New SDR radio does not seem to work

[cdev]

The device I have is a couple of years old and indeed, it does look a lot different. You should poke around on the Chinese ham web sites. I bet you could find some discussions there as whatever changes were made occurred.

I gather that you have not owned an RTLSDR before? Common mode chokes on the USB are necessary.

Yes, the one I have does let you send power to the antenna, as I explained its a functional but quite rudimentary "bias tee".

It can supply DC to the (VHF/UHF) antenna to power a LNA. Its enabled by changing a jumper. All in all, I thought it was a pretty good deal, considering I paid - maybe $28 for it.
I don't remember.

But you need a decent antenna and ground. And the device I have looks completely different from yours.

[WaveyDipole]

Yes, it was difficult to make out under the magnifier lamp, but under the microscope it was quite clear - just 2 characters: J3. I am trying to understand the difference between the two circuits, and the purpose of the semiconductor part. If its a transistor, that might explain the need for a 3.3v line input and there is a DC path back to GND via R3, however 10meg would seem rather high to provide sufficient base current to turn the transistor on? If it is a package with two TVS diodes, that might also make sense. I believe TVS diodes are sometimes used in hard drive PCBs and those I have seen are usually two pin devices. Do TVS diodes read like a regular diode on the DMM? This package reads like a regular transistor, i.e. like a diode from base to C/E and infinity between C and E. Red probe on B to get a reading of the usual approx 600ohms.

You are quite correct - I have not owned an RTL-SDR before and my first experience does not turn out to be a very good one! In your earlier post you mentioned that performance of the RTL-SDR holds its own against a Tecsun radio? The performance of this particular unit is a long way short of that unfortunately.

I was reading about bias tees following your mention the other evening and was just trying to confirm that the configuration here is, in fact, an example of a bias tee.

[chris_leyson]

Common mode noise is always going to be a problem with SDRs especially when using E-field antennas, you need a quite ground connection at the antenna input. The ground plane on the PCB is your reference point and any noise on the ground plane just gets added to the received signal. A shielded magnetic antenna might give better results.
I have a QS1R SDR and to get the best from it I have to use a linear bench supply to power it, no switching wall wart rubbish and a small notebook PC running off batteries or a linear bench supply. I could probably get much better performance with a tuned and screened mag loop but haven't got around to building it yet.

[cdev]

You may want to consider completely redoing the input stage with something different.

Since you have access to the two differential inputs without the risk of knocking it off its your oyster so to speak. There is a lot of opportunity to learn about RF visually with that setup.

Its really almost an ideal learning setup.

The input on the RTLSDR at pins 4 and 5 is around 3000 ohm differential input.

Its suceptible to ESD, so if you build your own whatever, keep that risk in mind.

Try to filter out everything above 24 MHz (since your dongle by itself can receive above 24 Mhz on its own.)

I would concentrate on the lower part of that range since we're near the minimum of the sunspot cycle.  An LC network can match the antenna impedance to the input impedance.

Especially do whatever it takes to electrically decouple your receiving setup somewhat from the noise thats often found riding on power lines.

In a pinch, even just a bunch of wire (i.e. an air core inductor) will work. 

But its best if you have an appropriate magnetic material you can use for a core.

Try to keep the length of wire between your unun and your ground connection as short as possible.

Another thing that may potentially help a lot is to decouple the coax from your receiver by wrapping it as many times as you can around a ferrite rod or toroid or even just make a tightly would coil using the coax.. creating an RF choke. before it gets to the receiver.

I have done this with RG-174 and Ive been able to get a lot of turns in there because its so thin. (The loss in RG-174 is still pretty modest for HF, so this helps a lot.)

If its convenient, try to put your ground rod somewhere where the soil stays wet year round. (like under a downspout)

An interesting antenna thats a fairly good choice for people without the room to erect a really long wire is the so called "mini-whip" e-field probe antenna.

Its an amplified antenna and it usually gets its power via its feedline.

