Category: News

A Multi-Channel Coherent RTL-SDR Product: For Passive Radar, Direction Finding and More

Coherent-receiver.com is a company which is a customer of our RTL-SDR V3 dongle and they have been working on creating a multi-channel coherent receiver product based on the RTL-SDR. An RTL-SDR multi-channel coherent receiver is at its most basic, two or more RTL-SDR dongles (multi-channel) that are running from a single clock source (coherent). A multi-channel coherent receiver allows signal samples from two different antennas to be synchronized against time, allowing for all sorts of interesting applications such as passive radar and direction finding.

The team at coherent-receiver.com have used the new expansion headers on our V3 dongles to create their product. In their receivers they attach a control board which has a buffered 0.1 PPM TCXO (buffered so it can power multiple RTL-SDR’s). They also added an 8-bit register and I2C connection capabilities which allows for control of future add-on boards. The I2C capability is useful because it means that several RTL-SDR dongles can be controlled and tuned from the same control signal. More information on the registers and build of the receiver control board can be seen on their technical support page.

A ten channel RTL-SDR coherent receiver.
A ten channel RTL-SDR coherent receiver.
The Coherent Receiver block diagram.
The Coherent Receiver block diagram.

One example application of a multi-channel coherent receiver is passive radar. Coincidentally, we’ve just seen the release of new GUI based Passive Radar software by Dr. Daniel Michał Kamiński in yesterdays post. Passive radar works by listening for strong signals bouncing off airborne objects such as planes and meteors, and performing calculations on the signals being received by two antennas connected to the multi-channel coherent receiver.

A second example is direction finding experiments. By setting up several antennas connected to a multichannel coherent receiver calculations can be made to determine the direction a signal is coming from. An interesting example of direction finding with three coherent RTL-SDRs can be seen in this previous post. A third example application is pulsar detection which we have seen in this previous post

Coherent-receiver.com sent us a prototype unit that they made with four of our V3 dongles. In testing we found that the unit is solidly built and works perfectly. We tested it together with Dr. Kamiński’s passive radar software and it ran well, however we do not have the correct directional antennas required to actually use it as a passive radar yet. In the future we hope to obtain these antennas and test the coherent receiver and the software further.

Currently they do not have pricing for these models as it seems that they are first trying to gauge interest in the product. If you are interested in purchasing or learning more they suggest sending an email [email protected] It seems that they are also working on additional RTL-SDR ecosystem products such as filters, downconverters, antennas and LNAs.

We hope that the release of this product and Dr. Kamiński’s software will give a boost to the development of coherent multi-channel receivers as we have not seen much development in this area until recently.

SDRDue running on the coherent-receiver.com unit.
SDRDue running on the coherent-receiver.com unit.

KiwiSDR Soon to Accept General Orders

Back in April 2016 the KiwiSDR was successfully funded on Kickstarter. Since then almost all the rewards have been mailed out and the number of worldwide receivers available on sdr.hu has increased. KiwiSDR is an SDR cape (add on) for the BeagleBone Black/Green embedded computer which covers 0 – 30 MHz with 30 MHz bandwidth. It’s main purpose is to be used as a web based remote receiver which can be publicly accessed by many users.

Over on the Kickstarter updates page we see news that Seeed Studio is taking over the production and distribution of the KiwiSDR, and soon you’ll be able to order the KiwiSDR cape directly from their online Bazaar. Seeed studio is the same company that produces several other capes for the BeagleBone and they also produce the BeagleBone Green which is needed to run the KiwiSDR. They write:

We are very pleased to announce an agreement for Seeed Studio to take over production and distribution of the KiwiSDR going forward. What does this mean? Until now Seeed only had a contract with us to produce the Kickstarter rewards and pre-orders. Now Seeed will add the KiwiSDR to their family of BeagleBone capes they manufacture and distribute. Very soon you’ll be able to order the KiwiSDR directly from Seeed’s online Bazaar, pay directly with a credit card or Paypal and use their shipping system.

For us, and you as Kiwi owners, this is a very positive development. It means soon we’ll be able to devote the majority of our time to software development and providing you support. And as you probably know there is a large list of bugs, feature requests, extensions, distributed experiments and educational material we’d like to be working on instead of worrying about shipping and manufacturing issues. Improving the software is the best way to differentiate ourselves in a crowded SDR marketplace.

