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RTL-SDR Blog V.3. Dongles User Guide
Version 3 of our customized RTL-SDR dongles brought out some new interesting features. In this guide we explain how to use those feature. If you are interested, we also have the V3 feature datasheet available here.
Feature 1: Direct Sampling HF Mode
This feature allows you to listen to HF signals between about 500 kHz to 28.8 MHz.
To use direct sampling mode
- Connect an appropriate HF antenna to the SMA antenna port (this is the same port where you connect your VHF/UHF antenna).
- In SDR# select the Q-branch in the configure menu (the cog icon next to the play button). (If it is greyed out make sure you stop the SDR first, by clicking the stop button in SDR#)
- Press Play and tune to 500 kHz – 28.8 MHz.
VHF antennas like small discones or short whip antennas will probably not pick up HF signals very well, if at all. If you have one of our dipole antennas, try connecting a long 5 meter or longer wire to the element connected to the coax center wire. To check which element is connected to the center coax you can open the lid on the black dipole base. Ideally you should use a 9:1 unun with the long wire antenna for optimal reception. Even more ideally you’d use an antenna tuner, though this is expensive.
HDSDR/GQRX and Other Software
Other software like HDSDR and GQRX can also support direct sampling. It may entail setting a device string, and for the Q-branch, the value should be 2 (or sometimes 3). In GQRX the device string would be «rtl=0,direct_samp=2» (without the quotes). In some installs that use different drivers it may be «rtl=0,direct_samp=3» instead. Make sure that there is no space after the comma. For SDR-Console please see this linked post for a modified ExtIO that can enable direct sampling. SDR-Touch on Android has a direct sampling option available in its settings page.
To go back to listening to frequencies above 28.8 MHz remember to change the sampling mode back to «Quadrature Sampling».
Note that this feature makes use of direct sampling and so aliasing will occur. The RTL-SDR samples at 28.8 MHz, thus you may see mirrors of strong signals from 0 – 14.4 MHz while tuning to 14.4 – 28.8 MHz and the other way around as well. To remove these images you need to use a low pass filter for 0 – 14.4 MHz, and a high pass filter for 14.4 – 28.8 MHz, or simply filter your band of interest.
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Modified rtl_tcp for direct sampling
The standard Osmocom version of rtl_tcp only allows for direct sampling on the I-branch, which is useless as we need direct sampling on the Q-branch. Please see our RTL-SDR-Blog Drivers for a version that includes a -D direct sampling flag. The Releases page has a Windows release.
Forcing Direct Sampling To be Always ON
In some cases you may want to force direct sampling to be always on. For example, not all programs expose the direct sampling controls to the user, so in those programs it can be impossible to turn direct sampling mode on. To force it on, use our RTL-SDR-Blog driver branch, and use set the direct sampling always on flag in the EEPROM with the command «rtl_eeprom -q y». To disable forced direct sampling do the opposite command «rtl_eeprom -q n»
Feature 2: Software Selectable Bias Tee
V.1. and V.2. of our dongles included a bias tee which could manually be enabled by opening the case and soldering two pads on the PCB together. V.3. introduces a 4.5V bias tee that can be toggled entirely in software. The bias tee can continuously pull up to 180 mA of current.
WARNING: Before using the bias tee please ensure that you understand that you should not use this option when the dongle is connected directly to a DC short circuited antenna unless you are using an LNA. Although the bias tee circuit is dual protected against accidental shorts with a thermal self-resetting fuse and overcurrent protection on the LDO, short circuiting the bias tee for an extended period of time (days) could damage the LDO or fuse permanently. Only use it while connected to an actual powered device, like an LNA, active antenna or the SpyVerter.
To make things clearer: DC Short Antenna -> LNA -> Coax -> V3(bias tee on) is absolutely fine. What’s not good and makes no sense anyway is DC Short Antenna -> Coax -> V3(bias tee on). DC Short Antenna -> Coax -> V3(bias tee off) is fine.
Note that the legacy DVB-T TV drivers will activate the bias tee by default. On Linux ensure that you have properly blacklisted the DVB-T drivers. More info on how to blacklist on the Linux section on the quickstart guide.
Optional Video: Bias tee tutorial by SignalsEverywhere available here.
To enable the bias tee in Windows:
- Download and extract all the files in this zip file to a folder on your PC. It contains two batch files that can be run.
- Make sure all SDR software like SDR#/HDSDR/SDR-Console etc is fully closed.
