Over on YouTube the electronupdate channel has posted a video showing the decapping of the R820T and RTL2832U chips. Decapping is the process of removing the plastic packaging on integrated circuit chips, thus exposing the internal circuits printed on the silicon die for viewing. In the video he shows microscope images of each of the decapped chips and explains a bit about what each part of the chip does.
Recently Luigi Tarenga wanted to do some work on developing RTL-SDR drivers, so he emailed Rafael Micro requesting some additional documentation about the chip. Usually previous requests to Rafael Micro for such information seem to have gone unanswered, but this time it seems they have decided to publicly released the Register description document for the R820T2 chip.
Previously the R820T datasheet was leaked/released to the public, but the information in the datasheet did not help much with driver development. This register description document describes the function and configuration bits for the registers on the chip, and may be useful for people wanting to develop better drivers for the RTL-SDR.
Over on YouTube RTL-SDR experimenter Adam 9A4QV has uploaded a video showing how the R820T dongle can fail to receive properly at frequencies above about 1.4 GHz as the temperature in the dongle rises. This is a known problem that may cause issues when trying to receive satellite signals like Inmarsat at 1.541450 GHz. In our own tests, the R820T2 chip appears to be much less prone to this behaviour when compared with the R820T, but still fails if the ambient temperature gets too hot, for example if left in direct sunlight. We’ve had several R820T2 RTL-SDR’s running at 1.5 GHz+ for over 48 hours when left in the shade, but not one R820T ran for more than a few minutes at those frequencies. Of course the E4000 tuner is the best RTL-SDR tuner for these GHz level frequencies, but that tuner is now rare and expensive.
Over on Reddit, some people have been discussing this issue, and have proposed that the likely cause is related to the PLL failing to lock properly at higher temperatures. A fix may be to apply a blob of solder to the vias underneath the R820T chip, and then attach a heatsink. The problem also does not occur on the Airspy, a higher performance SDR that also uses the R820T2 chip in its design. This may be due to better drivers for the Airspy, or better heat dissipation in the Airspy’s hardware design.
Peters first results show that the R820T2 has better reception and less spurious features at frequencies above about 1.45 GHz and improved frequency stability (with the newer R820T2 dongles that use the SMD oscillator). His second set of results explore issues that are more closely relevant to radio astronomy including observed spectra, Allan variance (frequency stability) tests and determining the shape of the R820T/2 internal bandpass filter.
In the conclusion of the paper Peter writes:
Two Newsky RTL2838U dongles were tested, the R820T2 device against the R820T. The evaluation results in a clear preference for the new RTL2838U/R820T2 dongle. In the L-band the new dongle is at least 2.7 dB more sensitive. According to the radiometer equation the effective system temperature is reduced by almost 50%. Most important for reliable radio astronomical observations are stability issues. Allan variance tests have shown that the R820T2 dongle is far better then the older version. The stability is comparable to that of professional radio astronomical devices. The tests have shown that using the full bandwidth of the RTL-SDR devices results in spurious baseline ripples. For a good performance it is recommended to use the dongles at reduced bandwidth. rtl power with the crop option -c 0.5 appears to be a good choice.
As seen in this previous post, the R820T tuner chip used on most RTL-SDR’s has a built in hardware tunable IF filter. Leif, the programmer of the Linrad SDR software has been experimenting with this filter and has uploaded a video of his experiments to YouTube.
In the video he shows how the R820T IF filter can be set to be as narrow as 300 kHz. Using a narrow IF filter can help to reduce the interference from strong nearby stations by up to 30 dB. Leif uses a modified RTL-SDR driver that comes with Linrad which allows the IF filter to be manually modified.
Addition of 100nF, 1nF and 100pF bypass capacitors on the power supply rail.
Added a common mode choke to the 5V line.
Added a MuRata NFM21 EMI suppression filter to the 5V line.
Replaced the oscillator with a 0.3 ppm temperature controlled oscillator (TCXO).
Disabled the internal RTL2832U 1.2V switching supply and provided external 3.3V and 1.2V supplies.
Replaced the MCX connector with an SMA female connector.
Enclosed circuit in a metal box.
In addition to the mods, Laidukas also made some measurements on the performance of the R820T2 on some metrics. In the first test he measured the input insertion loss or SWR. He found that the SWR was below 2 between frequencies of 25 MHz to 1076 MHz. At higher frequencies the SWR reached levels up to about 8.
Another test showed that with the LNA disabled the R820T2 had a lower noise floor by about 7dB, when compared to the R820T.
The programmer of Linrad (aka Leif sm5bsz) has uploaded a video to YouTube that compares several software defined radios on dynamic range and compression performance in the presence of strong nearby signals. In the video Leif tests the Airspy, BladeRF with B200, FDM-S1, Funcube Pro+, rtlsdr/E4000, rtlsdr/FC0013, rtlsdr/R820T, SDR-14 and SDRplay.
The main test works by tuning to a broadcast band FM frequency and then injecting a strong carrier signal at distances of 500 kHz, 1 MHz, 2 MHz and 5 MHz from the center frequency. The carrier signal strength is slowly increased until the SDR shows signs of complete degradation of reception of the FM signal. Better SDRs will tolerate stronger nearby signals without degradation.
The results are summarized at 34:20, 1:21:38 and 1:48:30. We have also taken screencaps of the results at 1:21:38 and 1:48:30 and they are shown below. The first column is when a higher gain is used, and the second column is when a lower but still barely copyable gain level is used. In the Levels for loss of performance columns smaller numbers are better and in the Dynamic range columns larger numbers are better. Finally, at the end of the video starting at 1:45:55 Leif also tests the spur performance of the SDRs.
The popular trunking decoding software Unitrunker now supports the RTL2832U R820T RTL-SDR directly in its new version. This means that extra SDR receiver software like SDR# is no longer required to use Unitrunker.
In a normal radio system, one company (or talkgroup) might use a single frequency for radio communications. However, this is very inefficient as the frequency may not be in use for the majority of the time. In a trunked radio system, a small set number of frequencies are shared between a large number of talkgroups. Each radio receives a special computer controlled control channel. The control channel determines a vacant frequency that a particular talkgroup should use. This helps to make radio frequency allocations more efficient.
Because a talkgroup might switch between various frequencies often, it can make listening to a conversation difficult for radio scanners. Unitrunker can be used to decode the control channel and follow a voice conversation as it hops across various frequencies. With two RTL-SDR dongles you can set up a trunking receiver station with just Unitrunker. What follows below is a tutorial on how to set this up.