We would appreciate it if you would continue to purchase from us until our stock is depleted. Seeed has already manufactured a significant number of units alongside our prior build and will be able to meet the demand immediately. We thank everyone at Seeed for their fantastic effort in making KiwiSDR a reality.

seeed_kiwi

Outernet rxOS Version 3 Released: Automatic Decompression, APRS, NOAA Weather Data, News Updates

Outernet is a new L-band satellite services which aims to be a “library in the sky”. Their satellite signal can be received from almost anywhere in the world, and they aim to constantly transmit data like news, weather updates, books, images/videos and other data files. The service is free and can be received with an RTL-SDR, LNA and patch antenna. We have a full tutorial on receiving their service available here.

The “rxOS” decoder, file management system and web interface GUI has recently been updated to version 3.0. This new version has several new features:

  1. Downloaded files are automatically decompressed after downloading, so they can be viewed directly in the Outernet web interface.
  2. An hourly transmission of APRS data which comes from the repeater on board the international space station. APRS messages can now be relayed across the world via the ISS and Outernet.
  3. This Monday they will begin transmitting NOAA weather data (we are unsure if this entails images or text data yet)
  4. Soon they should begin transmitting news data too.

More details on the update can be found on their forum post. To update the service on a CHIP or Pi 3, download the .pkg file from the links on the forum and choose this file in the Update Firmware section of the Outernet settings menu. 

An example of some received APRS messages from the Outernet.
An example of some received APRS messages from the Outernet.
outernet_aprs
APRS messages

Version 2.6 of GQRX Released

Version 2.6 of the popular SDR program GQRX has just been released (changelog). GQRX is a general signal browsing program similar to programs like SDR#, HDSDR and SDR-Console. However GQRX is designed to run on Linux, MacOS and Raspberry Pi 2 & 3. Note that v2.6 is still a work in progress for MacOS. Apart from the new features and bug fixes, one of the major improvements appears to be reduced CPU usage, meaning that it should run better on older PCs. The changelog is pasted below:

New features

  • 1-2-5 scaling on FFT axis.
  • Audio waterfall.
  • Remember AGC settings between sessions.
  • Right-click on FFT resets frequency zoom.
  • Separate dB ranges for pandapter and waterfall.
  • Raw I/Q mode.
  • Portaudio support.
  • Command line option to set Qt style (fusion, windows, …)
  • Binary packages for Raspberry Pi 2 and 3 (see below)

Bugs fixed

  • Stuttering audio with Pulseaudio backend.
  • Use system font on FFT plot (too small font on high res displays).
  • Broken FUNcube Dongle Pro+ support on Mac OS X 10.11.4.
  • Correct display of negative offsets between -1 and 0 kHz.
  • Reset frequency digits below the one that is being changed.
  • LNB LO could not be set from I/O configuration dialog.
  • Update squelch level when switching between demodulators.
  • Set correct filter range when loading bookmark.
  • White area on waterfall.
  • RFSpace Cloud-IQ support on Mac OS X, RPI binaries and in PPA.

Miscellaneous improvements

  • Input decimator performance.
  • SDRPlay integration through SoapySDR.
  • Only probe for devices when the program is started.
  • Allow user to enter ALSA device name.
  • Set default audio FFT range to -70…0 dB.
  • Restore audio FFT dB scaling between sessions.

GQRX

RTL-SDR.COM Broadcast FM Band-Stop Filter (88-108 MHz Reject) Now for Sale + RTL-SDR V3 Dongle Availability

In our store we have just released our new broadcast FM band-stop filter. The cost is $14.95 USD with free international air mail worldwide shipping. (Of course you may choose to upgrade to faster shipping if desired). This product is currently only shipping from our warehouse in China and is not available on Amazon for the time being.

The filter comes in a 2.8 cm x 2.8 cm x 1.3 cm aluminum enclosure and uses female SMA connectors on each end. Included in the package is also a SMA male to SMA male straight barrel adapter.

Click here to visit our store

Important Shipping Note: Please note that right now it is the long “Golden week” holidays in China (1 – 7 Oct), so this product will probably ship out next week and there may be some delays relating to heavy “catch-up” mail volumes around this time.

