Search results for: noise

SWLing Blog: Building a Homemade YouLoop (Noise-Cancelling Passive Loop) Antenna

Over on the SWLing Post Blog Thomas has uploaded an excellent tutorial showing how you can build your own YouLoop (aka a Noise-Cancelling Passive Loop). If you've been following our previous posts you'll know that we recently started selling the "YouLoop" which is designed and produced by Youssef from Airspy. The YouLoop is a passive loop antenna designed for HF reception, but also works well up until VHF. The main catch is that you need to use it with a receiver with a low noise figure front end, like the Airspy HF+ Discovery (SDRplay units should work well too). The RTL-SDR Blog V3 in direct sampling mode does somewhat work with it to an extent, but RTL-SDRs relying on upconverters for HF will probably see poor results.

We are selling the loop in our store for $34.95 including free shipping to most countries. Batch 2 is currently in preorder, but is almost sold out and should begin shipping soon. Batch 3 will also be available for preorder soon and is about 2 weeks away from shipping. We also expect there to be a high quality pre-amp available for sale in a few months too which will help those with higher noise figure radios or longer feed line runs. 

Alternatively, as the YouLoop is a relatively simple and openly shared design it is possible to homebrew your own if you want to. Over on the popular SWLing Post blog, author Thomas has written up a full tutorial on hombrewing your own. The parts you need include coax cable, a BN-73-302 wideband 2-hole ferrite core, magnet wire, heat shrink tubing and electrical tape. The guide takes you through the process of winding the balun and constructing the loop using simple tools and a soldering iron.

Demonstrating Solar Inverter Noise Cancelling with a Timewave ANC-4 and Airspy HF+

At his house W1VLF found that his solar inverter was causing huge amounts of interference on the HF bands, essentially making any hope at receiving shortwave or amateur radio signals impossible on his Airspy HF+ Discovery . However, over on his YouTube channel he's demonstrated a solution that allows him to almost completely cancel the noise.

The solution involves using a Timewave ANC-4 noise canceler, which is as analog noise cancelling device available from the manufacturer for US$209.95. To use the device you also need a noise probe which can be a small loop antenna. The noise probe is connected to the ANC-4 and placed near the source of the noise, which in W1VLF's case was just on the solar inverter enclosure mounted on the outside of his house. Then by tuning the gain and phase knobs on the ANC-4 the noise can be cancelled out of the signals received by the main antenna. 

In the video W1VLF demonstrates how effective noise cancelling with the ANC-4 can be by showing the before and after results with his Airspy HF+ Discovery.

Kicking Solar inverter noise in the A$$ with noise cancelling

Kickstarter for a Lower Cost Laboratory Grade RF Broadband Noise Generator

UPDATE: John wanted to add the following clarification:

In the article you wrote, you suggest that the two noise generators are equivalent. This is not the case. The noise generator you mentioned generates "Thermal Noise". Mine generates "Shot Noise".

The Zener in the generator you mentioned, contributes zero noise in the Gigahertz spectrum. Whereas mine generates Shot Noise roughly 20 db higher than the thermal noise floor all across the range.

The physics and the mathematics are as different as night and day.

Thank you to John Jackson from Jackson Research for writing in and sharing his Kickstarter for a laboratory grade RF broadband noise generator. John notes that he's discovered a semiconductor that is much cheaper than the expensive avalanche diodes used in other high end noise sources. This semiconductor has allowed him to create a noise source that works from 40 MHz up to 6 GHz. The target output power is 0 dB, with a flatness of less than 2 dBm over the entire bandwidth.

A noise source is useful for SDR/radio experiments as it is used in many applications such as scalar network analyzers, filter response measurements, satellite tuning, cellphone network tuning, antenna VSWR measurements, amplifier noise measurements and in jammers. If you're interested we have a previously posted tutorial about using an RTL-SDR and noise source for filter characterization and antenna VSWR measurements.

John is fundraising via Kickstarter in order to help fund development of the final product. The pricing is currently US$500. This might seem high, but John notes that comparable professional noise sources start at US$3,500 and go up to US$30,000. John writes:

There are a number of laboratory grade RF Noise generators on the market. They are all extremely expensive ranging in the thousands of dollars. This is beyond the reach of most individuals. In contrast, there are some low cost RF noise generator schematics floating around the Internet which are all based upon Zener diodes.

The problem with Zener diodes is the noise bandwidth. All the circuits I have seen have cutoff frequencies in the Megahertz range. The one noise circuit I saw used in the Gigahertz range was actually amplifier noise and had problems due to the several stages of amplification.

The alternative to Zener diodes is the avalanche diode specifically designed as a RF noise generator. These devices are difficult to acquire and have very high prices which often exceed the entire cost of all the parts and components needed to build the instrument.

Professor Jackson at Jackson Research has discovered a semiconductor that generates RF noise like an avalanche noise diode into the Gigahertz range, but at Zener diode prices. He has raw laboratory data and now needs to build up a professional grade instrument. To achieve this goal, he has launched a new KickStarter fund raiser to build up a number of units.

Jackson Research Laboratory Grade Broadband RF Noise Source
Jackson Research Laboratory Grade Broadband RF Noise Source

Reducing HF Electrical Noise by Using a Faraday Cage for Switch-Mode Power Supplies

Over on his blog, DXer OH2-2192 was frustrated by lots of local electrical noise showing up on the HF bands on his Airspy + Spyverter SDR receiver. He discovered that the majority of the noise he was seeing was coming from the switch mode power supplies that power the electronic devices used in his setup. Switch mode power supplies are very common in the modern world, with almost every electronic device using one to efficiently convert wall AC into DC power. However, they convert power by rapidly switching on and off, and these on/off square wave pulses cause a lot of RF noise especially on the HF spectrum.

