Re: [Discuss-gnuradio] USRP sampling rate (was: (no subject))

[Marcus Müller]

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

Email: address@hidden

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