The filter attached to our RTL-SDR.COM V3 dongle.
The filter attached to our RTL-SDR.COM V3 dongle.

This filter rejects signals between 88 – 108 MHz with around 50 dB or more attenuation. A broadcast FM band-stop filter is very useful for use with SDRs as in some areas broadcast FM signals can be so strong that they overload the SDR, causing very poor performance in other bands. You can tell if this is the case for you if you see images of BCFM stations or interference that looks like a WFM signal at other frequencies when you turn up the gain.

The filter is based on a simple 7th order Chebyshev design. The 3 dB roll off is at 76 MHz and 122 MHz. 88 MHz is attenuated by almost 60 dB, and 108 MHz is attenuated by 45-50 dB. Outside of the pass band the insertion loss is practically zero below 500 MHz, less than 0.5 dB from 500 MHz – 1 GHz, and below 1.5 dB between 1 GHz – 2 GHz. Between 2 – 3 GHz performance degrades slightly, but insertion loss remains below 1.5 dB for most frequencies. The filter can also pass up to 80 mA of DC current (probably can do more) and has negligible DC resistance.

http://ActiveArea

Active Area

http://InsertionLosses

Insertion Losses

http://BCFMFilterDisconnected

BCFM Filter Disconnected

http://BCFMFilterConnected

BCFM Filter Connected

Below we show a comparison of what a cheap $5 TV FM band-stop filter looks like on the network analyzer. You can see that the stop band is attenuated by significantly less, and the insertion losses on other frequencies are much much larger.

http://Cheap$5TVBCFMFilter

Cheap $5 TV BCFM Filter

http://RTL-SDR.COMBCFMBand-StopFilter

RTL-SDR.COM BCFM Band-Stop Filter

The Cheap $5 TV BCFM Band-Stop Filter
The Cheap $5 TV BCFM Band-Stop Filter

RTL-SDR v3 Availability Note: Our next batch of RTL-SDR v3 dongle stock should be available in 1-2 weeks (there is a delay with restocking due to the Golden week holidays). Production has completed, and we are only waiting for delivery of this stock. Batch 2 includes minor enhancements including slightly tweaked capacitor values, an additional decoupling cap and a fuse on the bias tee.

A Multi-Channel Coherent RTL-SDR Product: For Passive Radar, Direction Finding and More

Coherent-receiver.com is a company which is a customer of our RTL-SDR V3 dongle and they have been working on creating a multi-channel coherent receiver product based on the RTL-SDR. An RTL-SDR multi-channel coherent receiver is at its most basic, two or more RTL-SDR dongles (multi-channel) that are running from a single clock source (coherent). A multi-channel coherent receiver allows signal samples from two different antennas to be synchronized against time, allowing for all sorts of interesting applications such as passive radar and direction finding.

The team at coherent-receiver.com have used the new expansion headers on our V3 dongles to create their product. In their receivers they attach a control board which has a buffered 0.1 PPM TCXO (buffered so it can power multiple RTL-SDR’s). They also added an 8-bit register and I2C connection capabilities which allows for control of future add-on boards. The I2C capability is useful because it means that several RTL-SDR dongles can be controlled and tuned from the same control signal. More information on the registers and build of the receiver control board can be seen on their technical support page.

A ten channel RTL-SDR coherent receiver.
A ten channel RTL-SDR coherent receiver.
The Coherent Receiver block diagram.
The Coherent Receiver block diagram.

One example application of a multi-channel coherent receiver is passive radar. Coincidentally, we’ve just seen the release of new GUI based Passive Radar software by Dr. Daniel Michał Kamiński in yesterdays post. Passive radar works by listening for strong signals bouncing off airborne objects such as planes and meteors, and performing calculations on the signals being received by two antennas connected to the multi-channel coherent receiver.

A second example is direction finding experiments. By setting up several antennas connected to a multichannel coherent receiver calculations can be made to determine the direction a signal is coming from. An interesting example of direction finding with three coherent RTL-SDRs can be seen in this previous post. A third example application is pulsar detection which we have seen in this previous post

Coherent-receiver.com sent us a prototype unit that they made with four of our V3 dongles. In testing we found that the unit is solidly built and works perfectly. We tested it together with Dr. Kamiński’s passive radar software and it ran well, however we do not have the correct directional antennas required to actually use it as a passive radar yet. In the future we hope to obtain these antennas and test the coherent receiver and the software further.