Instead of changing to noise free linear regulators which are expensive, very heavy and big, OH2-2192 decided that he'd try creating a Faraday cage shield out of metal mesh to enclose all his switch-mode power supplies. Using a simple AM loop antenna and Airspy's Spectrum Spy software he measured the amount of noise produced by a switch-mode supply placed inside and outside of the cage. The results he saw were very promising with the shielded supply eliminating the noise almost entirely.

Faraday Cage Shield for Switch Mode Power Supplies
Faraday Cage Shield for Switch Mode Power Supplies (Right image shows the results of a switching supply placed outside and inside the Faraday cage)

Noise Cancellation using Linrad and the Phase Coherent Capabilities of an RSPduo

The RSPduo is the latest product from SDRplay, and it's main defining feature is it's dual RX channels. The two channels allow you to simultaneously listen to two stations that may be at opposite ends of the spectrum. Of course the same could be done cheaper with two separate RSP1A devices, however the advantage of the RSPduo is that the two channels are phase coherent. This opens up a lot of technical possibilities such as active noise cancelling/spatial filtering of an interfering signal or unwanted RF noise.

The basic idea behind phase coherent noise cancelling is that you have one antenna pointed towards your signal of interest, and the other pointing towards the interfering signal or coupled to the unwanted noise source. The unwanted signal/noise can then be subtracted from the signal of interest, resulting in a cleaner signal. This can only be done if the two channels are phase coherent like they are on the RSPduo.

Over on YouTube and his blog, ICAS Enterprises has been experimenting with noise cancelling on his RSPduo [Part 1] [Part 2]. The blog is entirely in Japanese, but all the relevant screenshots are in English, and we have provided Google Translated links. SDRplay have also provided a description of the process on their blog. The idea is to use Linrad to process the sound output of the two RSPduo channels from SDRuno. Linrad has capabilities that allow you to subtract two signals from one another.

In his experiments he receives a known 50 MHz CW beacon, and then injects an interfering test signal into the system. Then using Linrads 'signal adaption' feature he is able to completely remove the interference. 

RSPduoでのノイズキャンセルの実験 - Linrad使用

Video on using an RTL-SDR + Noise Generator as a Poor Man’s Network Analyzer

Over on YouTube user AE0AI has uploaded a video where he explains how he uses an RTL-SDR and a home made noise source as a poor man's network analyzer. A network analyzer is a tool that allows you to analyze the response of RF devices, such as filters. By using a noise source together with an RTL-SDR the same functionality as a network analyzer can be obtained, however of course with less accuracy.

In the video AE0AI shows us his home made noise generator, which is a based on a simple circuit that he found online. He then shows the noise generator connected to the RTL-SDR, which shows that his home made generator works up to about 40 MHz. Later in the video he tests a home made 40m filter with the noise source and RTL-SDR, and the response is easily visible. With the response visible he is able to tune the filter by adjusting the inductor windings.

We have a tutorial on the same concepts available here.

Poor Man's network analyzer for measuring filters (noise generator + RTL-SDR)

Measuring the Noise Figure of Airspy and HackRF SDRs in Real Time

The Noise Figure (NF) is an important metric for low noise amplifiers and SDRs. It's a measure of how much components in the signal chain degrade the SNR of a signal, so a low noise figure metric indicates a more sensitive receiver. The Noise Figure of a radio system is almost entirely determined by the very first amplifier in the signal chain (the one closest to the antenna), which is why it can be very beneficial to have a low NF LNA placed right at the antenna

Over on his blog Rowetel has been attempting to measure the noise figure of his HackRF and Airspy, and also with the SDRs connected to an LNA. He's managed to come up with a method for measuring the noise figure of these devices in real time. The method involves using a GNU Octave script that he created and a calibrated signal generator.

It’s a GNU Octave script called nf_from_stdio.m that accepts a sample stream from stdio. It assumes the signal contains a sine wave test tone from a calibrated signal generator, and noise from the receiver under test. By sampling the test tone it can establish the gain of the receiver, and by sampling the noise spectrum an estimate of the noise power.

As expected, Rowetel found that the overall noise figure was significantly reduced with the LNA in place, with the Airspy's measuring a noise figure of 1.7/2.2 dB, and the HackRF measuring at 3.4 dB. Without the LNA in place, the Airspy's had a noise figure of 7/7.9 dB, whilst the HackRF measured at 11.1 dB.

Some very interesting sources of noise figure degradation were discovered during Rowetel's tests. For example the Airspy measured a NF 1 dB worse when used on a different USB port, and using a USB extension cable with ferrites helped too. He also found that lose connectors could make the NF a few dB's worse, and even the position of the SDR and other equipment on his desk had an effect.

Noise figure measurement
Noise figure measurement

Identifying Noise Sources in the Shack using an SDR and an Active Receive Loop

Over on YouTube user SignalSearch has uploaded a video showing how he uses an active magnetic loop antenna indoors to identify local noise sources. Magnetic loop antennas are directional, meaning that they receive best when pointing towards a signal. This means that they also receive noise better when pointed at a noise source.  In the video SignalSearch uses a W6LVP receive loop antenna and demonstrates noise being emitted from his lightbulb, and from a plug in Ethernet over powerline adapter, which are known to be huge sources of HF noise.

If you are interested in the noise produced by these Ethernet over powerline adapters then we did a previous post on this problem over here.

Using an Active Receive Loop Indoors & utilizing Software Defined Radio to identify noise sources