- Run the biastee_on.bat file to turn the bias tee on. It will run and open a CMD prompt that will briefly say «Found Rafael Micro R820T Tuner». The CMD prompt will close soon after upon success.
- The bias tee is now on. To turn it off repeat steps 2 & 3, but instead run the biastee_off.bat batch file. Alternatively, simply disconnect and then reconnect the SDR to turn the bias tee off.
If you have multiple dongles connected you’ll need to edit the batch file to specify what dongle’s bias tee you want to activate. Open the bat file with any text editor, like Notepad, and add the dongle selector «-d» flag. For example to activate the bias tee on the dongle that was plugged in second you’d need to change it to «rtl_biast -b 1 -d 1».
If you get a Smart Screen message, click on More Info, and then on Run Anyway. Also note that some versions of Windows may fail to run batch files due to misconfiguration or aggressive antivirus software. If you cannot fix these problems with Windows or your antivirus, run the command manually on the CMD line.
To run it manually on the CMD line first browse to the directory where the bias tee software is stored using «cd» (e.g. cd C:\SDR\bias_tee_folder), and then run:
- ON: rtl_biast -b 1
- OFF: rtl_biast -b 0
- If needed select a particular RTL-SDR device with the -d flag.
To enable the bias tee in Linux:
In Linux or MacOS download the source from git, compile it the same way you do the regular RTL-SDR drivers, and then run ./rtl_biast -b 1 to turn the bias tee on and ./rtl_biast -b 0 to turn the bias tee off. The procedure is:
git clone https://github.com/rtlsdrblog/rtl-sdr-blog
cmake .. -DDETACH_KERNEL_DRIVER=ON
./rtl_biast -b 1
If you want to be able to run the bias tee program from anywhere on the command line you can also run «sudo make install».
If you have trouble running the bias tee check with a multimeter if there is 4.5V at the SMA port. Also check that your powered device is actually capable of receiving power. Remember that not all LNA’s can accept bias tee power. We recommend Adam 9A4QV’s LNA4ALL, as you can order this from his store with the bias tee power option enabled. If you need further help please contact us at [email protected]
Enabling the Bias Tee in PiAware
Please see this link for instructions, or see below to see how to force the bias tee to be always on.
Forcing the Bias Tee to be Always On
If you are using our RTL-SDR-Blog driver branch you can force the bias tee to be always on by setting a flag in the EEPROM. The rtl_eeprom command is «rtl_eeprom -b y». Run the opposite command «rtl_eeprom -b n» to disable the forced bias tee.
Feature 3: Selectable Clock & Expansion Headers
This is for advanced users who need to daisy chain clocks together for coherent experiments, or need to access other ports. You can either bridge the clock selector the directly with a solder bridge, or solder on a 1.27mm 2×2 header pin jumper.
To add a jumper to the CLK selector header.
- Carefully remove the 0 Ohm resistor.
- Very carefully solder a 1.27mm 2×2 header onto the clock selector pads.
- You can now select your clock input.
How to connect the CLK jumpers:
The first position allows you to output the dongles clock to the CLK pads. The second position allows you to input an external clock.
An example of CLK daisy chaining is shown below. One dongles TCXO is connected to two other dongles who have disconnected clocks.
Feature 4: Additional GPIO Ports
Please see the guide written by Rodrigo Freire here.
LF/MF Improvement / Bias Tee Disable Mod:
If you want to improve the performance at LF/MF (below 500 kHz) and do not require the bias tee, then you can remove the bias tee inductor at L13. Of course remember that if you are interested in VLF/LF, it might be a better idea to use an upconverter like the SpyVerter, which can be powered by the bias tee on the dongle.
Notes to be aware of:
I opened my RTL-SDR V3 dongle and found that the thermal pad has a small air gap between it and the enclosure, is this normal?
This is normal. The purpose of the thermal pad is to fix L-band VCO lock problem that are related to PCB heat build up. The RTL-SDR V3 only requires very minor heat sinking to overcome this issue, and a small air gap does not reduce the thermal transfer enough to cause issues. In fact the V3 PCB has already been redesigned to dissipate heat better, so the thermal pad is not strictly required, except in very warm climates.
My RTL-SDR V3 is getting hot.
Please remember that these units do get hot to the touch especially when used in warm climates. This is not an issue and is normal. The temperature will normally be about 20 – 25C above ambient. We have improved the thermal bonding and heat transfer between the chips and the metal case. This results in making the metal case hotter, but it keeps the chips much cooler, resulting in better performance. To lengthen the life of the dongle we recommend keeping the unit away from direct hot sunlight.