Currently they do not have pricing for these models as it seems that they are first trying to gauge interest in the product. If you are interested in purchasing or learning more they suggest sending an email [email protected] It seems that they are also working on additional RTL-SDR ecosystem products such as filters, downconverters, antennas and LNAs.

We hope that the release of this product and Dr. Kamiński’s software will give a boost to the development of coherent multi-channel receivers as we have not seen much development in this area until recently.

SDRDue running on the coherent-receiver.com unit.
SDRDue running on the coherent-receiver.com unit.

KiwiSDR Soon to Accept General Orders

Back in April 2016 the KiwiSDR was successfully funded on Kickstarter. Since then almost all the rewards have been mailed out and the number of worldwide receivers available on sdr.hu has increased. KiwiSDR is an SDR cape (add on) for the BeagleBone Black/Green embedded computer which covers 0 – 30 MHz with 30 MHz bandwidth. It’s main purpose is to be used as a web based remote receiver which can be publicly accessed by many users.

Over on the Kickstarter updates page we see news that Seeed Studio is taking over the production and distribution of the KiwiSDR, and soon you’ll be able to order the KiwiSDR cape directly from their online Bazaar. Seeed studio is the same company that produces several other capes for the BeagleBone and they also produce the BeagleBone Green which is needed to run the KiwiSDR. They write:

We are very pleased to announce an agreement for Seeed Studio to take over production and distribution of the KiwiSDR going forward. What does this mean? Until now Seeed only had a contract with us to produce the Kickstarter rewards and pre-orders. Now Seeed will add the KiwiSDR to their family of BeagleBone capes they manufacture and distribute. Very soon you’ll be able to order the KiwiSDR directly from Seeed’s online Bazaar, pay directly with a credit card or Paypal and use their shipping system.

For us, and you as Kiwi owners, this is a very positive development. It means soon we’ll be able to devote the majority of our time to software development and providing you support. And as you probably know there is a large list of bugs, feature requests, extensions, distributed experiments and educational material we’d like to be working on instead of worrying about shipping and manufacturing issues. Improving the software is the best way to differentiate ourselves in a crowded SDR marketplace.

We would appreciate it if you would continue to purchase from us until our stock is depleted. Seeed has already manufactured a significant number of units alongside our prior build and will be able to meet the demand immediately. We thank everyone at Seeed for their fantastic effort in making KiwiSDR a reality.

seeed_kiwi

Outernet rxOS Version 3 Released: Automatic Decompression, APRS, NOAA Weather Data, News Updates

Outernet is a new L-band satellite services which aims to be a “library in the sky”. Their satellite signal can be received from almost anywhere in the world, and they aim to constantly transmit data like news, weather updates, books, images/videos and other data files. The service is free and can be received with an RTL-SDR, LNA and patch antenna. We have a full tutorial on receiving their service available here.

The “rxOS” decoder, file management system and web interface GUI has recently been updated to version 3.0. This new version has several new features:

  1. Downloaded files are automatically decompressed after downloading, so they can be viewed directly in the Outernet web interface.
  2. An hourly transmission of APRS data which comes from the repeater on board the international space station. APRS messages can now be relayed across the world via the ISS and Outernet.
  3. This Monday they will begin transmitting NOAA weather data (we are unsure if this entails images or text data yet)
  4. Soon they should begin transmitting news data too.

More details on the update can be found on their forum post. To update the service on a CHIP or Pi 3, download the .pkg file from the links on the forum and choose this file in the Update Firmware section of the Outernet settings menu. 

