In previous post I told that I have few different RTL2832U based SDR dongles. I’m want to see what cheap Software Defined Radio technology can achieve. First one I tried was widely available version for 8-13 USD with RTL2832 + R820T chips. In this post I will look in to dongle with second generation tuner R820T2 which is available for 17-28 USD:
Inside it looks identical to first generation dongle except the different tuner chip. Most likely those chips are pin to pin compatible:
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:
It is also safe to drill some holes for mounting to the heat sink. With internal low noise amplifier (LNA) active RTL SDR dongle gets pretty hot. Even knowing that people are using dongle with no issue without any additional cooling I will feel better with dongle mounted to the heat sink:
I will add additional 100nF, 1nF and 100pF bypass capacitors in parallel to all existing power supply capacitors also will make external 3.3V and 1.2V power supplies because I read online that on-board 1.2V switch mode power supply adds some noise in HF band. Additionally filter raw 5V power supply going from USB connector to SDR dongle with common mode choke and MuRata NFM21… filtering solution. Last mod but certainly not least will be high precision TCXO 28.8MHz oscillator. eBay seller claims it’s +/- 0.3PPM acurate. That is aprox 8Hz error from 28.8MHz. I measured it with high accuracy system and found actual error fluctuating in the region of 1 to 3 Hz. Few shots from the process:
What insertion loss does antenna input have over full operational band? During first generation tuner measurements I found out that dongle antenna input does not have acceptable insertion loss (in other words SWR coefficient) over full declared operation band (25MHz to 1750MHz). I don’t expect it to be impressive as 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. But certainly not up to SWR coefficient of 8. 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 around 25MHz to 1076MHz. 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.
How accurate R820T2 dongle can be with high precision TCXO oscillator? In this measurement I fed 160.000.000 Hz carrier wave from very accurate RF generator which is clocked from rubidium frequency standard. With RTL SDR dongle + TCXO in the software I got the same 160.000.000 Hz. Well maybe we can argue that it is tiny bit off and I need to shift marker slightly to left by a few tens of a hertz but it is no way near to the error I got with original quartz resonator – 9905Hz, almost 10kHz:
The main selling point of second generation R820T2 tuners is better sensitivity. So it is even more interesting to see where is its the noise floor? And compare the result with first generation tuner. 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 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 -95.4 dBm. In comparison with first generation tuner I was able to go only down to -88.2dBm signal level. So second generation R820T2 tuner has about 7dB lower noise floor when LNA is disabled (0dBm gain):
As with the first generation tuner here I’m also interested is really amplitude 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 -85,2 dBm;
graticule -40 <> signal applied -74,8 dBm (applied signal step – 10,4 dBm);
graticule -30 <> signal applied -62,3 dBm (applied signal step – 12,5 dBm);
graticule -20 <> signal applied -53,5 dBm (applied signal step – 8,8 dBm);
graticule 0 <> signal applied -33,3 dBm (applied signal step – 20,2 dBm).
Since I only eyeballed the graticule with shown signal peak I call this close enough to real 10dB step though results are more scatered then with first generation dongle. This measurement was also made with LNA disabled (0dBm gain).
So now I have general idea about performance of first generation RTL SDR dongle with R820T tuner and second generation dongle with R820T2 tuner. In future I will do some measurements in other frequency bands.
Here are additional pictures, hope they will help to understand where you need to connect voltages and TXCO. It’s not necessary to make external voltage regulators and unsolder existing ones. I chose to use external 3,3V regulator to get better cooling by attaching voltage regulator directly to case, stock one on the PCB gets pretty hot since it is linear type. NOTE that external ones that I used are also liner to avoid RF noise that could be generated by switch mode type. It’s just that they are directly connected to the metal case. External 1.2V regulator was used because stock one was switch mode type and potentially can radiate RF noise which is noticeable in HF bands with up converter.
Current draw on 1.2V and 3.3V power rails with TXCO power from 3.3V rail:
with 0 dB LNA gain 1.2V rail draws 175mA and 3.3V 194mA;
with 50 dB LNA gain 1.2V rail draws 179mA and 3.3V 196mA.