EN: I bought few different RTL2832U based SDR dongles. I want to see what cheap Software Defined Radio technology can achieve. There are huge amount of information online but it’s more fun to do some measurements on the bench and see bay my self. I plan to add some filtering to power supply and add shielding. Then measure noise floor, frequency accuracy and maybe some other parameters.
More about SDR can be found on rtl-sdr.com. This is very informative site with tons of information.
LT: Programinio radijo (SDR) tema rašysiu anglų kalba, nes noriu, kad informacija pasiektų platesnę auditoriją. Kaip bebūtų iliustracijos gana aiškios ir manau nesudarys problemų net nesuprantantiems angliškai. Visada galite klausti komentaruose.
Plačiau apie SDR radiją grystą RTL chipu galima paskaityti rtl-sdr.com. Labai gera svetainė su begalo daug informacijos.
First one I tried was widely available version for 8-13 USD with RTL2832 + R820T chips.
Since I will not use dongle in original form factor I minimized it’s size. It is safe to unsolder IR remote control receiver and to cut off PCB as show in picture below:
Then added additional 100nF, 1nF and 100pF bypass capacitors in parallel to all existing power supply capacitors. On raw 5V power supply going from USB connector to SDR dongle I added NFM18… in line filtering solution from MuRata company. Later onI soldered more of those in other power supply nodes as an extra precaution against noise.
Scraped off some protective yellow PCB coating and soldered copper foil shield to the exposed ground:
I don’t think that these modifications will add much to dongle performance and make it entirely different beast but I will definitely sleep better by knowing that I did these mods. Sooner or later I will get another dongle without modifications and will try to compare.
First I am interested in antenna input return loss, how much signal is reflected back to the antenna. These dongles were meant to be used with 75 ohm impedance antennas for digital TV broadcasts so it will not perform as good on 50 ohm antennas, common in radio amateur world. Here is SWR plot of antenna input when LNA is disabled (0dBm gain):
Markers are set to SWR coefficient 2 meaning that inside these boundaries more then 89% of signal power will reach receiver and will not be reflected back to antenna. From this I see that SDR dongle best performs in the region from 44MHz to 1138MHz. To make things more complicated I can add that reflection coefficient changes with different LNA gain, here is archive with more measurements – VNA SWR archive. Lowest source signal I can configure in my VNA analyzer is -30dBm so maybe with higher LNA gain insertion loss is changing due to RF front end overload? In future I will do noise floor measurement over full band and note minimal signal I can see in spectrum window. We will see how it will compare to insertion loss.
With some LNA gain settings reflection coefficient slightly changes when frequency of operation is changed to 310MHz and any above that. It’s like something is changed inside the R820T tuner when working above that point. See archive above with more measurements for an example.
How bad is standard 28.8Mhz quartz resonator? In following measurement I fed 160.000.000 Hz carrier wave from very accurate RF generator which is clocked from rubidium frequency standard. And in the software I got 159.990.095 so error is 9905Hz, almost 10kHz:
Where is the noise floor? Since amplitude scale in RTL-SDR software packages is relative it is not straight forward to determine signal strength or noise floor. One of the way that came to my head first was to apply barely visible carrier wave from signal generator and call that close to noise floor. And that is what I did. In the picture below you see that in real time spectrum applied signal is not visible at all, only in waterfall spectrum it is possible to see traces of signal (click on picture to zoom in). This was signal strength of -88.2 dBm when LNA is disabled (0dBm gain):
The next interesting question is really graticule step 10dB? From the noise floor I started to increase RF signal generator output power until I reached graticule levels and noted the difference. This is what I came up with:
graticule -50 <> signal applied -73,9 dBm;
graticule -40 <> signal applied -64,1 dBm (applied signal step – 9,8 dBm);
graticule -30 <> signal applied -53,0 dBm (applied signal step – 11,1 dBm);
graticule 0 <> signal applied -23,6 dBm (applied signal step – 29,4 dBm).
Since I only eyeballed the graticule with shown signal peak I call this close enough to real 10dB step. This measurement was also made with LNA disabled (0dBm gain).
That’s it, next on the bench will be RTL SDR receiver from NooElec with R820T2 tuner.