Tagged: airspy HF+

Testing out the New Airspy HF+ Preselector

The Airspy team have recently been working on a preselector retrofit product for their HF+. The Airspy HF+ already has excellent dynamic range and sensitivity, but by adding a preselector they seek to improve performance enough to claim that the HF+ is as good as or even better than much more pricey SDRs like the Perseus by achieving dynamic range figures of more than 105 dBm.

A preselector is a filter or bank of filters that attenuates out of band signals. This is useful as radios can desensitize if an unwanted signal comes in too strongly. For example, if you are tuned to the 20m band, but there is a very strong MW signal, the SNR of your desired 20m band signal might be reduced. Radios with a natural high dynamic range design like the Airspy HF+ are less affected by this problem, but for the strongest of signals use of a preselector can still help.

The Airspy HF+ preselector needs to be soldered directly onto the HF+'s PCB, and once installed it automatically switches bands using GPIO expansion ports controlled automatically via tuning in SDR#, so no external switching is required.

The expected pricing of the HF+ preselector is US$49, and it will be ready for sale in a few weeks.

Measurements

We received a prototype of the filter a few days ago and have been testing it out. From measurements on a VNA, we found that the preselector features four bands of operation:

  • 0 - 5.2 MHz
  • 5.2 - 10 MHz
  • 10 - 17 MHz
  • 17 - 30 MHz

Airspy have also provided us with a block diagram schematic which we show below.

HF+ Preselector Schematic
HF+ Preselector Schematic

Insertion loss appears to be mostly below 3 dB with fairly steep skirts especially on the lower side. The top three filters do an excellent job at blocking out the broadcast AM band. Below are some VNA plots that show the filter responses.

VNA_180628_175816
VNA_180628_175907
VNA_180628_180008
VNA_180628_180057

Installation

The preselector comes in a small 3.2 x 1.7 cm sized PCB that is fully covered with a metal shielding can. To install it you need to carefully solder it onto the HF+ PCB. This can be a little tricky since the pads are so small, but if you're experienced with soldering it shouldn't be an issue.

  • First you need to open the HF+ and remove R3 from the HF+ PCB, which is a zero ohm resistor.
  • The preselector PCB can then be positioned and the two IN and OUT pads soldered in place.
  • Then you'll also need to connect the power and 2x GPIO lines to the preselector using wires.
  • Now you need to bridge the two shielding CANs with a thick bit of wire. We simply used two cuts of copper solder braid to do this.
  • Finally is also recommended to update the HF+ firmware to the latest version and download the latest version of SDR#.

Once soldered in place the preselector is ready to use, and the HF+ cover can be put back on. It is expected that the commercially sold versions of the preselector will come with detailed installation instructions. 

In the first photo below we removed the shield to see what was inside, and the second photo shows it installed on the HF+ PCB.

hf+presel_PCB
hf+presel_installed2

Using it on a RTL-SDR V3

Whilst the preselector is designed for the Airspy HF+, there's no reason why it couldn't also be retrofitted onto other SDRs, such as our RTL-SDR V3, for use in improving direct sampling mode performance.

The V3 has spare GPIO ports that can be used to control the filter, and 5V for powering the filter could be tapped off the PCB as well. Currently we're considering making a breakout PCB for the filter than might aide with this.

We did a quick test with the preselector connected to the RTL-SDR V3 running in direct sampling mode, and as expected performance is much better, especially above 5 MHz once the second filter kicks in. This is because the second, third and fourth filters all heavily attenuate the MW broadcast AM band, which is the main source of overload issues on HF.

The following screenshots show how much the filter was able to reduce the signal strength of AM broadcast when the second 5.2 - 10 MHz filter was turned on. This reduction was enough to prevent overload on all the higher bands.

Preselector OFF

Preselector OFF

Preselector ON

Preselector ON

Preselector OFF

Preselector OFF

Preselector ON

Preselector ON

HF+ Results

For the HF+ we tested by injecting a strong signal into two HF+ SDRs, one with the filter installed and the other without. The HF+ with the filter was routinely able to withstand much higher signal powers without any signs of overload occurring, and no degradation due to insertion loss was observed.

The screenshots below show an experiment with a weak desired signal injected at 14.2 MHz, and a strong unwanted signal being injected at 1.5 MHz. With the unwanted signal at 5 dBm, the filtered HF+ showed no signs of overload, whilst the unfiltered HF+ had the AGC kick in to increase the front end attenuation, reducing the signal strength by about 20 dB and raising the noise floor.

