Tech Topics: Review Of An SDR Dongle

The SDR Dongle 

SDR = software defined radio

Having already a conversation with VE7TI (John) about an older generation of SDR dongles I felt compelled to buy a new one, in 2018, a much smaller one, also from China. Most probably what I bought is a knockoff of a NooElec micro dongle. It was in sale at the time, for $7.87 CAD, shipping and taxes included, from aliexpress.com. It came with a remote control, an antenna and a CD with drivers. I discarded all those accessories, which are totally unusable if somebody wants to use the SDR dongle as a general receiver, and not as a DVB-T PC adapter, as intended.

I would like to start my review by underlining exactly that, the SDR dongle I am reviewing was not designed to be a general receiver, as I use it.

My first action was to install it on the computer, on a USB port, and to install drivers and software for it. I followed the instructions from www.rtl-sdr.com. It is tricky to have the drivers work in Windows 10, but if the instructions are followed exactly as in the given website, it works.

Some conclusions

• 
  The only software that completely works in Windows 10 is SDR sharp. It has various useful plugins, like a plugin for detecting the CTSS tones. Many plugins do not work with the last version of SDR sharp. It is free. A close competitor is HDSDR, which does not know how to decode stereo FM. All other programs I tried partially worked (they do not know all modulations types, have unclear settings, and so on).
  
  
• It has to be connected on the USB computer port with an extender, otherwise the electric noise generated by the computer makes it unusable, completely deaf for useful radio signals. I used my own accessories, in order to adapt the MCX antenna connector from the dongle to my antennas: 
  
  
• Caging the SDR dongle does not help much; if it is not case to case to the electric noise generator, but several centimeters apart, it is fine. I tried to cage it in metal and it did not make any difference in various test situations. I suspect it is already somehow shielded or partly shielded inside.
  
  
• In the commercial FM band it is a cheap stereo and more important, a RDS (radio data system) receiver. It knows how to display the name of the station, the songs that are played at that moment and whatever digital info the station sends in addition to the analog signal. The sensitivity in FM is way worse than 2 microvolts. Any dedicated commercial receiver-amplifier, including my roommate’s Yamaha 2 microV, every single FM radio in the apartment we have, including clock radios, and MP3 portables (the radio part) are more sensitive than the SDR dongle. I am using a proper horizontal dipole antenna on the balcony measuring 71 cm each leg, connected with coax cable to the SDR dongle, while all other 7 receivers have just a small piece of wire as antenna. I estimate the sensitivity in the 88 – 108 MHz band somewhere at 30 microvolts . It is expected the SDR dongle would be less sensitive in the FM band, due to the wide frequency bandwidth. I limited the bandwidth from 250 KHz to 180 KHz and there was a slight improvement.
  
  
  
• The sound in the FM band is not great. Even at 250 KHz, wide band FM (maximum in SDR sharp program), has audio quality that is just bearable. This is not exactly acceptable. I will not replace any of the radios with this SDR dongle, even though it displays data.
  
  
• The characteristics differ very much on the Rx bands and require adjustment at the RTL dongle settings. That means RF Gain; RTL AGC; Tuner AGC. 
  
  
• It is stable. I did not feel the need for a more stable oscillator. It did require adjustment in the software, 218 ppm as in the above picture for my dongle. This is considered a huge adjustment. I verified this with encapsulated quartz oscillators (32 MHz, 125 MHz, 150 MHz, the 28.197 CW beacon), and indeed it needs that huge adjustment.
  
  
• It seems it does not like the 50 MHz band, and the sensitivity is not great in this band. I confirmed the poor reports as everybody writing about this issue on the Internet experienced the same result, although I hear some local ham radios almost every evening. They never say their callsigns, so I just presume they are ham radios since they are in a ham band.
  
  
• On the 144 MHz band, with a good dipole, it receives everything the Kenwood 7950 and the Chinese walkie-talkie receives. It likes this band and it has a good sensitivity. All repeaters from Victoria, Port Angeles, Nanaimo, and Cowichan are 59. Probably the path is more important than the sensitivity in this case, too. I am at 103 meters above sea level. There are some images for strong local repeaters.
• It also likes the marine band, air traffic band and the weather band. They are all around 150 MHz and once the settings are done for one station, they can be kept for the weather, marine, 2 meter bands.
  
  
• The CB band and the beacon on 28.197 MHz (VE7MTY, Pitt Meadows, continuous, CW) are in a band where the RTL dongle is not so sensitive. The beacon (nearby me) booms in on my SONY ICF7600G portable radio, with its telescopic antenna. The SDR dongle with a CB whip on the balcony receives it almost OK, but only because I was hunting for the beacon and I knew where it was. The beacon’s signal barely produces a trace in the display spectrum, and I am nearby it (exactly 13.89 km).
  
  
• There are images everywhere. The FM band (88 – 108 MHz) can also be received on 30-50 MHz. The worse thing to do is to use an upconverter, as I saw so many do on the Internet, with a NE612 integrated circuit, and wide non-tuned input. I tried, and the images kill any useful signal. In the end I did 2 converters, in order to cover 3.5 MHz to 30 MHz, one for the lower part and one for the upper part. I used an NE612, attached to an amplifier with a BF998 in front. I have a tuning circuit just at the antenna, and a 2.2K resistor + coil in the output of the BF998’s drain. The source terminal is connected directly at the ground and the BF998 is supplied with 9 Volts (12 V is max in the datasheet, and it does burn after 12 V). The oscillator is an encapsulated 3.3 V powered oscillator, in a socket, to easily change it. The best the dongle worked for me is in the 150 MHz band, to stay away from the FM commercial band and to upconvert the shortwave into a sensitive band that the SDR dongle likes. I can adjust the signal from the oscillator to the value from the NE612 datasheet, but it actually does not make any difference, even if it is attached with 2 Volts (NE612 has a buffer in it before the mixer).

• The noise of the first element in the SDR dongle must be better than that in the  BFR91A. I tried a wide range untuned amplifier with one BFR91A, and it did not bring anything new, just noise. 
  
  
• The situation changed when I put a SAW 88 – 108 MHz (3 pin filter) in front of the BFR91A, and it helped.

• It does not run hot. Whatever other users noticed with old SDR dongles is no longer an issue with my 2018 SDR small dongle.

Final conclusions

• The SDR dongle is the cheapest 2 meter receiver a Ham can buy, and works as receiver on par with dedicated equipment, which is generally limited by line of sight, not by sensitivity. A beginner can listen to the weekly nets for some $8–11 CAD, shipping and taxes included.
  
  
• The SDR dongle is the cheapest FM commercial RDS receiver one can have, capable of displaying the digital data continuously and transmitted by almost all stations in Vancouver. 
  
  
• The SDR dongle was not meant as a general coverage receiver. It was designed as a DVB-T television European standard receiver, and it is probably better for that purpose.

A Postscript…

In the NI Multisim schematic you see a LED with a big resistor in series. It is not a mistake.

All LEDs I use are from China. The 3 mm ones I bought extremely cheap (I think they were 200 or 500 in the bag). The white ones are the most sensitive, and light at several microamps. I need to use resistors between 150 K (for BLUE) and 300 K (for WHITE) in series with the LEDs for 12 Volts power supply.

Now I understand what kind of LEDs they use in the portable lit antennas for walkie-talkies they sell for Baofeng. They light OK. Smaller resistances means burned LEDs. I tried the old values from various published schematics, and NO, they are not OK for the bags of LEDs I have.

~ Daniel VE7LCG

18/12