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Friday, 20 October 2017

Why not use a RTL2832U dongle for a faster spectrum refresh rate?

Hi,

The RTL2832U dongles are very popular for a cheap entry to SDR (Software Defined Radio).

People who used SDR# (sdrsharp), are amazed by the super fast beyond real time refresh rate of the spectrum - not to mention the high resolution!

Why not use such an SDR dongle as a spectrum analyser? What is the advantage of the SMA/NWT devices?

Well, first off, some quick explanation: the cheap RTL2832U SDR dongles will digitize 2.4MSPS (Mega Samples per Second) and then perform a Fast Fourrier Transformation (FFT) to convert the discrete samples obtained in TIME DOMAIN (signal level in time) into FREQUENCY DOMAIN (signal level in frequency). Because 2.4MSPS are used, this will allow for a span of aproximately 2MHz.

The SMA/NWT devices, however, will capture samples across the frequency range specified, efectively sweeping the band, which is why they are SSA (Sweep Spectrum Analysers) - well sort of, as they are much simpler than "real" spectrum analyser.

Anyway, they manage to get 500 samples in 1-2 seconds. Considering that you can well display a span of 1GHz in 500 pixels, this gives a refresh rate of 1-2 frames per second.

Now, I am surely not the first who thought about using the FFT in 2MHz steps to eventually conver a broader span.

This is what you can do with QSPectrumAnalyzer, a Linux implementation for SDR devices.

Here is the result:


Note that each screen refresh takes about 70 seconds! The reason is fairly simple: a 1GHz range requires 500 2MHz steps for the FFT operation - there goes the fantastic speed...

Here the same signal rendered with the SMA/NWT device using my software:


In this case, the screen refresh takes around 1 second!

My intention is by no means to bash the RTL2832U devices - I love them! I just wanted to explain the difference.

Some notes of interest:

  1. If you look closely at the first image, you will notice that I use Ubuntu with a VirtualBox virtual machine under Windows 10 and it sees the RTL2832U dongle just fine!
  2. Don't buy the cheapest RTL2832U dongles you can find on eBay. Some will suffer from frequent crashes, which will really take any fun of using them.
  3. I just bought the NooElec Smart Premium.It is reasonably priced (around 25 $/£/€) and features a propper design.
  4. The first stop for any RTL-SDR related information is of course: https://www.rtl-sdr.com/
  5. Need some guidance for RTL-SDR under Linux? Look here: https://ranous.wordpress.com/rtl-sdr4linux/
Cheers,
Vitor

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