Its feedline is used in part of the antenna but it needs to have a big RF choke to prevent the nise indoors from making its way out to it.

The active unit is tiny, its basically a capacitive probe that sticks up into the air as high as possible. The antenna itself has a very high impedance preamp.

Its performance depends on it being put in a quiet location.. (outside of a house, and as far away from e-stuff as possible.)

Then ground the coaxial cable outside before it enters the house, then put a common mode choke on the coax before it gets to the receiver.

https://www.google.com/search?hl=en&as_q=pa0rdt+mini+whip

[Beamin]

I now cost is an issue but the difference in a dongle and an SDRplay is night and day difference below 50MHz. For example at night every single channel on the MW band has two to three stations every 10 KHz apart from 540 to 1600 on it from all the skip. Every multiple of allocated AM broad cast has a huge sharp peak on it. Some times its fun just to start one end of the band and count how many stations total you can pick up to two to three hundred different AM stations with a big long wire antenna. Lets see your car radio do that.

This thread makes me want to take my old dongle apart but I have a feeling it going into to little rasp pi box and do something useful like a self contained SDR with a 3" screen if I can find all off the shelf parts. If you only care about one frequency you could mount the pi/ dongle and battery/ solar cell in some high remote location with a small amp and have it broad cast the audio back. The pi gets choked with CPU usage when you add bandwidth to it.

[WaveyDipole]

I decided to mod the antenna circuit to make it similar to the BA5SRA circuit by adding a torroid 1:4 transformer which I wound as per the recommendations here:

http://mikikg.files.wordpress.com/2012/08/rtl2832u-dc-mods.pdf

Furrther information regarding transformers can be found here under the heading 'Balun/Matching Transformer Improvement':

https://www.rtl-sdr.com/rtl-sdr-direct-sampling-mode/

It states that the transformer should be wound on a FT37-43 core using 5 turns wound in trifilar fashion. Since I only had a couple of FT37-61 ferrites left over from a previous project, I had to adjust the number of windings. It so happens that this other project was originally designed for FT37-43 cores but came supplied with FT37-61 cores and required the same adjustement and the number of turns required for both types of core were provided. Calculating for 5 turns proportionally worked out to 13-14 turns.

I first removed the two links I had put in from the main board to RTL2832 pins 4 & 5. I then also removed the series capacitor (C4 on previous diagram) and also the 47ohm resistor (R1). I then used a couple of spare pads to connect the primary of the transformer. The secondary was connected to the two pads on the SDR dongle board. Finally I added two protection diodes (1N4148) to the inside of the antenna connector.

The difference was huge! The MW band is now alive with stations, Radio 4 came in clear on 198kHz LW and I can pick up various foreign stations on SW. I could almost call this 'holding its own' against the Tecsun. The dongle still has more noise than the radio and further experimentation with filtering may improve things, but reception is now quite acceptable and connecting the antenna earth no longer has a detrimental effect. I also found in gqrx, that using the narrow filter width on AM helped considerably with filtering out background noise.

The second reference I linked above mentions a T16-6T-KK81 transformer. So far I haven't found one available online to the UK, but I if I could find one then I wouldn't mind giving it a try to see whether it improves things further.

In the meantime I still await the replacement board and will report back when it arrives, however it would seem that this newer blue box appears to have an inferior HF circuit layout to the BA5SRA design.

[cdev]

This is great!


Glad to hear this!

I don't know what kind of bypass caps he uses in his board but I had good luck adding a few additional caps to my board. I added a number of additional large value SMT cps across the power rails in different points and a tantalum cap across the 3.3 volt output of the voltage regulator.

There are a bunch of good ideas here if you scroll down to "Reception of VLF frequencies using RTL820T2 Dongle":
http://www.g8jnj.net/softwaredefinedradio.htm

(also his "active Antennas" page collects a lot of good links to limited space antennas which would work well with a VLF enabled dongle.. Also look at what Martin says about the transformer, right at the bottom.