Filtered HF+

Filtered HF+

Unfiltered HF+

Unfiltered HF+

Other Reviews

Other reviewers have also received the preselector and have been testing it. Fenu radio has uploaded a short review, and plans to write more in the future. He's also made his HF+ with preselector available for public use via SpyServer (details in his post). In the video below Leif SM5BSZ reviews the preselector and runs through several tests while comparing it against the Perseus. His results seem to show that the Persues gets a +25 dBm IP3, whilst the HF+ with the latest firmware and preselector is able to obtain a respectable +10 dBm IP3. 

hfpluspresel2
hfpluspresel2

Conclusion

For most people, the dynamic range of the HF+ is probably already more than enough, but if you are receiving very strong signals, the preselector can help get you get more performance out of the HF+. Of course the preselector cannot help if you have strong signals within the filter bands.

If you're looking to get the most out of your HF+ then the filter at only $49 is a pretty good deal. Just take note that you'll need to open the HF+ and be comfortable with soldering onto the PCB. 

Testing out the New Airspy HF+ Preselector

The Airspy team have recently been working on a preselector retrofit product for their HF+. The Airspy HF+ already has excellent dynamic range and sensitivity, but by adding a preselector they seek to improve performance enough to claim that the HF+ is as good as or even better than much more pricey SDRs like the Perseus by achieving dynamic range figures of more than 105 dBm.

A preselector is a filter or bank of filters that attenuates out of band signals. This is useful as radios can desensitize if an unwanted signal comes in too strongly. For example, if you are tuned to the 20m band, but there is a very strong MW signal, the SNR of your desired 20m band signal might be reduced. Radios with a natural high dynamic range design like the Airspy HF+ are less affected by this problem, but for the strongest of signals use of a preselector can still help.

The Airspy HF+ preselector needs to be soldered directly onto the HF+'s PCB, and once installed it automatically switches bands using GPIO expansion ports controlled automatically via tuning in SDR#, so no external switching is required.

The expected pricing of the HF+ preselector is US$49, and it will be ready for sale in a few weeks.

Measurements

We received a prototype of the filter a few days ago and have been testing it out. From measurements on a VNA, we found that the preselector features four bands of operation:

  • 0 - 5.2 MHz
  • 5.2 - 10 MHz
  • 10 - 17 MHz
  • 17 - 30 MHz

Airspy have also provided us with a block diagram schematic which we show below.

HF+ Preselector Schematic
HF+ Preselector Schematic

Insertion loss appears to be mostly below 3 dB with fairly steep skirts especially on the lower side. The top three filters do an excellent job at blocking out the broadcast AM band. Below are some VNA plots that show the filter responses.

VNA_180628_175816
VNA_180628_175907
VNA_180628_180008
VNA_180628_180057

Installation

The preselector comes in a small 3.2 x 1.7 cm sized PCB that is fully covered with a metal shielding can. To install it you need to carefully solder it onto the HF+ PCB. This can be a little tricky since the pads are so small, but if you're experienced with soldering it shouldn't be an issue.

  • First you need to open the HF+ and remove R3 from the HF+ PCB, which is a zero ohm resistor.
  • The preselector PCB can then be positioned and the two IN and OUT pads soldered in place.
  • Then you'll also need to connect the power and 2x GPIO lines to the preselector using wires.
  • Now you need to bridge the two shielding CANs with a thick bit of wire. We simply used two cuts of copper solder braid to do this.
  • Finally is also recommended to update the HF+ firmware to the latest version and download the latest version of SDR#.

Once soldered in place the preselector is ready to use, and the HF+ cover can be put back on. It is expected that the commercially sold versions of the preselector will come with detailed installation instructions. 

In the first photo below we removed the shield to see what was inside, and the second photo shows it installed on the HF+ PCB.

hf+presel_PCB
hf+presel_installed2

Using it on a RTL-SDR V3

Whilst the preselector is designed for the Airspy HF+, there's no reason why it couldn't also be retrofitted onto other SDRs, such as our RTL-SDR V3, for use in improving direct sampling mode performance.

The V3 has spare GPIO ports that can be used to control the filter, and 5V for powering the filter could be tapped off the PCB as well. Currently we're considering making a breakout PCB for the filter than might aide with this.

We did a quick test with the preselector connected to the RTL-SDR V3 running in direct sampling mode, and as expected performance is much better, especially above 5 MHz once the second filter kicks in. This is because the second, third and fourth filters all heavily attenuate the MW broadcast AM band, which is the main source of overload issues on HF.

