My previous post describes the $16 dual channel rtl_sdr dongle hack. In the last few days I’ve done some more testing and it turns out I can use the system for passive radar! I didn’t expect this, because the receiver only has 8 bits and passive radar requires a lot of dynamic range.
|Airplanes and occasional specular meteor echoes.|
I hooked up the two channels into yagi antennas that we have used with Echotek and USRP receivers for passive radar. One of the antennas was measuring the transmit waveform, and the other was measuring the echoes. I ran a measurement, and to my great surprise, it worked just fine.
I did tweak the signal levels a bit in order to ensure that I optimally use the dynamic range. I also had the bandwidth set to 2.4 MHz, giving me about 4.5 bits extra dynamic range after filtering the signal to 100 kHz in single precision floating point.
|Two log periodic antennas used to passive radar with the dual coherent RTLSDR R820T dongle.|
This really does give us a glimpse of the future where high end digital receivers will cost $10 per channel. The low end ones are already in that price range. Think of all the potential science that can be done!
I have been playing around with the cool RTL dongles (more on rtl-sdr dongles on superkuh’s web page or rtl-sdr.com) that you can buy on e-bay for about US$8 (including shipping). These are very capable 8-bit digital receivers that have up to 2.4 MHz bandwidth and can tune anywhere between 24 MHz and 1850 MHz
I recently came up with a trivial hack to build a receiver with multiple coherent channels using the RTL dongles. I do this basically by unsoldering the quartz clock on the slave units and cable the clock from the master RTL dongle to the input of the buffer amplifier (Xtal_in) in the slave units (I’ve attached some pictures).
I originally drove the master crystal with both dongles, which also worked. However, Ian Buckley pointed out to me that a more typical way of doing this is feeding the signal into Xtal_In (in the pictures below). So I tried that too, and it also worked. I’m still not sure what the optimal setup is, as there is no schema for the dongle, but both methods I’ve tried so far have worked in practice.
|This is how you make a dual coherent channel digital receiver with $16. The clock drive probably won’t be enough for many of these, but this can be fixed with a buffer or some other active splitter.|
|The oscillator is wired using a piece of 75 Ohm antenna coax that came with the dongle. It’s like they designed the dongle for multi-channel coherent applications.|
This has some implications for low cost geophysical instruments. It will be possible to use this receiver for the 150/400 MHz beacon satellite receiver, as this only requires that the receivers have clocks that are locked with each other. Interferometry and passive radar are other application examples. With more than two locked channels, applications such as imaging start to become possible.
I’ve made some relative phase noise measurements, and the systems don’t have detectable sample drift over two hours, and their relative phase is also pretty stable.
|Spectrum at 1 Hz sample rate of the relative z_1/z_2 phase signal going into two receivers.|
|IQ plot of the z_1/z_2 relative phase signal over ~6000 seconds at 1 Hz sample rate.|
And oh, by the way, I found this nice usb hub, which I’m going to use to hopefully get a 7 channel coherent rtl system.
|Hub with the right usb port orientation for rtl dongles.|
Stay tuned for more results. I already have some pretty nice passive radar results using the system, which I’ll be posting in a few days.
Update: Apparently three dongles will also run fine from one master clock. I know the clock isn’t split correctly, but adding any components would increase the total cost and the whole point of this exercise is to determine what the lower bound is for software defined radios.
|Three channel coherent RTLSDR receiver.|
By Nick G0CWA
Hi this is my latest design for a RTL-SDR communications receiver. The set amounts to little more than a switchable receive up converter. Although designed for the Realtek RTL2832U Chipset + Elonics E4000 tuner, it will work with any of the other compatible tuners. I would personally recommend SDR# as the control package for the dongle. http://sdrsharp.com/
The frequencies from DC to ~1.7GHz are covered in two ranges: High range ~60MHz to 1.7GHz (depending on tuner chip) with the normal gap around 1.1GHz And Low range DC to 60MHz.
The main aim was to build a version of my designs to comfortably fit into a Lap top bag and be fully portable and powered from the PC USB port. The completed unit including a dongle fits into a small die-cast instrument case measuring only 55x25x125mm. To keep the circuitry size to a minimum I have not included any extras e.g. pre-amp or antenna switching etc.
There is nothing clever or particularly original in the design with one exception the use of a 125MHz conversion oscillator so avoiding the VHF broadcast band on the output and reducing any sensitivity issues. The 5V supply is taken off the dongles USB connector by opening up the dongle case, carefully, and soldering a fine insulated wire to the appropriate USB connector pin taking this out through a small opening cut in the side of the dongle case.
