Tagged: decimation

An RTL-SDR ExtIO with Decimation and Tuner Bandwidth Controls

RTL-SDR experimenter Hayati Ayguen ( on GitHub) has released a version of rtl_tcp and an ExtIO module that allows access to normally hidden tuner settings. The ExtIO allows you to manually set the tuner bandwidth of the R820T/2 chip. It can be set almost anywhere between 350 kHz and 8000 kHz. In the standard drivers the tuner bandwidth is normally set at a size appropriate for the sample rate, but in this driver using a smaller bandwidth allows you to filter out unwanted signals whilst taking advantage of the decimation a higher sample rate gives you.

In addition to the tuner bandwidth settings, the ExtIO module also implements decimation controls. This allows you to reduce the displayed bandwidth without sacrificing the sample rate. At the same time the R820T/2 tuner bandwidth is reduced to the visible sample rate, improving filtering.

Hayati writes in the Readme file:

Tuner bandwidth is supported with E4000 and R820T tuners.
Changing bandwidth with R820T looks good and shows immediate differences in spectrum.
E4000 is not so nice, i can’t see any difference, for the bandwidths below 3400 kHz.
I also put in the wider bandwidths, even wider than the samplerate, which will produce aliasing!
These might be interesting for wide amplitude modulated signals, e.g. ADS-B.

Decimation of Samplerate is an option for very old and slow computers:
Before decimation, a very simple and fast low-quality filter (sum) is applied:
y(k) = sum over x( k * (1..decimation) ) ; x() = input samples from rtl_tcp
; y() = output samples to SDR app (HDSDR)
The sum produces values requiring more than 8 bit.
In addition to this soft-filter, minimize the tuner-bandwidth!
You will still receive aliases!!!, but a bit damped compared to directly sampling at the slower speed.
The reception’s center frequency (LO) is the most alias-free region.

The ExtIO interface. Enables tuner bandwidth and decimation controls.
The ExtIO interface. Enables tuner bandwidth and decimation controls.

A precompiled release of the ExtIO dll can be downloaded at github.com/hayguen/extio_rtl_tcp/releases. As the ExtIO connects via an rtl_tcp server, you will also need to download Hayati’s modified rtl_tcp server from github.com/librtlsdr/librtlsdr/releases/tag/win32_with_biasTee. To use the driver, first run rtl_tcp locally simply by double clicking on it, then open HDSDR and select the new ExtIO_RTL_TCP.dll file.

We tested out the tuner bandwidth functions and it appears to work well. For instance in the following image there is a strong pager signal near 157.9 MHz. At first the RTL-SDR is overloaded by the strong signal causing multiple images to appear on the spectrum. But after reducing the tuner bandwidth to 350 kHz the left side of the spectrum is attenuated, and the interfering pager signal is no longer too strong. 

The modified rtl_tcp software (and rtl_(adsb|fm|power|tcp)) also support access to the software activated bias tee on our V3 dongles via the -T flag.

Tuner Bandwidth Selection Example.
Tuner Bandwidth Selection Example.

Hayati has also uploaded some slides which show some of the effects of using various different tuner bandwidths.

New SDR# RTL-SDR Driver with LNA/Mixer/VGA Gain Settings and Decimation

A new SDR# driver for the RTL-SDR that comes with manual settings for the LNA/Mixer and VGA gain stages as well as software decimation (in Russian use Google Translate) has been released on rtl-sdr.ru. The three seperate gain stages offer much greater control over optimization of signal SNR and interference management. The decimation feature allows you to sacrifice some bandwidth for increased ADC bit resolution. This means that using the decimation feature can significantly improve the SNR of a signal. Update: It turns out that decimation is already used in the existing SDR# code, so using decimation in the new driver won’t improve the real SNR, only the visible SNR – so there will be no audible improvement. It will however allow you to effectively zoom in on the spectrum without loosing visible resolution. Also it allows you to use a higher sample rate (2.8 MSPS) which will improve SNR, whilst maintaining a reasonably sized (small enough) visible bandwidth for narrowband signals.

The driver is based on Oliver Jowet’s modified driver so it should also have the capability to tune down to 13 MHz or lower. To use the plugin simply download the zip file and copy the files into the SDR# folder, replacing all existing files. Previously we posted about another driver with similar capabilities, but this driver was never released to the public. There was also another driver with just the three gain settings released earlier which this driver appears to be based on.

New RTL-SDR Driver with LNA/Mixer/VGA Gain Settings and Software Decimation
New RTL-SDR Driver with LNA/Mixer/VGA Gain Settings and Software Decimation

Demo of a New R820T Driver with extra Gain, Decimation and IF Filter Adjustments

Over on YouTube user mm6dos has uploaded a video showing a new driver he helped develop for the R820T tuner which is used in the most commonly purchased RTL-SDR dongles. He writes:

A short demonstration of a new non-gpl RTL2832U / R820T driver specifically written for SDR#. The R820T tuner actually contains a configurable IF filter and 3 separate gain stages. Unfortunately Osmocom’s implementation has fixed this filter and one of the gain stages. Aliasing and overloading is significantly reduced using this driver.

Currently we don’t have a source for the download of this driver, but we assume that it will be released soon. The videos below show the driver in action, with the first video showing the IF filter adjustments and the second video showing the software decimation feature.