The following screenshots show how much the filter was able to reduce the signal strength of AM broadcast when the second 5.2 - 10 MHz filter was turned on. This reduction was enough to prevent overload on all the higher bands.

Preselector OFF

Preselector OFF

Preselector ON

Preselector ON

Preselector OFF

Preselector OFF

Preselector ON

Preselector ON

HF+ Results

For the HF+ we tested by injecting a strong signal into two HF+ SDRs, one with the filter installed and the other without. The HF+ with the filter was routinely able to withstand much higher signal powers without any signs of overload occurring, and no degradation due to insertion loss was observed.

The screenshots below show an experiment with a weak desired signal injected at 14.2 MHz, and a strong unwanted signal being injected at 1.5 MHz. With the unwanted signal at 5 dBm, the filtered HF+ showed no signs of overload, whilst the unfiltered HF+ had the AGC kick in to increase the front end attenuation, reducing the signal strength by about 20 dB and raising the noise floor.

Filtered HF+

Filtered HF+

Unfiltered HF+

Unfiltered HF+

Other Reviews

Other reviewers have also received the preselector and have been testing it. Fenu radio has uploaded a short review, and plans to write more in the future. He's also made his HF+ with preselector available for public use via SpyServer (details in his post). In the video below Leif SM5BSZ reviews the preselector and runs through several tests while comparing it against the Perseus. His results seem to show that the Persues gets a +25 dBm IP3, whilst the HF+ with the latest firmware and preselector is able to obtain a respectable +10 dBm IP3. 

hfpluspresel2
hfpluspresel2

Conclusion

For most people, the dynamic range of the HF+ is probably already more than enough, but if you are receiving very strong signals, the preselector can help get you get more performance out of the HF+. Of course the preselector cannot help if you have strong signals within the filter bands.

If you're looking to get the most out of your HF+ then the filter at only $49 is a pretty good deal. Just take note that you'll need to open the HF+ and be comfortable with soldering onto the PCB. 

Going Portable with the Airspy HF+, Raspberry Pi and 7-Inch Touch LCD

Over on the swling blog we've seen a post where contributor 'Tudor' demonstrates his Airspy HF+ running nicely on a Raspberry Pi 3, 7-inch touchscreen LCD, and USB power bank. The video shows GQRX running very smoothly on the Pi, and how the setup is able to receive various HF signals. Tudor writes:

I bought the RPi to use it as a Spyserver for my Airspy HF+ SDR.

My main radio listening location is a small house located on a hill outside the city and there is no power grid there (it’s a radio heaven!), so everything has to run on batteries and consume as little power as possible.

My first tests showed that the Raspberry Pi works very well as a Spyserver: the CPU usage stays below 40% and the power consumption is low enough to allow it to run for several hours on a regular USB power bank. If I add a 4G internet connection there I could leave the Spyserver running and connect to it remotely from home.

Then I wondered if the Raspberry Pi would be powerful enough to run a SDR client app. All I needed was a portable screen so I bought the official 7” touchscreen for the RPi.

I installed Gqrx, which offers support for the Airspy HF+. I’m happy to say it works better than I expected, even though Gqrx wasn’t designed to work on such a small screen. The CPU usage is higher than in Spyserver mode (70-80%) but the performance is good. Using a 13000 mAh power bank I get about 3.5 hours of radio listening.

On the swling blog post comments Tudor explains some of his challenges including finding a battery that could supply enough current, finding a low voltage drop micro-USB cable, and reducing the noise emanating from the Raspberry USB bus. Check out the post comments for his full notes. 

Airspy HF+ and Gqrx running on Raspberry Pi
Airspy HF+ and Gqrx running on Raspberry Pi

More Reviews and Discussion on the Airspy HF+

Recently a few more reviews of the HF+ have been released and we list some of them below for those thinking about purchasing one.

SDRPlay RSP-1A vs. Airspy HF+ on Shortwave and Medium Wave

In this video icholakov compares the RSP-1A with the HF+ on shortwave and medium wave reception. He writes:

Comparing reception of two popular SDR Receivers using the same antenna at 5 PM local time. Short wave and medium wave frequencies. Using the same SDR Console 3 software for both. I have not ced enough variances using different usb cables and different host laptops to say that in this case the two are pretty much on par. The laptop running RSP-1A happened to have a better audio profile but that's the laptop not the sdr. I only see a noticeable difference when receiving the low power 10 Watt Travel Information radio from the Florida Turnpike on 1640 kHz. I assume that it is coming via ground wave.