Current Known Issues:
We’re constantly trying to improve our units and we always make note of what issues exist and how to fix them.
No known issues.
2020 and earlier units:
Note that the following problem has been fixed in newer batches with a new design.
0.2 – 0.3% of units may have a faulty RTL2832U chip. This is characterized by higher than normal USB currents (normal is 0.28A – 0.3A), and often random disconnections from the USB as well as increased heat. The same problem affects all brands of RTL-SDR.
A small number of these units (
approx 300 units) had faulty bias tee LDO chips which caused the bias tee to be permanently on. The cause was bad silicon in the LDO chip. These units run normally in all other ways, except that the bias tee cannot be turned off. They can continue to be used normally, without the bias tee. The thermal fuse will protect against short circuits.
If you have one of these, feel free to contact us at [email protected] for a replacement, or if the bias tee is not important to you and you can solder, removing the L13 inductor will fully disable the bias tee.
Known V3 Batch 1 Issues (limited quantity batch, no longer shipping):
- Increased sideband noise on very strong narrowband signals. This should not be a significant problem as it only affects very strong signals. The hardware fix is to add about 100-220uF of capacitance on the 3.3V power line. Batch 2 will reduce this noise.
- The bias tee when turned on adds a large spur in direct sampling HF mode. This may be problematic only if you intend to use a bias tee powered HF LNA in direct sampling mode. This can be fixed by adding about 2.2uF of capacitance to the output of the LDO, before the inductor. Batch 2 will fix this.
- The bias tee can be damaged by accidentally short circuiting the output for a few seconds while it is on. This damage only occurs on USB3.0 and USB2.0 ports that can provide up to 1A or more or current. Batch 2 will add a resettable fuse to prevent damage.
AltIMU-10 v3 Gyro, Accelerometer, Compass, and Altimeter (L3GD20H, LSM303D, and LPS331AP Carrier)
The Pololu AltIMU-10 v3 is an inertial measurement unit (IMU) and altimeter that features the same L3GD20H gyro and LSM303D accelerometer and magnetometer as the MinIMU-9 v3, and adds an LPS331AP digital barometer. An I²C interface accesses ten independent pressure, rotation, acceleration, and magnetic measurements that can be used to calculate the sensor’s altitude and absolute orientation. The board operates from 2.5 to 5.5 V and has a 0.1″ pin spacing.
Clearance: This board is being replaced by the newer AltIMU-10 v5.
|Description||Specs (8)||Pictures (7)||Resources (20)||FAQs (1)||On the blog (5)|
Clearance: This board is being replaced by the AltIMU-10 v5, which features newer sensor ICs (the LIS3MDL 3-axis magnetometer, the LSM6DS33 3-axis accelerometer and gyro, and the LPS25H barometer). The new v5 version is pin-compatible with the older v3 and v4 versions, but software written for the older versions will need to be rewritten to work with the v5. This product will be discontinued when stock runs out and is not recommended for new designs where continued part availability is important.
The only difference between this v3 version and the AltIMU-10 v4, which is also currently on clearance, is the altimeter. The v4 and v5 feature the newer LPS25H barometer, which has better accuracy and lower noise than the LPS331AP barometer on the v3.
The Pololu AltIMU-10 v3 is a compact (1.0″ × 0.5″) board that combines ST’s LPS331AP digital barometer, L3GD20H 3-axis gyroscope, and LSM303D 3-axis accelerometer and 3-axis magnetometer to form an inertial measurement unit (IMU) and altimeter; we therefore recommend careful reading of the LPS331AP datasheet (453k pdf), L3GD20H datasheet (3MB pdf), and LSM303D datasheet (1MB pdf) before using this product. These sensors are great ICs, but their small packages make them difficult for the typical student or hobbyist to use. They also operate at voltages below 3.6 V, which can make interfacing difficult for microcontrollers operating at 5 V. The AltIMU-10 v3 addresses these issues by incorporating additional electronics, including a voltage regulator and a level-shifting circuit, while keeping the overall size as compact as possible. The board ships fully populated with its SMD components, including the L3GD20H, LSM303D, and LPS331AP, as shown in the product picture.
Compared to the original AltIMU-10, the v3 version offers a number of improvements arising from the use of newer MEMS sensors, including a wider maximum magnetic sensing range and better gyroscopic accuracy and stability. This version also adds a pin for changing the sensor slave addresses, allowing two AltIMUs to be on the same I²C bus. The AltIMU-10 v3 is pin-compatible with the original AltIMU-10, but changes in I²C addresses and configuration registers might require some changes to software written for the older version (this should not be an issue if you are using our Arduino libraries).