The optimization of the dongle for VLF will bring improvements to HF reception too. Its kind of a worst case scenario for noise and interference.

Once you have the transformer working at its best, then look at reducing the DC to DC converter noise..

Its even possible to eliminate it and replace it with several diodes. See the link to "Toshi"s web site in japan for a schematic.  http://ggtoshi.at.webry.info/201406/article_6.html

Its amazing how much can be done and learned, with a cheap dongle.. Really, they must break some kind of record for most bang per buck educational tool.

People from all around the world have been adding to the fun.

http://userdisk.webry.biglobe.ne.jp/019/963/01/N000/000/000/140317102568976083225_RtL2832U.PNG

http://userdisk.webry.biglobe.ne.jp/019/963/01/N000/000/000/140317075718660037225_R820T_20140619183917.PNG

http://blog.livedoor.jp/bh5ea20tb

Small improvements in bypassing bring big improvement in signals on HF.

Do you have an oscilloscope? If you do that will make it easier figuring out if some change represents a a likely improvement and why.

Its impossible to do this without a good connection to the board.
i would solder on some grabber-friendly loops of wire to use as test points to the points where the various voltages are used on the board to make it easy to attach your probes.

I think I am going to put a dip socket in there so i can swap out transformers easily.

I have always thought I could improve the low frequency response with a better transformer but its scary because the connection is so iffy..

The dongle kit came with a lot of parts but like everything it was made to sell at a certain price point and make a profit. So a little bit of investment in a better transformer will probably pay off.

See the images below, which are all kind of old but they use the Newsky dongle- likely some changes have been made since then and also for this specific dongle but the general schematic is still likely very similar.


There is also a bunch of helpful info here - but you will need to use Google Translate: http://ggtoshi.at.webry.info/201501/article_1.html

Also, here...

http://blog.livedoor.jp/bh5ea20tb/archives/cat_120700.html

also look at Chris Trask's varactor tuned active antenna design and especially its double transformer..

http://home.earthlink.net/~christrask/

[WaveyDipole]

I managed to obtain a mini 1:16 transformer which I tried today. This thing is tiny and the piece of ferrite inside must be miniscule - significantly smaller than my ferrite ring. It performed reasonably well and gain seemed to be increaed a little but the noise level was also considerably increased. I tried re-placing the series capacitor, reconnecting the 3.3v supply and placing the transformer across the original link points to the RTL2832 IC but curiously this did not work at all - I got no signal whatsoever. I then re-instated my orignal mod and hand wound transformer as on the whole that gave me the best performance. I have also ordered some clip on ferrite beads to put on the USB cable.

[cdev]

If you have not had the clip on ferrite beads up until now you'll likely see a big improvement once they arrive and you put them on there!

Also, more filtering/bypass capacitors help. At some point however it refuses to start up (when the cap isnt charged up so you need to discharge it to see this- because it will draw current at the beginning) eventually you will have added too much and it then wont work, the USB circuitry doesnt like it, then back off a bit. (this is from memory and its been a while but thatsbasically what I remember)

It seemed when I was looking at your boards design that it perhaps had fewer add on caps than mine.. anyway, more bypassing if its short path to ground and there are not loops will bring improvement. Tantalum capacitors work the best in my experience but thre may be better. SMD caps are better than through hole because of lead length. As the 'manual' says a tantalum is a good addition.. if you can an additional .22 uf (or .33 uf) tantalum cap and put it across the 3.3 volt rail thats good.. Make sure its the right polarity!

I am in the process of installing a little dip socket in my modded rtlsdr so that I can plug in different transformers without tearing out the wires.

I then re-instated my original mod and hand wound transformer as on the whole that gave me the best performance. I have also ordered some clip on ferrite beads to put on the USB cable.

This web page contains some good ways to build HF transformers:

http://www.qsl.net/in3otd/ham_radio/160m_transformers/160m_trafos.html