Designing a Low Noise UHF Front End with Sharp Filtering for SDR

Most wideband SDR’s do not come with any front-end filtering built in. This limits their ability to receive weak signals in the presence of strong signals. Recently Sivan, a reader of wrote in to let us know about a paper he published through the ARRL detailing how to design a concrete front-end unit for SDR use. A front-end helps to filter out signals that are outside of the desired passband, thus reducing interference from nearby strong signals significantly. Although Sivan uses a USRP with WBX daughtercard in his paper, he writes that the same front-end design principals can be applied to the RTL-SDR as well.

In the paper he designs a 431 – 435 MHz front-end using low cost SAW filters, a low noise amplifier (LNA) and a limiter to protect the radio. He writes that the design could easily be adapted for other bands as well.

A Selective and Robust UHF Front-End
A Selective and Robust UHF Front-End

SDR# Frequency Scanner Plugin Updated

The popular frequency scanner plugin for SDR# has recently been updated (in Russian, use Google Translate). The frequency scanner plugin allows you to quickly scan through a definable slice of spectrum, stopping to listen on active signals. The new features are outlined below

1. Added the selection of multiple ranges to scan. Ranges can be different modulation and bandwidth scanning step. To select multiple ranges, use Ctrl .

2. The scanner operates completely independently of native squelch SDR #. To determine the end of the activity, a new parameter Hysteresis – is the level below which the signal is considered inactive. When using the option Use Mute , you can disable the squelch SDR #.

3. In setting the added options – choose the scanning signal with the maximum level and the width of the spectrum used.

4. Parameter Detect moved from the window in the panorama scanner settings.

Updated fast scanner plugin.
Updated fast scanner plugin.

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.

Compressing and Filtering I/Q Data

One great feature of SDR software like SDR# is the ability to make I/Q recordings of the entire received bandwidth for later analysis. The problem is that these recordings can take up massive amounts of hard disk space. Jay Moore, the author of the RTL-SDR DX blog has recently been experimenting with methods for compressing I/Q files.

Jay tried compressing a test I/Q file with 7-Zip LZMA, Zip Deflate and FLAC. His results showed that FLAC compression was by far the fastest and also compressed the most in a test I/Q file with low amounts of redundant information.

Jay also discovered that it is possible to record “filtered” I/Q recordings using the RAW setting in SDR#. The RAW setting allows you to record an I/Q file from the tuned bandwidth selected within SDR# which allows for a much smaller file size.

I/Q Data interpreted by an audio editor
I/Q Data interpreted by an audio editor

Updates to Removing the 1.2V Switching Power Supply on the RTL-SDR

A few months ago we posted about how a Japanese RTL-SDR modder disconnected the noisy 1.2v switching power supply on the RTL-SDR and replaced it with three diodes. The diodes reduced the voltage from the 3.3v line to 1.2v. It seems that most of the improvements seen from the previous post were inconclusive, as soon after he realized that there are minor dongle to dongle variations in sensitivity, so he could not be certain if the improvement was due to his modifications.

However, he has continued to modify the RTL-SDR using the same principal (Japanese, use Google Translate). He discovered that to fully suppress the 1.2v switching regulator pin 27 on the RTL2832U chip must be connected to ground. Once this is done the switching noise present at 1 MHz disappears, although there does not seem to be any significant improvement in noise anywhere else.

The first screenshot shows an unmodded dongle with the switching regulator still connected. There is some noise present at around 1.026 MHz. The second image shows a modded dongle, the noise at 1.026 MHz is gone.

Switching regulator on. Noise can be seen at around 1.026 MHz.
Switching regulator on. Noise can be seen at around 1.026 MHz.
Switching regulator off. Noise at 1.026 MHz is gone.
Switching regulator off. Noise at 1.026 MHz is gone.
The 3 diode mod.
The three diode mod.

Listening to Trunked Radios with One RTL-SDR

Usually to listen to trunked radio systems, two RTL-SDR dongles are required. One for decoding the trunking control channel and another for listening to the audio channel. However if the audio channels are within the same chunk of received bandwidth as the control channel it is possible to use just one dongle to follow trunked conservations.

Recently Pawel of pewusoft wrote in to to let us know about a tutorial he uploaded showing how to get trunking to work with just one RTL-SDR dongle. His method uses Unitrunker and SDR# together with the AuxVFO plugin and a new plugin that he wrote for interfacing with Unitrunker.

Although there is already a Unitrunker interface plugin for SDR#, Pawel wrote a new plugin based on serial port commands as he found that the original interface plugin did not work properly for him.

Unitrunker Serial Port Interface SDR# Plugin
Unitrunker Serial Port Interface SDR# Plugin

Listening to NXDN with SDRSharp, the AuxVFO Plugin and DSD+

Over on YouTube user John Miller has uploaded a video showing how he receives NXDN digital audio using a combination of SDR#, the AuxVFO plugin and DSD+. He writes:

I have it set with 5 auxiliary VFO’s one for each channel of the Christian Co NXDN system from the Kelly Towers. I use VAC to route the audio from each VFO to DSD+ each VFO has it own DSD+ running. I then have all the DSD+ go into one output VAC and use that to run a feed on Broadcastify, The secret to running multiple DSD+ is to have separate install of it, so I have 5 DSD+ folders.

Christian Co NXDN

SDRSharper: A modified version of SDR#

What appears to be a third party reskinned version of SDR# has been released on the *removed* Facebook discussion page. The modified software is called “sdr sharper” and changes the look and feel of SDR#. The software also includes several plugins ready to go including a scope, audio notch filters, audio equalizer, audio waterfall and s-meter. The sdr sharper software call be downloaded from the file *removed* over at *removed*.

Important Note: We are currently unsure of the legality of this software as it seems to be based on the licensed SDR# code base. If it turns out to be illegal software we will remove all download links.

Update 28/01/2015: As suspected SDRSharper is illegal and infringes on SDR#’s licensed code and trademark as pointed out by Youssef, the author of SDR# in the comments section of this post. In response we have removed the direct download link. Please respect the SDR# licence and Youssef’s hard work by not using illegal copies.

The majority of added features can be added to the original SDR# with the plugins listed on this page.

Reskinned SDR#: SDRSharper
Reskinned SDR#: SDRSharper

Reducing USB Shield Interference on RTL-SDR Dongles

Leif, the programmer of Linrad has recently uploaded a video to YouTube showing how he shields his RTL-SDR dongles to prevent the USB shield from acting as an antenna and picking up unwanted signals. In the video he also measures how well various other SDRs are at avoiding USB shield interference.

From the video we can see that Leif’s shielding method uses the following steps:

  • The metal box is RF tight and held together by many screws.
  • The USB shield and external antenna connector shields are both connected to the box.
  • The USB 5V line is decoupled to the GND line.
  • The USB 5V line has a ferrite on it.
  • The internal coax connection between the external connector and RTL-SDR antenna connection has a ferrite on it.
  • The USB cable has a ferrite where it enters the box.

With this shielding effort the interference is significantly reduced.

HackRF Controlling a Quadcopter

Over on YouTube user Mike has uploaded a video showing a quadcopter being controlled by the HackRF, a low cost transmit capable software defined radio. Mike uses a Hubson X4 quadcopter and controls it with a USB joystick coupled with GNU Radio. According to a tweet by Micheal Ossmann (the inventor of the HackRF), there were initially USB latency issues that caused problems, but have since been fixed by Mike.

HackRF quadcopter control