRX2 daughterboard?
>     It's the sampling rate that the 100MS/s from the ADC get decimated
>     to. For example, you can set a 10MS/s sampling rate, and the USRP
>     will internally take the 100MS/s from the ADC, and filter them down
>     to 10MHz and decimate to 1/10th of the original rate.
>     The TVRX2 is a bit special compared to all other daughterboards
>     because it actually uses a real IF, all other daughterboards are
>     direct converters that give complex baseband. The samples that the
>     USRP sends to the host are all "pseudo-complex", ie. (sample_value +
>     j*0).
>
> >     Or is this the bandwidth that is displayed on the QT Sink?
>     No, not inherently: You can configure any "sampling rate" parameter
>     in the QT Sinks, and that will do nothing but change the numbers on
>     the frequency axis. It doesn't have an effect on the signal, just on
>     the labeling.
> >     I'm asking this because whenver I set it to say 400 kHz in
> >     sampling rate, it give me a bandwidth of 400 kHz.
>     Not quite sure about that: As the TVRX2 is a 2x real IF device, the
>     USRP's FPGA is configured to give you a real sample stream --
>     meaning that the 400kS/s are real samples, and that the bandwidth
>     would be 200kHz.
>     For all IQ-demodulating direct conversion daughterboards (ie.
>     everything but TVRX, TVRX2, BasicRX/TX and LFRX/TX) you get complex
>     sampling, where f_sample == b_nyquist, but for real signals, Nyquist
>     dictates that the representable bandwidth is half the sampling rate,
>     remember?
>
> >     If this sets the sampling rate in the usrp, it should only make
> >     the plot nicier, not increase its bandwidth. If I am not mistaken.
> >     Thanks in advance.
>     I don't understand -- obviously, increasing the sampling rate will
>     increase the bandwidth of a digital signal. That's the mathematical
>     effect of sampling.
>     However, the TVRX2 is among the very few daughterboards with
>     adjustable analog bandwidth -- you can select between 1.7 MHz, 6
>     MHz, 7 MHz, 8 MHz and 10 MHz of analog filter bandwidth.
>
>     Best regards,
>     Marcus
>
>     On 15.09.2015 18:11, Gerome Jan L wrote:
> >     Hi guys,
> >
> >     What does the sampling rate in this usrp block does?i'm talking
> >     about the uhd:usrp source block. Is this the one that samples the
> >     IF from my TVRX2 daughterboard? Or is this the bandwidth that is
> >     displayed on the QT Sink? Or is this the bandwidth that needs to
> >     be sample. As far as I know, sampling rate is fixed at 100 MSps by
> >     the ADC.
> >     I'm asking this because whenver I set it to say 400 kHz in
> >     sampling rate, it give me a bandwidth of 400 kHz. If I increase
> >     does, it increases the bandwidth too. If this sets the sampling
> >     rate in the usrp, it should only make the plot nicier, not
> >     increase its bandwidth. If I am not mistaken.
> >     Thanks in advance.
> >
> >
> >     --
> >
> >     Best,
> >
> >     *Gerome Jan M. Llames *
> >     Aspiring Technopreneur | Tech Savvy | Curious
> >     Mobile: +639271525124 <tel:%2B639271525124>
> >     Email: <mailto:address@hidden>address@hidden
> >     <mailto:address@hidden>
> >
> >     "/Because the people who are crazy enough to think they can change
> >     the world, are the ones who do./"  - Steve Jobs
> >
> >
> >
> >
> >
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