Tagged: direct sampling

New Comparison Videos from Leif SM5BSZ: Airspy vs SDRplay vs Several Other SDRs

Over on YouTube Leif SM5BSZ has uploaded two new videos. The first video shows a set up that compares the Airspy and the SDRplay RSP on several lab tests that test for dynamic range performance at various frequency offsets. The Airspy definitely shows better results, but Leif notes that the differences are fairly small. The Airspy and SDRplay are two SDRs that compete in the mid range SDR price bracket.

Smaller is better, where each value represents the amount of attenuation required before saturation
Smaller is better. Each value represents the amount of attenuation used (in dBm) that causes a 3dB loss from reciprocal mixing

As lab tests can only approximate real world performance, in the next video Leif does a HF reception comparison on a real world antenna. In this video he compares our RTL-SDR.com V3 in the special direct sampling HF mode, a Funcube Pro+, SDRplay RSP, Airspy+Sypverter, Afedri Net, and an FDM-S1. The test injects an artificial signal and combines signals from a real antenna via an adjustable attenuator. Leif adjusts the attenuator to increase the antenna signals until the test signal strength is degraded by 3dB from reciprocal mixing/overload. That attenuation setting is then recorded.

The results for the daytime and nighttime results results rank the SDR’s in order from best to worst: FSM-S1 ($400 + shipping), Afedri ($259 + shipping), Airspy+Spyverter ($218 + shipping/$149 + shipping (mini)), SDRplay ($129 + shipping), Funcube Pro+ ($155 + shipping), RTL-SDR.com V3 direct sampling ($20 incl shipping). Interestingly the performance seems to correlate nicely with the unit cost. Of course the V3 in direct sampling mode can be significantly improved by using filtering on the front end, or just by using an upconverter and quadrature mode instead.

At the end of the video Leif also shows a final ranking of the HF performance of all radios tested in his previous videos.

Night time reception SDR ranking
Night time reception SDR ranking
Daytime reception SDR ranking
Daytime reception SDR ranking
Final Ranking
Final Ranking

RTLSDR4Everyone: Review of the Nooelec SMArt SDR, Direct Sampling and Generic vs Premium Dongles

RTL-SDR enthusiast and blogger Akos has recently uploaded three new articles. In his first article he discusses what he believes is the differences and advantages of Generic vs Premium branded RTL-SDR dongles.

In his second article he shows how easy it can be to perform the direct sampling mod on newer dongles, as most have the direct sampling break out pads. He shows how it can be as easy as sticking a wire into these holes. Please note that if doing this we would caution you to take decent ESD precautions as these pins are not ESD protected.

In the third article he reviews the recently release Nooelec SMArt dongle. The SMArt is a new RTL-SDR variant which comes in a smaller black case, cooling via thermal pads and with an SMA connector. With these modifications it is very similar to our RTL-SDR.com units, however the one advantage of the SMArt is that it is small enough to fit two side by side on closely spaced USB ports, like on the Raspberry Pi. In the post he shows what is inside the SMArt and discusses various points such as heat generated, included antennas and performance.

Inside the new Nooelec SMArt RTL-SDR dongle.
Inside the new Nooelec SMArt RTL-SDR dongle.

Using a direct sampling enabled Airspy as a Panadapter for a Yaesu FTDX-5000

Tim Havens is an avid CW operator on the ham bands and primarily uses his Yaesu FTDX-5000 transceiver for this purpose. At the same time he also uses a software defined radio coupled with an upconverter as a panadapter by connecting the SDR to the 9 MHz IF output of the Yaesu.

However a problem Tim encountered was that the frequency drift of any SDR he tried was too large, even with a TCXO based software defined radio (like a modded RTL-SDR or the Airspy), and that it was a constant hassle to recalibrate. Furthermore, he noticed that the upconverters he used introduced their own drift which just added to the overall frequency drift.

To get around this Tim decided to use the Airspy in a special configuration. First he used the external clock input of the Airspy to connect to his Jackson Labs “Fury” GPSDO. This device uses GPS satellites to generate a very accurate 10 MHz clock, with almost zero drift. Secondly, to get around the need for an upconverter with it’s own frequency drift he used the ADC1 direct sampling input ports on the Airspy to connect to the 9MHz IF output of his FTDX-5000 through an extra band pass filter and LNA.

Tim writes that he will soon update his post with more images and a video.

Airspy with external GPS clock and ADC1 output connected.
Airspy with external GPS clock and ADC1 output connected.

Optimizing Reception of VLF frequencies on an RTL-SDR Dongle

Over on his web site, Martin (G8JNJ) has uploaded a new article discussing how to optimize reception of VLF (<10 kHz) signals on a direct sampling modified RTL-SDR. He writes that the main problem with the RTL-SDR receiving VLF frequencies is that there is significant overloading caused by broadcast stations operating on the LF and MF bands.

So in order to optimize VLF reception, Martin built an external frequency equalisation network which has the following components and functions:

  • 2MHz Low pass Filter – to minimise alias signals originating at 30MHz
  • 20dB Variable attenuator – to set the overall signal level fed into the dongle
  • Switched LF roll-off – to optimise the performance at frequencies around 10KHz in the presence of strong lightning surges
  • -10dB notch at 198KHz – to reduce level of BBC R4 broadcast station in the LF band
  • -10dB notch at 800KHz – to reduce level of local broadcast stations in the MF band

His screenshot results show that his filters work well and significantly reduce the effect of lightning pulse noise at 9 kHz. With the filters in place and properly optimized with the attenuator and various switches, he is able to receive Russian Alpha navigation signals at frequencies around 12 and 14 kHz and the 300 to 500 kHz aeronautical and maritime navigation bands.