Follow my track layout and use top quality miniature co-ax for maximum sensitivity I used silver plated. Do NOT use single strand wire for any RF signals as these dongles are very sensitive to mis-matches and unless you are very lucky will shut down and work poorly, if at all.
SDR Schematic (Larger View)
You may notice that D1, D2, C11 and C12 do not appear on the PCB top they are actually soldered on the underside of the board to save space. There are also two wire links on the top of the PCB to add.
The RF input to the dongle is via a connector shown in the “real world” PCB layout diagram. This connector was “scavenged” from a cheap plastic bodied co-ax plug which is soldered to the PCB in a small slot. TAKE NOTE the connector is soldered on both sides bridging the two earth pads to complete the ground track to both sides of the PCB; the centre pin is connected via a small piece of copper wire to the PCB.
The only problem I had was when switching between the receive bands sometimes the dongle would lock up, this was due to the current demands of the relays and smoothing caps I managed to “cure this problem” by reducing the values of C11 and C12 anything above 100 nF should be ok.
The actual PCB dimensions and cut-outs suited both my box and gave sufficient clearance for the dongle, antenna socket and switch. Adjust these to suit your case etc.
The coils are all 6.5 turns of ~1mm diameter copper wire 0.25 inch diameter by 0.4 inch long, the relays are both NEC/TOKIN UB2-5NE 5V Coil DP Ch-over types although any similar equivalent ones may be used. The regulator is an LM1117T 3.3V unit. The crystal oscillator is a 3.3V 125MHz crystal oscillator module, any oscillator module frequency above 60MHz can be used but I can’t guarantee the receivers performance or immunity to de-sentisation/interference from the VHF broadcast bands.
For any further information about operation and performance check my MK2 design write up also on this site.
Enjoy this design hope you find it of use.
The Radio board and QRZ forums With any questions, please only contact me via these routes if you have any questions, I can’t guarantee to reply otherwise as I can’t see every reference to my sets. Enjoy the design 73 for now Nick G0CWA
Take a look at Mike’s RTL SDR website. This is quite a popular project!
Please note I do not supply kits, parts, PCB’s or build boards for my projects but am more than willing to help talk you through a build or fault finding via my normal contact methods, or even SKYPE if required for direct contact.
Well, sometimes you have disconnected all too soon, such as reception of ADS-B, and it has been extended with a USB repeater cable and USB extension cable tuners original. It is the effective means to directly below the antenna reception, but it will not make very troublesome need to be recognized again connect or disconnect the tuner at the site contact failure occurs once. When you received by the antenna directly under, let’s enough measures.
Refer to a tuner directly to the PC It is big no-no. You can find information fitted in. However, you should consider using a USB extension cable tuner also serves as prevention of leak noise from the PC of course … is loose.
I’ll try to raise the issue of the original USB connector.
Contact instability ⑤ disconnect number of times, which is also what is being corrosion flux adheres to the plating of ④ cover software freezes backlash ② connection after contact ① connector weak touches the big ③ running tuner will increase it will not recognize it becomes
Basic performance … the problem is good Hey there and well, and will teach you the improvements we want to try them. Earlier to say that defect report TV28Tv2DVB-T USB Stick of (R820T) Some of them are introduced but.
I’ll try to increase the improvement of the USB connector.
In the tape winding fixed (degree of difficulty ★) ① USB connector connection part (USB extension cable)
This will not fix the problem, but it is about prevention of freeze touch.
You are using a USB extension cable ② brand item (Difficulty ★ ★)
Resolve only budget, but the degree of contact is slightly better
Will be replaced with a high-quality ③ USB connector (Difficulty ★ ★ ★ ★ ★)
The difficulty is in removing the USB connector …
The connection to the solder and the USB cable ④ tuner board (Difficulty ★ ★ ★ ★ ★)
Most reliable if there is confidence in the arm! And to say that, I tried to replace the USB connector most solid.
(A type, male) was used as a replacement is a surface mount USB connector Akizuki Denshi Tsusho. Actual product photos and product introduction of the HP is slightly different.
(Shape of the rib is different, but is independent of the connection. Reprint from HP Akizuki e’s)
Press working (rib), differences in appearance, comes with a form etc. Chigae in general, those that do not come with a part of country C made often seen. The connector just for exchange … It is cost down, pressing of the cover, plating is also beautiful to resin portion of the interior has also been shaped clean.