SDRPlay RSP-1A vs. Airspy HF+ on Shortwave and Medium Wave
SDRPlay RSP-1A vs. Airspy HF+ on Shortwave and Medium Wave

Airspy HF+ vs Elad FDM-S2 Weak Signal Comparisons

In this guest post on the swling.com blog Guy Atkins put up a number of audio samples recorded from the HF+ and FDM-S2. The audio samples were not labelled with the radio they came from and he asked readers to vote on which audio sample sounded better. A week later he released the results which showed that the HF+ and FDM-S2 had mostly 50/50 votes, indicating that one did not really sound better than the other.

Airspy HF+ Review - A Nice SDR Receiver

In his blog post Roi Huberman shows a few examples of the HF+ in action and briefly compares it against his SDRplay noting the better dynamic range.

Airspy HF+ Broadcast FM Selectivity Check

In this video by YouTube user stereo11 the selectivity of the HF+ is tested by attempting to receive weak far away stations that are very close to a powerful local station on the frequency spectrum. The HF+ and the SDR# software is able to easily reject the strong station once the IF is adjusted. 

Airspy HF+ SDR - selectivity check next to local
Airspy HF+ SDR - selectivity check next to local

Overview of the HF+

This YouTube video by Radio-Noticias revista española de radioafición is in Spanish, but can be decently autotranslated using YouTube captions to English. It provides a good overview of the HF+ design and features.

Escucha la HF a tope con el receptor SDR Airspy HF+
Escucha la HF a tope con el receptor SDR Airspy HF+

Airspy HF+ MW Overload Problems

All the reviews we've seen so far have praised the HF+ heavily, but it's worth noting that it seems that a few HF+ owners are experiencing significant overloading problems. These are typically users that live very close to powerful MW AM transmitters.

However, the good news is that it seems that a recent firmware patch fixes this issues. The firmware update with instructions can be found at the bottom of the HF+ page on the Airspy site. The firmware update involves opening the case and briefly shorting two pads so it is only really something to do if you are experiencing problems in the first place. It also appears that performing a simple hardware mod helps too.

Previous Reviews

In the past we reviewed the HF+ ourselves and that review can be found here. You can also search our previous posts for various other HF+ reviews that we've posted about before.

Some more information about the Airspy HF+ R3 Bypass Modification

At the beginning of this month we posted a review by Bjarne Mjelde who reviewed the MW DXing performance of the Airspy HF+. One thing he found was that by performing a simple capacitor bypass modification, the performance of the HF+ below 15 MHz could be significantly improved. Over on the SWLing blog we've seen a post that gives a bit more information for those wanting to perform the R3 bypass modification on their HF+'s. There Yousseff, head of the Airspy team wrote:

During the early phases of the design R3 was a place holder for a 0 ohms resistor that allows experimenters to customize the input impedance. For example:

  • A 300 pF capacitor will naturally filter the LW/MW bands for better performance in the HAM bands
  • A 10µH inductor would allow the use of electrically short antennas (E-Field probes) for MW and LW
  • A short (or high value capacitor) would get you the nominal 50 ohms impedance over the entire band, but then it’s the responsibility of the user to make sure his antenna has the right gain at the right band
  • A custom filter can also be inserted between the SMA and the tuner block if so desired.

R3 and the nearby resistors have been intentionally left outside of the RF shield, and their size was picked to be big enough to allow anyone to play with them. You will notice the size difference with the rest of the components.

In general, unless one knows what he’s doing, it’s not recommended to alter a working system. “If it’s working, don’t fix it”. But, we are hobbyists, and not doing so leaves an uncomfortable feeling of something unachieved. Most brands addressing the hobby market leave some tweaks and even label them in the PCB.

The main purpose of the HF+ is the best possible performance on HF at an affordable price. This is to incite HAMs to get started with this wonderful technology while using an SDR that isn’t worse than their existing analog rig.

The MW/LW/VLF crowd may have slightly different requirements, but that can be addressed by shorting a resistor.

The HF+ Mod (Edited by Bjarne, original photo by Nils Schiffhauer)
The HF+ Mod (Edited by Bjarne, original photo by Nils Schiffhauer)

A MW DXers Review of the Airspy HF+

Medium Wave DXer Bjarne Mjelde has recently written up his experiences with the new Airspy HF+ software defined radio. If you weren't already aware, MW DXing is the art of attempting to receive extremely weak and distant broadcast AM stations which may be close to powerful local stations. Generally a radio with high end dynamic range specifications is required for this task. The HF+ is a new low cost SDR that aims to meet those very needs.

