Thursday, November 7, 2013

Yet Another Post About SDR Using A $20 "RTL" Dongle

There's been a lot of chatter on various blogs and forums about using the $20 "SDR-RTL" dongles for wide-band receivers. For $20, and FREE software, you can hardly go wrong, as long as you keep in mind some of the limitations on these little guys, such as dynamic range, about 50 dB compared to 80~100dB for a "real" receiver, and selectivity. Their sensitivity is more than enough for casual use, and they're really amazing for $20.

These little guys plug in to, and are powered by your PC's USB port, have an antenna connector on the other end, and were originally designed to receive DVB-T (Digital Video Broadcasting-Terrestrial) television signals in countries outside the US. See the article "What Is DVB-T" for more info.

The basic structure of the dongle is a tuner, followed by a detector, usually a Quadrature Sampling Detector, which acts as an Analog-to-Digital converter. The most popular ones use an Elonics E400 tuner (favored because it has the widest tuning range, 52~2200MHz, with a gap between 1100~1250MHz), followed by a Realtek RTL2382U QSD/ADC/USB "Data Pump", which outputs 8-bit I and Q samples to your PC, where the software does it's magic, and out comes audio.

If you live in an area with LOTS of strong AM, FM, and Paging Systems, you might want to look into some filters (more correctly called "Preselectors") to put between the antenna and dongle to prevent the "front end" of the little radio from being overloaded with strong signals.

The dongle I bought from "nooelec" on eBay, and the two software packages I've been experimenting with are Gqrx for Linux, and SDR# for Windows.

From the little bit of experimenting I've been doing, the "best" settings for running these with SDR# seems to be with the RF Gain control set to minimum (-1dB), and the  "RTL AGC" box checked. These settings are reached by using the "Configure" button just to the left od the frequency display.

Here's a screenshot using one of the $20 USB dongles you can buy on ebay, connected to the Discone antenna I use for my scanner, and running under Gqrx on my Linux PC.


The little "radio" is tuned to a local FM radio station, KLOS, on 95.5 MHz.


I've run this same dongle with both packages, and here's a screenshot of the dongle running under SDR# on my Windows 7 machine.



Both programs are displaying the same 2MHz swath of spectrum. The Windows program has more adjustments in terms of what types of filters, sampling rates, and noise reduction is used.

I've also used it with HDSDR, but that program seems to be more suited to "Communications Quality" audio, and although it has a TON of adjustable "stuff" to play with, I couldn't figure out how to get the bandwidth high enough for Broadcast FM.



For the more "hardcore" Linux users there's Linrad, and if you're really a masochist, you can get GNURadio, which I've never been able to figure out, as it's more configurable than EMACS!

I also have a FUNCube Pro dongle, which has since been replaced by the FUNCube Pro+ version. These are much higher quality, with a much wider tuning range, and at a much higher price, currently about $201, depending on the exchange rate with Great Britain.

SO, if you're interested in using one of these for a receiver, you can't hardly go wrong for $20, and some FREE software. They won't replace a good communications receiver, or a scanner, but you can have a lot of fun experimenting with them.

5 comments:

  1. So I'm having fun playing with the little $20 SDR, and appreciate your input on settings.

    How much better is the FUNCube? Can you get noise floor plots of it?

    Are there ways to look at broader swaths of spectrum to help find transmitters? I've played with the SDR-14 from RF Space, and while it would only demod in a narrow BW, it would display a lot more spectrum.

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  2. Aye, there's the rub!

    Either my FUNCube dongle is defective, or the design is flawed. One of the extra things the FUNCube has is an LNA ahead of the tuner chip, but I've read it's not a particularly good LNA. The NF is around 4dB, which is better than just the tuner, but unless you use some front-end filtering, it tends to drive the tuner too hard. I experimented last night with adding some attenuation to the antenna input, and it seemed to help, along with cranking down the gain adjustment in the software, but FM stereo sounded really bad.

    The $20 DVB-T dongle appears to be far superior to the FUNCube, at least on broadcast FM.

    No, you can't get them to display any more spectrum than they do, as the sample rate used limits it to about 2 MHz.

    The RF Space boxes are really neat, but YOW! You sure pay for it!

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  3. Just because I can't think of another place to post this, I fired up the ham station yesterday with the RTLSDR running at this end of the house, to see if the USB demod was any good. I put my home station in a place on 6m SSB where nobody tends to hang out and ran tests.

    Bottom line is the demod worked fine, but the RTL was severely overloaded by the nearby 100W transmitter. Had to turn the gain and AGC off, and then run the rig power down to QRP levels (I think it does 5W PEP with the "RF Gain" all the way off.

    So in case someone was wondering if the USB Demod works, there you go!

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  4. Yep, they're extremely easy to overload.

    Have you ever connected yours to a large external antenna?

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  5. I figured out why the FUNCube Dongle doesn't receive broadcast FM!

    It only grabs 92kHz of spectrum, of which about 80kHz is "usable".

    Commercial FM is much wider than that, which explains why it sounds so sucky......

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Keep it civil, please....