Terminal differences are: (contact). The USB connector of the original, contact will be really weak for terminal is flat, but the terminal of the USB connector you have replaced, contact resistance has improved contact part has a convex shape. It looks terminal is lower than the resin part If you look at the terminal part of the original. Contact also I think in a weak for that? The backlash seems to be less for a replacement, resin part because it can be slightly larger for the original.
Software freezes in haste to the touch even a little USB part in running the case of the original tuner and try to actually use. However tuner after the replacement, did not freeze after compressing or pry or twist a little. Also, I had to freeze or between always If used for a long time, but does not have it! It was not a poor contact even after repeated insertion and removal. And I realized feeling when plugged anyway and is solid.
Why not? It is the important part to perform data communication and power supply, but the accuracy is too bad too. It’s not that say it can not be used in its original state, but it is a very important part as the electronic equipment. It is recommended that you try to check once. Such as there often is that the tuner can not be recognized in particular? Maybe you no longer recognize? I think if symptoms get such, it is good when I suspect the USB connector.
You will be omitted replacement procedure, but because it only install by removing the connector at the end.
Simple xtal oven for accurate clocks
A one-transistor xtal oven gives stable xtal temperature for very accurate clocks.
What does it do?
This is a very simple and easy to make temperature controller and heater to be attached to a xtal (crystal). The xtal is normally used to clock a microcontroller (PIC or ATMEL etc).
Normally xtals provide an accurate clock to 50 or 100 PPM (parts per million) making them useful for real-time clocks, like in your wristwatch. However their frequency output changes with temperature.
This circuit keeps the xtal at a constant temperature – commonly called a “xtal oven” also called an “Oven Controlled Xtal Oscillator” or “OCXO”. Hence the xtal error is reduced to 1 – 10 PPM and the clock will keep almost perfect time.
Thsi circuit can be built with very common cheap parts and means you don’t have to find or buy an expensive xtal oven.
Coupled with my 1-sec PIC timer algorithm HERE you can use ANY value of xtal to build a very high accuracy clock.
How does it work?
Three small resistors are glued to the body of the xtal. These act as “heater elements” and get warm when current is passed through them and heat the entire xtal body. This current is controlled by a little darlington transistor. The temperature is sensed by a cheap NTC thermistor, which controls the current; increasing current if too cold, decreasing current if too hot. Simple negative feedback.
Beacuse the time constant is slow, determined by the thermal mass (mechanical time constant) the circuit stabilises to a regulated temperature at about 35’C (about body temp, slightly above room temp) and remains at that temperature at all times, unless exposed to a temperature extreme that it cannot cope with. This is designed for indoor use and with the parts shown has plenty of range to cope with typical indoor temperatures.
Clever Roman Black-style minimum-parts shortcuts; A single transistor seems a poor choice for a temperature controller as it’s performance changes with temperature! However by including the transistor itself in the thermal mass, the transistor is now kept at a constant regulated temperature like the other parts!
To look into this further; the NTC thermistor (and the other resistors) act to reduce the heater current as temp increases. This is the desired effect. However the transistor tries to increase the current with temp increase, the opposite to the desired effect! I was worried that this may be a problem until I had done enough testing with the actual device, but there was no problem! It acts like a balance; the thermistor (NTC of I) tries to do the “right thing” and the transistor (PTC of I) tries to do the “wrong thing” and because the thermistor has a MUCH greater effect on operation the whole circuit acts globally as (NTC of I) and it works.
How to make it!
- the xtal
- TO-92 (small) NPN darlington transistor (any type, or 2 normal NPN’s wired as darlington)
- cheap NTC thermistor (I used DickSmith 100k type, =55k @ 40’C)
- 220k resistor
- 0.01 to 0.1uF small capacitor (value not critical!)
- 3x 39 ohm resistors
- araldite (5 minute epoxy)
- (for adjustment) 150k resistor, 220k trimpot, multimeterINSTRUCTIONS;1. Glue the 3 heater resistors and thermistor to the xtal body with TINY spots of superglue. Glue the thermistor in good contact with the xtal body, and touching the 2 heater resistors on that side of the xtal. Cure the superglue for a few minutes under a warm desk lamp.