The AltIMU-10 v3 is pin-compatible with the MinIMU-9 v3 and offers the same functionality augmented by a digital barometer that can be used to obtain pressure and altitude measurements. It includes a second mounting hole and is only 0.2″ longer than the MinIMU-9 v3. Any code written for the MinIMU-9 v3 should also work with the AltIMU-10 v3.
Side-by-side comparison of the MinIMU-9 v3 with the AltIMU-10 v3.
The LPS331AP, L3GD20H, and LSM303D have many configurable options, including selectable resolutions for the barometer and dynamically selectable sensitivities for the gyro, accelerometer, and magnetometer. Each sensor also has a choice of output data rates. The three ICs can be accessed through a shared I²C/TWI interface, allowing the sensors to be addressed individually via a single clock line and a single data line. Additionally, the SA0 pin is accessible, allowing users to change the slave addresses and have two AltIMUs connected on the same I²C bus (For additional information, see the I²C Communication section below).
The nine independent rotation, acceleration, and magnetic readings provide all the data needed to make an attitude and heading reference system (AHRS), and readings from the absolute pressure sensor can be easily converted to altitudes, giving you a total of ten independent measurements (sometimes called 10DOF). With an appropriate algorithm, a microcontroller or computer can use the data to calculate the orientation and height of the AltIMU board. The gyro can be used to very accurately track rotation on a short timescale, while the accelerometer and compass can help compensate for gyro drift over time by providing an absolute frame of reference. The respective axes of the two chips are aligned on the board to facilitate these sensor fusion calculations. (For an example of such a system using an Arduino, see the picture below and the Sample Code section at the bottom of this page.)
Visualization of AHRS orientation calculated from MinIMU-9 readings.
The carrier board includes a low-dropout linear voltage regulator that provides the 3.3 V required by the LPS331AP, L3GD20H, and LSM303D, allowing the module to be powered from a single 2.5 V to 5.5 V supply. The regulator output is available on the VDD pin and can supply almost 150 mA to external devices. The breakout board also includes a circuit that shifts the I²C clock and data lines to the same logic voltage level as the supplied VIN, making it simple to interface the board with 5 V systems. The board’s 0.1″ pin spacing makes it easy to use with standard solderless breadboards and 0.1″ perfboards.
- Dimensions: 1.0″ × 0.5″ × 0.1″ (25 mm × 13 mm × 3 mm)
- Weight without header pins: 0.8 g (0.03 oz)
- Operating voltage: 2.5 V to 5.5 V
- Supply current: 6 mA
- Output format (I²C):
- Gyro: one 16-bit reading per axis
- Accelerometer: one 16-bit reading per axis
- Magnetometer: one 16-bit reading per axis
- Barometer: 24-bit pressure reading (4096 LSb/mbar)
- Sensitivity range:
- Gyro: ±245, ±500, or ±2000°/s
- Accelerometer: ±2, ±4, ±6, ±8, or ±16 g
- Magnetometer: ±2, ±4, ±8, or ±12 gauss
- Barometer: 260 mbar to 1260 mbar (26 kPa to 126 kPa)
A 1×6 strip of 0.1″ header pins and a 1×5 strip of 0.1″ right-angle header pins are included, as shown in the picture below. You can solder the header strip of your choice to the board for use with custom cables or solderless breadboards or solder wires directly to the board itself for more compact installations. The board features two mounting holes that work with #2 or M2 screws (not included).
Using the AltIMU-10 v3
A minimum of four connections are necessary to use the AltIMU-10 v3: VIN, GND, SCL, and SDA. VIN should be connected to a 2.5 V to 5.5 V source, GND to 0 volts, and SCL and SDA should be connected to an I²C bus operating at the same logic level as VIN. (Alternatively, if you are using the board with a 3.3 V system, you can leave VIN disconnected and bypass the built-in regulator by connecting 3.3 V directly to VDD.)
Pololu AltIMU-10 v3 gyro, accelerometer, compass, and altimeter pinout.