An example of some received APRS messages from the Outernet.
An example of some received APRS messages from the Outernet.
outernet_aprs
APRS messages

Version 2.6 of GQRX Released

Version 2.6 of the popular SDR program GQRX has just been released (changelog). GQRX is a general signal browsing program similar to programs like SDR#, HDSDR and SDR-Console. However GQRX is designed to run on Linux, MacOS and Raspberry Pi 2 & 3. Note that v2.6 is still a work in progress for MacOS. Apart from the new features and bug fixes, one of the major improvements appears to be reduced CPU usage, meaning that it should run better on older PCs. The changelog is pasted below:

New features

  • 1-2-5 scaling on FFT axis.
  • Audio waterfall.
  • Remember AGC settings between sessions.
  • Right-click on FFT resets frequency zoom.
  • Separate dB ranges for pandapter and waterfall.
  • Raw I/Q mode.
  • Portaudio support.
  • Command line option to set Qt style (fusion, windows, …)
  • Binary packages for Raspberry Pi 2 and 3 (see below)

Bugs fixed

  • Stuttering audio with Pulseaudio backend.
  • Use system font on FFT plot (too small font on high res displays).
  • Broken FUNcube Dongle Pro+ support on Mac OS X 10.11.4.
  • Correct display of negative offsets between -1 and 0 kHz.
  • Reset frequency digits below the one that is being changed.
  • LNB LO could not be set from I/O configuration dialog.
  • Update squelch level when switching between demodulators.
  • Set correct filter range when loading bookmark.
  • White area on waterfall.
  • RFSpace Cloud-IQ support on Mac OS X, RPI binaries and in PPA.

Miscellaneous improvements

  • Input decimator performance.
  • SDRPlay integration through SoapySDR.
  • Only probe for devices when the program is started.
  • Allow user to enter ALSA device name.
  • Set default audio FFT range to -70…0 dB.
  • Restore audio FFT dB scaling between sessions.

GQRX

RTL-SDR.COM Broadcast FM Band-Stop Filter (88-108 MHz Reject) Now for Sale + RTL-SDR V3 Dongle Availability

In our store we have just released our new broadcast FM band-stop filter. The cost is $14.95 USD with free international air mail worldwide shipping. (Of course you may choose to upgrade to faster shipping if desired). This product is currently only shipping from our warehouse in China and is not available on Amazon for the time being.

The filter comes in a 2.8 cm x 2.8 cm x 1.3 cm aluminum enclosure and uses female SMA connectors on each end. Included in the package is also a SMA male to SMA male straight barrel adapter.

Click here to visit our store

Important Shipping Note: Please note that right now it is the long “Golden week” holidays in China (1 – 7 Oct), so this product will probably ship out next week and there may be some delays relating to heavy “catch-up” mail volumes around this time.

The filter attached to our RTL-SDR.COM V3 dongle.
The filter attached to our RTL-SDR.COM V3 dongle.

This filter rejects signals between 88 – 108 MHz with around 50 dB or more attenuation. A broadcast FM band-stop filter is very useful for use with SDRs as in some areas broadcast FM signals can be so strong that they overload the SDR, causing very poor performance in other bands. You can tell if this is the case for you if you see images of BCFM stations or interference that looks like a WFM signal at other frequencies when you turn up the gain.

The filter is based on a simple 7th order Chebyshev design. The 3 dB roll off is at 76 MHz and 122 MHz. 88 MHz is attenuated by almost 60 dB, and 108 MHz is attenuated by 45-50 dB. Outside of the pass band the insertion loss is practically zero below 500 MHz, less than 0.5 dB from 500 MHz – 1 GHz, and below 1.5 dB between 1 GHz – 2 GHz. Between 2 – 3 GHz performance degrades slightly, but insertion loss remains below 1.5 dB for most frequencies. The filter can also pass up to 80 mA of DC current (probably can do more) and has negligible DC resistance.

http://ActiveArea

Active Area

http://InsertionLosses

Insertion Losses

http://BCFMFilterDisconnected

BCFM Filter Disconnected

http://BCFMFilterConnected

BCFM Filter Connected

Below we show a comparison of what a cheap $5 TV FM band-stop filter looks like on the network analyzer. You can see that the stop band is attenuated by significantly less, and the insertion losses on other frequencies are much much larger.

http://Cheap$5TVBCFMFilter

Cheap $5 TV BCFM Filter

http://RTL-SDR.COMBCFMBand-StopFilter

RTL-SDR.COM BCFM Band-Stop Filter

The Cheap $5 TV BCFM Band-Stop Filter
The Cheap $5 TV BCFM Band-Stop Filter

RTL-SDR v3 Availability Note: Our next batch of RTL-SDR v3 dongle stock should be available in 1-2 weeks (there is a delay with restocking due to the Golden week holidays). Production has completed, and we are only waiting for delivery of this stock. Batch 2 includes minor enhancements including slightly tweaked capacitor values, an additional decoupling cap and a fuse on the bias tee.

