Tagged: noise

Making a Cheap Noise Figure Indicator with an RTL-SDR

Amateur radio hobbyists Frank Schmaling (DL2ALF), Wolf-Henning Rech (DF9IC) and Alexander Kurpiers (DL8AAU) have uploaded a pdf document containing slides which show how they made a cheap noise figure indicator using an E4000 RTL-SDR dongle. A noise figure indicator can be used to check the noise figure of various components used in RF applications.

The hardware of the device consists of an RTL-SDR, a MGZ 30889 preamp, a noise source, a 28V boost converter to power the noise source and a serial to USB converter to control the noise source. They also created their own custom software in C# to go along with the hardware.

Their results showed that this setup was comparable to a professional noise figure test set.

RTL-SDR based Noise Figure Indicator
RTL-SDR based Noise Figure Indicator
Noise Figure Indicator Software
Noise Figure Indicator Software

The Effect of Noise Produced by the RTL-SDR

Over on YouTube Adam, the creator of the LNA4ALL, LNA4HF and UP100 upconverter has uploaded a video showing that the noise that is produced by the RTL-SDR dongle itself can degrade performance when combined with an LNA and/or upconverter.

Most commonly we’ve seen people mount the RTL-SDR dongle together with an upconverter and/or LNA in the same shielded box right next to each other. However, these results show that the RTL-SDR should be shielded separately from the LNA and upconverter for best performance.

Unshielded DVB-T dongle may cause the problems

Tip to Reduce Radio Interference on the RTL-SDR

A few months back we posted about a tip to reduce RFI (Radio Frequency Interference) on the RTL-SDR. Now Akos from the SDR for mariners blog has tried this tip for himself and written about his experience with it, after getting inspired to by a post in the rtl-sdr.com/forum.

To reduce RFI, the tip recommends disconnecting the shield connection of the USB cable from the ground connection of the RTL-SDR dongle. This overcomes a design flaw in the RTL-SDR which allows the shield of the USB extension cable to act as an antenna, causing unwanted RFI.

What Akos did was to remove the metal part of the USB extension cables connector to prevent any ground connection. This already reduced an interfering signal by 10dB. He also found that wrapping the connection point in foil further reduced the noise. Connecting coax to the ground then coiling it up and putting the RTL-SDR in the center of the coil also appears to significantly reduce RFI.

Update: Akos has also tried using ferrite chokes on the USB cable, and also found they significantly reduce interference.

Check out the full post by Akos for more information here

RFI noise reduction in the RTL-SDR

Tip: Effect of Power Sources on the Ham It Up Upconverter

The Ham-It-Up upconverter uses a 5V USB power input. I discovered that different 5v power sources can cause significant interference with this upconverter, and the same effect will probably occur in other upconverters as well.

When the upconverter was powered by mains power via a phone charger, the signals were almost completely drowned out in noise. Powering it with a PC USB port was better, but the PC USB power introduced some other strong noise sources. Powering it with a battery (used a mobile phone with OTG cable) was the best option. There are still some strong noise sources present, but I can probably solve them with better shielding.

Click continue reading to see some comparison images.

Continue reading

Reducing Interference in the RTL-SDR

Luis Colunga has posted an interesting tip on his blog on how he reduced broadcast FM interference in the rtl-sdr dongle. He writes

Many people have noticed that even if there is not an antenna connected to the RTL SDR FM Stations still come strong. This is not good at all because these are signals that are getting into the PCB via another way that is not the antenna.

He then goes on to explain a method he used to significantly reduce the interference, which involves removing the male USB connector from the dongle PCB, and fitting your own USB cable leaving the shield floating, which he then wraps with aluminium foil.

Interference Reduction Mod

Check out the rest of his post here.