What is simplest software that will direct sample with an appropriate sample frequency?

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ajforrest
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Joined: Fri Nov 13, 2020 3:38 am

What is simplest software that will direct sample with an appropriate sample frequency?

Post by ajforrest » Fri Nov 13, 2020 3:47 am

I have the RTL-SDR BLOG V.3 dongle and I wish to capture some raw data from somewhere in the single-digit MHz range (e.g. 4 MHz).

Various GUI apps seem to be "too helpful" and I don't end up with the number of samples I expect.

I switched to rtl_tcp with the direct sampling option from the blog's github account but it rejects sample frequencies of 8 MHz or more (up to 28.8).

This would appear to severely limit the utility of the direct sampling mode if the sample frequency is so low. How can I get 8 MS/s from this device?

[edited to correct desired sample rate]
Last edited by ajforrest on Fri Nov 13, 2020 2:03 pm, edited 1 time in total.

alanzfq
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Re: What is simplest software that will direct sample with an appropriate sample frequency?

Post by alanzfq » Fri Nov 13, 2020 8:25 am

I have not got any idea of what you are trying to do but the fundamental fact is that you will not sample at much more than 2MHz from a RTL.
More than that requires a different SDR. Up to 30MHz and beyond is available but not quite so cheaply.
Alan

ajforrest
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Joined: Fri Nov 13, 2020 3:38 am

Re: What is simplest software that will direct sample with an appropriate sample frequency?

Post by ajforrest » Fri Nov 13, 2020 1:50 pm

Thanks for the reply Alan.

I was basing this understanding on this text from the v3 user guide:
Note that this feature makes use of direct sampling and so aliasing will occur. The RTL-SDR samples at 28.8 MHz, thus you may see mirrors of strong signals from 0 - 14.4 MHz while tuning to 14.4 - 28.8 MHz and the other way around as well. To remove these images you need to use a low pass filter for 0 - 14.4 MHz, and a high pass filter for 14.4 - 28.8 MHz, or simply filter your band of interest.
While I have seen documentation that aligns with what you've said, I had assumed that applied either to models other than the v3 or modes other than direct sampling. Have I misunderstood?

rtlsdrblog
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Re: What is simplest software that will direct sample with an appropriate sample frequency?

Post by rtlsdrblog » Fri Nov 13, 2020 9:24 pm

I should clarify that to note that it's the ADC sampling at 28.8 MHz. However after the ADC the RTL2832U down samples that to a 3.2 MHz MAX BW.

hotpaw2
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Re: What is simplest software that will direct sample with an appropriate sample frequency?

Post by hotpaw2 » Sat Nov 14, 2020 2:46 am

It's not a software issue. It's a hardware interface issue. The RTL2832 chip in the RTL-SDR directly samples at 28.8 MHz, but it can't send that much data across its USB 2.0 port. So the chip mixes down to baseband and filters a slice of the full directly sampled spectrum, to just 240k to 3 MHz IQ samples per second.

For much higher bandwidths, you need a more expensive SDR with a higher speed interface, for instance the LimeSDR PCIe, or USB 3.0 devices.

ajforrest
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Joined: Fri Nov 13, 2020 3:38 am

Re: What is simplest software that will direct sample with an appropriate sample frequency?

Post by ajforrest » Sun Nov 15, 2020 1:46 am

Thanks to all who have responded.

I've since learned that apparently I can get a bandwidth B centered at frequency F sampled, presumably by appropriate filtering and aliasing. Off to figure that out now.

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