G8JNJ's LF Band Equalizer Circuit
G8JNJ’s LF Band Equalizer Circuit

Direct Sampling 14 MHz Low Pass Filter and Galvanic Isolator for the RTL-SDR now for sale in Japan

Previously we posted about Japanese RTL-SDR experimenter Nobu and his work in prototyping a new 14 MHz low pass filter and galvanic isolator for use with the RTL-SDR. The low pass filter improves reception when using the RTL-SDR in direct sampling mode by reducing out of band interference and the galvanic isolator (isolation transformer) reduces computer and other noise when using the RTL-SDR with an upconverter.

The products can be bought from the Japanese Amazon store [Galvanic Isolator] [Low Pass Filter], however to purchase from outside of Japan you will need to use a third party shopping service available at http://agent.jzool.com/.

If you are interested in Nobu’s products, he also has direct sampling modified RTL-SDR dongles and dongles with 10 ppm crystals available, as well as his own upconverter for sale in Japan.

Galvanic Isolator
Galvanic Isolator
14 MHz Low Pass Filter
14 MHz Low Pass Filter

Low Pass Filter for RTL-SDR Direct Sampling Mode

Over on his blog (in Japanese) Nobu has been working on prototyping a 14 MHz low pass filter (LPF) product for direct sampling modified RTL-SDR dongles (in Japanese, use Google Translate). Direct sampling mode is a hardware modification that allows the tuner chip in RTL-SDR dongles to be bypassed, allowing reception of signals between 0 – 14 MHz. However, after performing this mod there is no filtering and images from higher frequencies such as broadcast FM can be problematic. To fix these problems a low pass filter is required.

Another product Nobu is working on is an isolation transformer (aka Galvanic Isolator) which can be used together with an upconverter to help reduce noise generated from common ground sources such as the PC. The isolation transformer is inserted between an upconverter and antenna.

Low Pass Filter (Top), Isolation Transformer (Bottom)
Low Pass Filter (Top), Isolation Transformer (Bottom)

In the image below Nobu shows the effect of inserting the LPF . An interfering FM broadcast band signal is removed after inserting the LPF.

Effect of inserting the Low Pass Filter
Effect of inserting the Low Pass Filter

The image below shows the effect of the isolation transformer showing a clear decrease in noise floor and increase in signal strength.

Effect of an Isolation Transformer when used with an Upconverter
Effect of an Isolation Transformer when used with an Upconverter

Airspy Direct Digital Converter (DDC) Modification

The Airspy software defined radio comes with breakout pins that connect directly to the analogue to digital converter (ADC) inputs. By connecting an antenna to these inputs together with experimental software it is possible to receive frequencies between 0 – 10 MHz. This is the same trick used in the RTL-SDR direct sampling mod. However, to get decent reception and to keep the Airspy circuitry safe, a low pass filter and isolation transformer are required.

Over on YouTube W9RAN discusses this mod in his latest YouTube video.


New Experimental R820T RTL-SDR Driver that Tunes down to 13 MHz or Lower

Over on the Osmocom mailing list, Oliver Jowett an RTL-SDR experimenter has posted about his new experimental driver for the R820T RTL-SDR which extends the tunable range down to around 13 and up to 1864 MHz (previously 24 – 1766 MHz). Oliver writes

You can get these changes from https://github.com/mutability/rtl-sdr/(you’ll need to build from source yourself). There should be no application changes needed, just tune as normal. (gqrx needs the “no limits” option turned on) These changes work by limiting the tuner to a range of frequencies that it can reliably tune to, then allowing tuning beyond those bounds by making the 2832’s downconverter do the final bit of tuning. This can add up to 14.4MHz to each end of the range. Also, the tuner is switched to low-side mixing at the top of the range which gives a bit more range there. The practical range is limited by the width of the IF filter and aliasing effects at the extreme edges of the downconverter’s range. I’ve been able to pick up broadcast AM and amateur CW/SSB down to around 15.5MHz without too much trouble. I’d be interested to know how this works for others. Also.. these changes are likely to have broken offset tuning, direct sampling mods, and tuners other than the R820T, as it touches all those areas but I only have an unmodified R820T to test against. If you have different hardware and are willing to spend some time testing then please let me know. I expect that the range of the other tuners can be extended in the same way with not much trouble.

Over on the Reddit RTL-SDR discussion board there has been talk about this patch. Most users are reporting that it works well down to around 15 MHz, but some people are reporting that they have been able to receive signals down to around 4 MHz. Testers also report that this modified driver works much better than the no-hardware direct sampling mod patch released a few months ago. Update: Commenter J.B has been kind enough to upload a ready to go Windows binary for SDR#. Download here https://db.tt/0JuVpWBL. Simply copy the files in the zip into the SDR# folder.

Reddit user gat3way was able to take this screenshot showing AM reception at 9.5 MHz
Reddit user gat3way took this screenshot showing AM reception at 9.5 MHz