New ThumbNet RTL-SDR Receiver Released: F-Connector, TCXO, External DC Power, No Switch-Mode Power

ThumbSat is a company that aims to help experimenters design and launch experiments on their mini satellites (10x smaller than a regular cubesat with most of the same functionality) into orbit. They write that for about $20k they will fully design a satellite based experiment and launch it into orbit – all you need to do is provide the orbital experiment that you would like done.

To aide with the reception, they also have the ThumbNet project which aims to setup a network of satellite receivers around the world. They do this by providing school students around the world with low cost satellite receivers. The satellite receivers consist of modified/upgraded RTL-SDR dongles and satellite antennas. 

Today the ThumbNet project announced the latest iteration of their RTL-SDR dongle, called the ThumbNet N3 SDR Receiver. This receiver has some interesting design changes when compared to any other dongle that we’ve seen so far. The biggest change appears to be that this dongle uses an external power port for power. They also replaced the 1.2V switching regulator with a 1.2V linear regulator for lower noise operation. This is useful because switching regulators can cause noise, whilst linear regulators are much cleaner. However, using a linear regulator increases the power consumption significantly, and the new dongle draws 450mA of current (vs 250-280 mA on standard or our V3 dongles), meaning that some USB ports may be unable to power the device unless the external power supply port is used.

The other interesting change is that they have changed the PCB form factor, and it can now fit into a common 1455 aluminum case. Also, similarly to our V3 RTL-SDR dongles, they have decided to add a common mode choke to the USB lines, which significantly reduces USB noise. To add ESD protection they also added a static bleed resistor. Finally, like their previous receivers they continue to use a F-type RF connector and a TCXO for frequency stability.  

The price is $25.75 each plus flat rate global shipping of $4.50 and the receivers are expected to ship in mid-October. While we have not yet tested this model, it looks to be like a good receiver for those who need very low noise, or external power options.

They write:

The next Generation, ThumbNet N3 is designed from the ground up to be as simple to use as older generation dongles, but with powerful hardware features for advanced hobbyists and experimenters.

We removed all of the excess components that were sources of noise or interference in other dongles, and optimized the circuit for simplicity, sensitivity and selectability. Then we added a port to use a cable with the extremely common mini-USB connection so that the N3 is less prone to noise from the host computer than a traditional dongle. Finally, the use of standard Surface Mount 0603 or larger components makes it simple for testing or modification.

We built them for our own use, then decided to offer them to everyone.

A quick list of the features of the N3:

- Full backward compatibility with existing RTL-SDR dongles and software
- High stability TCXO (+/-0.5ppm) (ensuring rock-solid stability from start-up and over a wide range of temperatures)
- Standard R820T2 + RTL2832U (plus 24C02 EEPROM) chipset
- Improved/enhanced decoupling. (Common-mode choke on USB port)
- Low-noise, linear only power regulation (separate 1.2v and 3.3v regulators)
- External DC (+5v, 450mA) supply connector
- Mini-USB connection (allows easy separation of the RF unit from the noisy PC)
- F type RF connector (very common and compatible with existing ThumbNet tracking stations)
- Large (6x4cm) contiguous ground-plane (for better thermal dissipation)
- Static drain-away resistor on the RF input (1K to ground)
- All unnecessary parts (IR receiver, high-current LED etc.) eliminated to reduce parts count and noise
- Circuit board can be mounted into a common 1455 case

Ideal for experimentation:

- Can be connected to an external power supply for very clean power
- All of the important tracks are visible on the top side of the board for easy access
- All of the RF parts are on the top of the board (only regulators and decouplers on the back)
- Logical, simple layout using 0603 (or larger) SMT parts
- IF port break in connector (between front end and IF/USB chip) provided

While not required for operation, the N3 receiver is designed to be able to utilize a clean source of power from an external 5v power supply, instead of using the noisy power line coming from the computer’s USB port. This gives a tremendous advantage to the purist or experimenter who wants to utilize power from the N3 to power any external experiments. (When the external power supply is active, no power is drawn from the USB port to power the N3.)