In his review Bjarne noted that the MW band sensitivity of the HF+ was good, but not quite as good as the more expensive Perseus and Cloud-IQ SDRs. He also noted that the LW band was more attenuated than expected. However, he discovered that there is an optional hardware modification for the HF+ that involves simply bypassing a capacitor on the PCB with a short circuit. After performing this mod Bjarne found that the sensitivity was significantly improved on the MW and LW bands. Also although sensitivity above 15 MHz was expected to be reduced, Bjarne found no noticeable detrimental effects.

Bjarne concludes that the HF+ is a very capable receiver that after modding satisfies the needs of a demanding MW DXer, although he does note the drawback of the limited 660 kHz of bandwidth. In other previous reviews of low cost SDRs on his blog, Bjarne  reviewed the SDRplay RSP1A, ColibiriNano and the Airspy R2 + SpyVerter. Basically he found that none really satisfied his MW DXing needs, with the RSP1A being suprisingly good but failing with strong signals, the Airspy R2 + SpyVerter having too high of a noise floor, and the ColibriNANO being okay, but with a high internal noise level.

The HF+ Mod (Edited by Bjarne, original photo by Nils Schiffhauer)
The HF+ Mod (Edited by Bjarne, original photo by Nils Schiffhauer)

OpenWebRX now Supports the Airspy HF+

Thanks to Stefan Dambeck for letting us know that there is now a fork of libairspyhf made by DL9RDZ which contains an adapted version of airspyhf_rx (the raw IQ generator). This enables the Airspy HF+ to be easily integrated into OpenWebRX.

If you weren't aware, OpenWebRX is a browser based SDR interface and server software that allows an SDR to be used by multiple people at the same time over the internet. It performs audio demodulation and compression on the server side allowing for very low and efficient network usage.  In this way it is different to Airspy official server solution SpyServer which sends the IQ data over the network. So an OpenWebRX server uses significantly less network bandwidth and might be more suitable for those on slower or capped internet connections.

At the moment we're not seeing any public HF+ servers available on the OpenWebRX database at sdr.hu, but this may change in the future.

Airspy HF+ Running on the OpenWebRX Web Browser Interface
Airspy HF+ Running on the OpenWebRX Web Browser Interface

Leif Compares the Airspy HF+ with the Airspy+Spyverter Combo

Over on YouTube Leif (sm5bsz) has uploaded a video where he compares the Airspy HF+ with the Airspy+Spyverter combination. In the test he compares the two radios at 7 MHz. The signals come in from an antenna, are amplified and then passed into a notch filter which notches at 7.198 MHz. The antenna signal is then passed into an attenuator, and then through a directional coupler and then split into the two radios. A signal generator is used to inject a signal via the directional coupler at the notch frequency, and this signal is used to compare the two radios. This method stops antenna noise from appearing at the notch frequency and so any non-linearities appearing in the notch must be a problem with the radio.

The results that Leif finds are quoted below. They show that although the Airspy HF+ has good linearity, it can still be significantly improved in tough environments by adding a front end filter for the band of interest.

The Airspy HF+ and the Airspy+Spyverter are compared on 7 MHz with and without a band pass filter on the input. Without the filter the HF+ is a little better than the Airspy+Spyverter combo, but when the filter is inserted, the HF+ is MUCH better than the combo.

hfplusfilter
hfplusfilter

In an earlier video Leif also compares the two Airspy units on FM broadcast and the 2 meter band. Again he shows that the Airspy HF+ is better than the standard Airspy, but adding a filter to block out the broadcast FM can still help fairly significantly when trying to listen to the 2M band on the HF+.

airspies
airspies

Video Comparing the Airspy HF+ and KiwiSDR

Over on YouTube user icholakov has uploaded a video comparing the Airspy HF+ with the KiwiSDR. The Airspy HF+ and KiwiSDR are both high performance yet low cost SDR platforms. The differences are that the Airspy HF+ is normally connected directly to a PC (but can be run remotely too) whereas the KiwiSDR is designed to be run remotely only, and so can only be accessed through a browser platform. In addition the HF+ only has maximum live bandwidth of 660 kHz whereas the KiwiSDR samples the entire 30 MHz of the HF band. Both are very sensitive and fairly resistant to overloading, but the HF+ should be better in both regards.

In his video icholakov does side by side comparisons with each radio. He writes

Comparing short wave and medium wave reception from Airspy HF+ SDR Console 3 and KiwiSDR with its built in web server. Using the same 80m dipole antenna. No special noise cancelling on the Airspy HF Plus.

Airspy HF+ vs. Kiwisdr SDR Radio comparison
Airspy HF+ vs. Kiwisdr SDR Radio comparison