Two Pololu AltIMU-10 v3 modules in a breadboard.
|SCL||Level-shifted I²C clock line: HIGH is VIN, LOW is 0 V|
|SDA||Level-shifted I²C data line: HIGH is VIN, LOW is 0 V|
|GND||The ground (0 V) connection for your power supply. Your I²C control source must also share a common ground with this board.|
|VIN||This is the main 2.5 V to 5.5 V power supply connection. The SCL and SDA level shifters pull the I²C bus high bits up to this level.|
|VDD||3.3 V regulator output or low-voltage logic power supply, depending on VIN. When VIN is supplied and greater than 3.3 V, VDD is a regulated 3.3 V output that can supply up to approximately 150 mA to external components. Alternatively, when interfacing with a 2.5 V to 3.3 V system, VIN can be left disconnected and power can be supplied directly to VDD.|
|SA0||3.3V-logic-level input to determine I²C slave addresses of the three ICs (see below). It is pulled high by default through 10 kΩ resistor. This pin is not level-shifted and is not 5V-tolerant.|
The CS, DEN, data ready, and interrupt pins of the LPS331AP, L3GD20H, and LSM303D are not accessible on the AltIMU-10 v3. In particular, lack of the CS pin means that the optional SPI interface of these ICs is not available. If you want these features, consider using our LPS331AP carrier, L3GD20H carrier, and LSM303D carrier boards.
The above schematic shows the additional components the carrier board incorporates to make the LPS331AP, L3GD20H, and LSM303D easier to use, including the voltage regulator that allows the board to be powered from a single 2.5 V to 5.5 V supply and the level-shifter circuit that allows for I²C communication at the same logic voltage level as VIN. This schematic is also available as a downloadable pdf: AltIMU-10 v3 schematic (207k pdf).
The LPS331AP’s barometer, the L3GD20H’s gyro, and the LSM303D’s accelerometer and magnetometer can be queried and configured through the I²C bus. Each of the four sensors acts as a slave device on the same I²C bus (i.e. their clock and data lines are tied together to ease communication). Additionally, level shifters on the I²C clock (SCL) and data lines (SDA) enable I²C communication with microcontrollers operating at the same voltage as VIN (2.5 V to 5.5 V). A detailed explanation of the protocols used by each device can be found in the LPS331AP datasheet (453k pdf), the L3GD20H datasheet (3MB pdf), and the LSM303D datasheet (1MB pdf). More detailed information about I²C in general can be found in NXP’s I²C-bus specification (1MB pdf).
The L3GD20H, LSM303D, and LPS331AP each have separate slave addresses on the I²C bus. The board connects SA0 pins of the three ICs together and pulls them all to VDD through a 10 kΩ resistor. You can drive the SA0 pin low to change the slave address. This allows you to have two AltIMUs (or an AltIMU v3 and a MinIMU v3) connected on the same I²C bus. The following table shows the slave addresses of the sensors:
|Sensor||Slave Address (default)||Slave Address (SA0 driven low)|
|LSM303D (accelerometer and magnetometer)||0011101b||0011110b|
All three chips on the AltIMU-10 v3 are compliant with fast mode (400 kHz) I²C standards as well as with the normal mode.
We have written a basic LPS331 Arduino library, L3GD20 Arduino library, and LSM303 Arduino library that make it easy to interface the AltIMU-10 v3 with an Arduino or Arduino-compatible board like an A-Star. They also make it simple to configure the sensors and read the raw pressure, gyro, accelerometer, and magnetometer data.
For a demonstration of what you can do with this data, you can turn an Arduino connected to a AltIMU-10 v3 into an attitude and heading reference system, or AHRS, with this Arduino program. It uses the data from the AltIMU-10 v3 to calculate estimated roll, pitch, and yaw angles, and you can visualize the output of the AHRS with a 3D test program on your PC (as shown in a screenshot above). This software is based on the work of Jordi Munoz, William Premerlani, Jose Julio, and Doug Weibel.
The datasheets provide all the information you need to use the sensors on the AltIMU-10 v3, but picking out the important details can take some time. Here are some pointers for communicating with and configuring the LPS331AP, L3GD20H, and LSM303D that we hope will get you up and running a little bit faster:
- The pressure sensor, gyro, accelerometer, and magnetometer are all off by default. You have to turn them on by setting the correct configuration registers.
- You can read or write multiple pressure sensor, gyro, or accelerometer registers in a single I²C command by asserting the most significant bit of the register address to enable address auto-increment.
- Compared with previous LSM303-series sensors, the register interface to the magnetometer in the LSM303D is much more consistent with the accelerometer interface, and its accelerometer and magnetometer share a common I²C address instead of acting as two separate slave devices on the same bus.
- The pressure sensor has a 24-bit pressure reading. The gyro, accelerometer, and magnetometer all output readings in a 16-bit reading (obtained by combining the values in two 8-bit registers for each axis).
We carry several inertial measurement and orientation sensors. The table below compares their capabilities:
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