PLEASE NOTE: The N3 draws approximately 450mA of current and care should be taken, even when using a powered USB hub, as it could possibly exceed the current limit of the USB port.

The ThumbNet N3
The ThumbNet N3
The ThumbNet N3 inside its optional 1455 aluminum enclosure.
The ThumbNet N3 inside its optional 1455 aluminum enclosure.

New RTL-SDR Blog Units Now Available in Store: HF via Direct Sampling, Software Switchable Bias Tee, Less Noise/Spurs

A few months ago we brought out a poll asking readers of this blog what they might like to see in a revised RTL-SDR dongle. We’ve now taken some of those suggestions and implemented them into a brand new dongle. For now the price of the new dongle will remain the same as before at $24.95 USD for the dongle + antenna kit and $19.95 USD for the dongle only, but we may need to increase the price by $1 – $2 within the next few weeks due to our slightly increased manufacturing costs. Worldwide shipping remains free from the Chinese international warehouse, and US customers can order either from the Chinese international warehouse or from Amazon who will give you free shipping if you are a Prime member, or spend over $49. The Chinese warehouse is currently stocked and ready to ship, and Amazon is now stocked and should be ready to ship by the end of this week.

Please go to our store page at rtl-sdr.com/store for information on purchasing.

RTLSDR_Front

RTLSDR_PCB

Here is the short version of the biggest changes:

1) HF support via direct sampling. Connect an HF antenna directly to the SMA connector and tune from 500 kHz – 24 MHz with the direct sampling mod. (No hardware modding or soldering required)
2) Lower internal noise. Less spurs, lower noise floor etc.
3) Software switchable bias tee. No need to do any soldering to enable the bias tee. Can be turned on and off in software.

We call this version three of our RTL-SDR Blog dongles. The first was version zero and was simply the standard MCX dongles with better antennas. Next came version 1 with the bias tee and SMA connector, and version two introduced the metal case.

Here is the long list of improvements and changes, and why they were made:

1) Improved ESD protection on the radio front end. The BAV99 diode which is used on most dongles is not a true ESD rated diode. We have added a real ESD rated diode for better protection. The BAV99 remains in the circuit as a strong signal clipper, to prevent damage to the R820T2 from overly strong signals. Please remember that not even this will save your radio from a lightning strike, and any permanently outdoor mounted antenna system must have its own lightning protection.

2) Longer SMA connector. One or two customers had problems with the shorter SMA plugs which could not fit some of their antenna connectors. The longer shaft fixes this and also allows us to add a nut to fasten it to the aluminum body which provides a better low impedance connection (although this is not strictly needed as the PCB side ground tracks already provide a good connection).

3) Improved front end circuit. The standard matching circuit on the RTL-SDR was designed for DVB-T use, and tends to attenuate signals above ~1 GHz. The new matching circuit has less attenuation above 1 GHz and similar performance below. We used very high quality, high SRF, high Q inductors in this circuit.

4) Added a software switchable 4.5v bias tee. In previous versions of our units the 4.5v bias tee needed to be activated manually, by soldering a bridge between two pads on the PCB. However we found that many customers who want to use the bias tee do not have the skills or tools to be able to perform this mod. The new unit makes use of a low noise LDO and one of the GPIO pins on the RTL2832U to activate the bias tee in software. This of course requires a modification to the drivers, but we will shortly upload a program called rtl_biast and batch files (available now) to turn the bias tee on and off in Windows and Linux.

This bias tee is great for powering a remote LNA (like Adams PSA5043+ based LNA4ALL) or something like the SpyVerter upconverter. We’ve tested it with both and found them to be running just fine. 

Warning: The bias tee LDO can be damaged if you short circuit it. Before turning on the bias tee, ensure the circuit to be powered is not shorted, or that the RTL-SDR is not connected to a DC shorted antenna!

5) Added several access pads on the PCB. Access pads for the unused GPIO pins, CLK in/out, 3.3V, GND and I2C pins have been added. The CLK input/output is disconnected by default (see change 6). Access pads for the I branch have also been added as some users and industrial customers are using these in special projects. These pads are only for advanced users who need them for special projects. Take care as these pins are not ESD protected.

