Thumbnet N3 First Impressions

Guess which part of waterfall images correspond to Thumbnet's N3:





Thumbnet is manufacturing RTL-SDR dongles for its own purposes, and sells surplus to the general public (official info here). Their latest receiver is called N3, shipping around mid-October.
I had an engineering/prototyping model for testing.
This is not a review or evaluation. Customers will get a different product, which, I fervently hope, will be probably very similar - I'll write a full review of a production N3 after it starts shipping.

Also, the N3 is not a commercially oriented RTL-SDR receiver, it is designed for the Thumbnet network. Just like the FlightAware ProStick and HackRF, be happy that it works like an RTL-SDR, and be merry that profit from sales go to good causes.
RTL-SDR architecture and components: engineers took a standard RTL2832U with R820T2 tuner and TCXO, then placed components on a 6x4 cm printed circuit board with alternative power supply options - N3 is twice as large as a full-size RTL-SDR dongle for better heat dissipation.
External DC power can be supplied via two pins (officially called a Phoenix connector, I used RasPi female jumper pins), takes 4.5V-5.5V, mobile phone or RasPi chargers (which are 5V), and power banks can be used.



Only data to the host computer with external power (= less noise from PC).
Important note: All testing has been conducted with external power, either a 4.75V 700mA charger or 20,000 mAh battery bank. Performance will probably be different if USB power from host computer is used, but my prototyping sample did not have that option.
F-connector: the only standard easily and cheaply available worldwide.
Just like external hard drives and higher-class SDRs, it uses a separate USB cable for data and power connection to the host computer. I still cannot fathom why every manufacturer feels compelled to reinvent the wheel every single time by not using MicroUSB: seriously, it's like the 90's for mobile phone chargers, move on fellas, every major consumer device uses MicroUSB (sans iStuff). Anyway, Thumbnet at least sells a suitable cable for a few dollars in the online store.
There's a metal dome (RFI shield/screen) above electronic components, which further increases heat dissipation area and acts as a Faraday cage / protects components from prying fingers.
Needs a metal case (official one is 4 dollars extra), electronic components on bottom of PCB are in a vulnerable position. Large corner holes take multiple screw options, and F connector means mounting into a makeshift box is easy with a nut and washer.



I had the distinct impression it's built to last, there's enough solder on the board to assemble 3 generic dongles.
Performance is similar to any dongle with a R820T2, but with much less noise on screen than any other RTL-SDR. Worked on shortwave with Ham-It-Up v1.3 upconverter, and on every frequency other dongles could tune. Only tested up to 1090 MHz, no issues, PiAware recognized the N3 as a regular RTL-SDR dongle and started sending data straight away.



Audio quality was superb on commercial FM and for SWLing. Can't comment on reliability, obviously, was powered on for a week whilst connected to outdoor antennas and had no problems.
Compared to: can't really compare it to anything, as it was not a final product, but from price alone it's clear the premium and satellite-oriented RTL dongle club has a new applicant. Can't wait to put it up against a v.3, SMArt and XTR+.

Pricing



Preorder only at the moment, (link to store) bare board costs $25.75, flat rate shipping for 4.5 dollars worldwide.
Realistically, you'll add an RFI screen (+$2) and a metal case (+6), ending up with enough change from $40 to buy an F to SMA female adaptor.
Receiver only. No antennas, power/data cable, adapter, bias-T, LNA, direct sampling hole etc, but it's unlikely the target audience will care.


Who is it for?



I think the N3 will find favor at the advanced / enthusiast end of the market. $35 for a metal cased, RFI screened R820T2 TCXO dongle with external DC power is not too bad.


Don't draw conclusions



I had an engineering model, so production units will be different; I doubt major changes will happen in the next few weeks until it ships.
The N3 is based on the RTL-SDR platform, so compatible with existing software, and because of its architecture, offers several benefits and ...  and I'm waiting for a production sample in a metal case as well, because as I really don't know how final units will perform. If close to this, I'm sold.

7 comments:

  1. Great review.

    My comment: RasPi chargers are switching power supplies. The concept of the N3 separate power input is to prevent switching power supplies from feeding the SDR. So you should avoid such chargers. Build a linear power supply with a LM317T or 7805 or use batteries to take benefit of the concept.

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    1. I agree, after such big work to have a clean power supply using AC power adapters is a criminal offence. Even common power banks use cheap switching circuits to supply 5V output. The best solution is to make your power bank using 2x18650 battery series and a 7805 linear regulator.

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    2. Totally agree, but a linear PSU costs ~200 dollars where I live, and four rechargeable batteries die in 2-3 hours with the N3. Tried, didn't include in the post, it was a prototyping model.
      I could use regular batteries in parallel, but I will not, because I will not throw away batteries (=waste landfill) after a few hours. Criminal offense or not, mobile chargers work, visible noise is better, but as emphasized above many times, mine was a prototyping model.

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  2. Akos, i was not thinking to your test when i wrote but to the regular use; i wouldn't have written that way if i was. A 12V 2A linear PSU is not that expensive; they were widely used for CB radios but they are getting hard to find. I am thinking to put a used car battery under my desk with a circuit for trickle charge activated when not in use. However i am concerned about safety when the negative pole ends up connected to the mains ground. I have to further investigate about this.

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    1. I used, and recommend the car battery route; problem is, the N3 AFAIK uses 5V, and wasn't in the mood to smoke it.
      The LNA4ALL can be powered with a 12V car battery, something I did for years before bias-T became widespread. Tried the 12V to 5V route, but that's the same switched mode PSU or worse as in a charger.
      The additional advantage of a car battery is that the negative terminal can be used as an artificial ground, this has been explored in the reduce noise post.

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  3. Heck, I am going to order one today! Cheers!

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