2. Glue the transistor (or 2 if you make your own darlington) to the xtal body.
3. Using pointy pliers gently bend the legs to the right position and trim to length for neat construction. Put a small spot of solder on the joins and check with magnifying glass. Above you can see the green thermistor on the right (between the 2 resistors) and the black transistor and 1 resistor on the left.
4. Trim the capacitor leads and solder it across the thermistor.
5. Hook up 2 short power wires; the +5v end of the TOP heater resistor (this is the +5v connection) and the 0v (gnd) is connected to the emitter of the transistor (see schematic).
6. Connect the 100k and 220k trimpot in series (chain) and connect them from +5v to the base of the transistor. See schematic.
7. Turn the trimpot to centre and connect REGULATED +5v power through a 50 mA ammeter (multimeter on 200 mA range will do).
8. (See above photo – testing and adjusting) Adjust trimpot to about 15mA total current draw. Allow temperature to stabilise (may take 30 seconds) and adjust the trimpot again if needed. The trimpot sets the temperature of the xtal, you can measure it if you have a thermometer (I use an infrared optical thermometer), otherwise 15mA raises the xtal about 8’C above room temperature so you can work from that.
9. Now disconnect power and disconnect the trimpot and measure it, then replace with a fixed resistor to reduce size and make it neater and more reliable.
10. Test it still works with just the fixed resistor, I chose 220k which regulated at about 35’C. Test that it draws less current when warmed by a desk lamp, and draws more current when cooled by a fan. If it all tests ok, shorten all the leads and make it more compact and neat ready to cover with epoxy.
11. Put a thick layer of araldite (5 minute epoxy) all over the whole thing but make sure your 2 (or 3) xtal leads and the 2 power leads come out dry so you can solder to them later! The araldite forms a decent thermal conductor and makes the entire circuit into one thermal mass.
12. Cure the araldite under a desk lamp (see above) at “warm sun” temp (50’C) for half an hour or so. Then test it again and hope you didn’t mess it up! You can clearly see above the green thermistor surrounded by 2 heating resistors.
13. Cover the entire xtal oven with generous layer of styrofoam insulation. This reduces its power consumption. Seal any air gaps with more epoxy or silicone etc.
14. Cover with aluminium foil or other RF shield, wrap a ground wire around that and glue in place (optional).
Note! I haven’t covered this one with styrofoam yet until I build the clock because I want to check size and clearances etc. I did wrap it in a large loose wad of tissue paper (for testing) and current consumption dropped a few mA as expected. Overall, the xtal oven does seem to work really well! Heating or cooling it performs exactly as expected; the heater current responds appropriately and my infrared thermometer says it remains at 35’C at all times. Success!
Uses for the xtal oven
If you are making your own clock, you will need to calibrate your software to match the new exact xtal frequency. I suggest using a GPS, these display (and serial output) a time code that is locked to the atomic clocks on the GPS satelites. Adjust it once a week and if it gains/loses less than a second a week that will be less than a minute a year error. With a little effort to calibrate it, your xtal oven clock should be accurate to a few seconds a year.
You could also retrofit this circuit to an existing clock, or a test instrument like a frequency meter. Or certain amateur radio (HAM) equipment that needs a stable frequency. All it requires is a +5v regulated supply and 0 – 40 mA, (usually less than 20mA).
Adapting my design for SERIOUS USE
If you need more serious temperature control, especially for outdoor use with large temperature variations, the circuit needs to be improved. I suggest using more current through the heater resistors (reduce their resistance), obviously to cope with lower (MIN) temperatures you need more heater power. Good thermal insulation will help both in performance and to reduce total current needs.
To cope with higher (MAX) temps you need to run the xtal at a higher temp than any temp the device will experience. If it is to run in a car or robot in the hot sun this might be a considerable temperature. Again this may involve increasing the power to the heater resistors.
Finally you might want to switch to a higher precision temperature controller, ie use a 8-pin comparator chip instead of a single transistor! Either way you can expect to do a lot of temperature testing. It might be easier just to buy a commercial OCXO providing it can handle your temp range needs.
So why not just BUY a xtal oven??
Where’s the fun in that?? 🙂 And anyway, I built this in about an hour from $1 worth of parts. A commercial OCXO costs a LOT more than that with postage and would have taken a week or more to get here.
My circuit is also more energy efficient than commercial OCXO products and only needs a few mA as it was tailored for my needs (ie low power indoor use within a limited temp range).
And this way I got to pick my own xtal frequency too.
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