6) Added a clock selector jumper. By soldering in a 4 pin 1.27mm pitch jumper header and removing the default 0 Ohm resistor, one can now easily select between the onboard clock, an external clock, or having the on board clock be the output for another dongle. This is for advanced users only who want to experiment with things like passive radar, and coherent receivers.

7) Reduced noise with a modified PCB design. This significantly reduces spurs and noise pickup due much lower impedance grounding and blocking of interference. Also added a USB common mode choke to reduce USB noise, several ferrite chokes on the PCB, and a lower noise LDO. A larger ground plane also improves on heat dissipation. 

8) Added an experimental HF direct sampling circuit, which is diplexed out from the SMA connector. This has little to no effect on VHF/UHF operation, but allows us to make use of the Q branch on the RTL2832U chip for direct sampling, which allows us to receive from about 500 kHz to about 24 MHz. (Below 500 kHz is unavailable due to attenuation from the bias tee circuit). We used a ~10dB 50 Ohm preamp as a buffer and to overcome losses in the transformer and filter. We also added a strong 24 MHz low pass filter, and added an impedance matching transformer coil to ensure good direct sampling performance.

Of course direct sampling can never be as good as using an upconverter. It can overload easily if you have strong signals since there is no gain control. And you will see aliasing of signals above 14.4 MHz due to Nyquist. But this should at least give the majority of users a decent taste of what’s on HF. If you then find HF interesting, then you can consider upgrading to an upconverter like the SpyVerter (and the SpyVerter is of course compatible with our bias tee for easy operation).

We’re still classing this mode as experimental (and will be interested to hear any feedback on results), but we have had good results in our testing of this mode when receiving signals that are not too strong, getting sensitivity as good as an upconverter. We found that very good reception was obtainable with a long wire antenna and 9:1 unun combination.

9) Antenna bases now come with a stronger magnet and a conductive copper sticker on the bottom. The stronger magnet adds very good stability when using our large 1.5m antenna and the copper sticker ensures that good electrical contact can be made between the base and whatever piece of metal you use underneath as the ground plane. This significantly improves the antenna’s performance as a quarter wave ground plane.

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10) Added corner mounting holes for those who want to stack PCBs. Some customers have been building devices that require multiple RTL-SDR dongles, and these standoff holes should aid in stacking.

As from the previous innovations the units still come with:

1) SMA connector – The most common connector in the radio world. Easy to adapt to other connectors and low loss over a wide range of frequencies.
2) Thermal pad – A thin thermal pad allows heat to transfer from the PCB to the metal case easily. The metal case then cools off to the surrounding air. This helps to solve L-band insensitivity problems.
3) Metal case – Helps block out interference and provides cooling.

We now have a V3 users guide available which explains how to use the new features such as the bias tee, HF mode and CLK jumpers.

What’s coming next?

We think that our unit is now pretty much at the peak of how good a cheap R820T2 RTL-SDR can be, so apart from minor tweaks this is likely to be our last major revision of this model of the RTL-SDR. In a 1-2 months we hope to bring out a FM bandstop filter with metal enclosure and SMA plugs with a target cost of $14.95 shipped. Further into the future we also hope to bring out supporting products like a wideband bias tee powered LNA and wideband antennas. These supporting products will of course be compatible with other SDR’s like the Airspy or SDRplay, or other RTL-SDR dongles.


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RTL-SDR Blog SDR Unit Sale!

As many of you know we have been working on releasing a new revision of our RTL-SDR Blog SDR units for the last few months. We are a few weeks away from being able to release news about this new unit and begin sales.  At the moment we are still confirming the features and testing the prototypes so cannot release any news, but if you want a hint at what features might be coming you can take a look at our previous poll asking RTL-SDR users what they wanted in a low cost SDR. The new units may include some of these features/improvements.

For now we are selling off our current batch at reduced prices. The dongle only package is reduced from $19.95 to $17.95, and while the dongle + antenna kit is sold out in our international store, we have reduced its price from $24.95 down to $23.95 on our Amazon USA store.

You can purchase our units from our store at www.rtl-sdr.com/store.

These prices will only last until this batch of stock runs out, and there are only a few hundred units remaining. This special offer also combines with our 5% off deal if you buy more than two items from our store.

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