Re: [Discuss-gnuradio] samp_rate in GNU Radio

Hello Priyanka,

As mentioned in the previous e-mails, you are not dealing with the IF stage at all. Yes, the tuners of the RTL-SDR dongles may have some IF stages, but that is an internal thing and the data you are getting is in baseband. That intermediate frequency means nothing to the operator (you) and should not be included in the calculation (except some very special cases).

Also, as mentioned by Carles Fernandez, you cannot achieve 38.192MS/s with an RTL-SDR dongle - or with any USB 2.0 based SDR receivers/transceivers. That is just too much data for USB 2.0. The minimum requirement for a such sampling rate is USB 3.0, PCI-e, or 10GBit/s Ethernet.

I do not deal with GPS signals, so I do not understand where you got that sampling rate, but if you need that sampling rate, you are simply out of luck. You need to get a better SDR (USRP B200 or LimeSDR alternatives, for minimum). Alternatively, you can look for things that you can do without using a such high sampling rate. (Maybe there are some GPS signals that you can cover with bandwidth smaller than 2.6MHz - which can be done with a RTL-SDR. Again, I do not deal with GPS signals so I do not know much about their signals.)

Finally, the sampling rate that you set on your software may not necessarily match the sampling rate of the ADCs - the sampling rate that you set is the effective sampling rate (=the incoming rate of the data from the SDR to the PC), and the ADC may sample at a faster rate. The sampled data will be then re-sampled to the sampling rate that you requested and then get passed to you.  This is a very common practice, and RTL-SDR dongles actually do this as well. You usually do not have to worry about this, but you may have to consider that if you are writing any reports about this.

Regards,

Kyeong Su Shin

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보낸 사람: PRIYANKA PRIYADARSHINI <priyankapriyadarshini081@gmail.com> 대신 Discuss-gnuradio <discuss-gnuradio-bounces+ksshin=postech.ac.kr@gnu.org>
보낸 날짜: 2018년 4월 9일 월요일 오후 10:14:31
받는 사람: Carles Fernandez
참조: Discuss-gnuradio@gnu.org
제목: Re: [Discuss-gnuradio] samp_rate in GNU Radio

 

Hi,

Thank you for your response, I got your point. But I need to know this:

I want to receive a GPS L1 signal (1575.42MHz) and I studied that the local oscillator of RTL-SDR has the intermediate frequency of 3.57MHz-4.57MHz. So when the signal with this IF goes to ADC of SDR, I want my ADC to sample this incoming signal at a sampling rate of 38.192MHz (as per the requirement). But when I try to put this sample rate in RTL-SDR, it starts dropping samples. I want to know that is there anything am I doing wrong OR it is the limitation of my RTL-SDR.  

Thank you.

On Mon, Apr 9, 2018 at 11:57 AM, Carles Fernandez <carles.fernandez@gmail.com> wrote:

Dear Priyanka,

According to https://www.rtl-sdr.com/about-rtl-sdr/ :

The maximum sample rate is 3.2 MS/s (mega samples per second). However, the RTL-SDR is unstable at this rate and may drop samples. The maximum sample rate that does not drop samples is 2.4 MS/s, however some people have had luck with 2.8MS/s and 3.2 MS/s working well on some USB 3.0 ports.

Thus, something between 2.0 and 2.4 MS/s would work for you.

Maybe this paper could be of your interest, although it is a little bit outdated: 

http://www.cttc.es/wp-content/uploads/2013/09/Turning_TV_into_GNSS_Rx1.pdf

Hope this helps,

Carles

On Mon, Apr 9, 2018 at 11:38 AM, PRIYANKA PRIYADARSHINI <priyankapriyadarshini081@gmail.com> wrote:

Hi,

I am trying to receive GPS signal using RTL-SDR with GNU Radio. I am interested in L1 band of GPS signal (1575.42MHz). But I am confused about my sampling frequency. What value should I choose as samp_rate in RTL-SDR block of gnuradio? 

Thank you

Priyanka

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