Tagged: IF filter

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

Testing the built in IF Filter on the R820T RTL-SDR

As seen in this previous post, the R820T tuner chip used on most RTL-SDR’s has a built in hardware tunable IF filter. Leif, the programmer of the Linrad SDR software has been experimenting with this filter and has uploaded a video of his experiments to YouTube.

In the video he shows how the R820T IF filter can be set to be as narrow as 300 kHz. Using a narrow IF filter can help to reduce the interference from strong nearby stations by up to 30 dB. Leif uses a modified RTL-SDR driver that comes with Linrad which allows the IF filter to be manually modified.

Update: Leif has posted some extra details in the Osmocom mailing list. He writes that the modified driver can be found at https://github.com/dl8aau/librtlsdr and a Windows version is at https://drive.google.com/file/d/0B9m6SAaGpewQeWtyS0lzdW1xVmM/view?usp=sharing.

R820T with IF filtering

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.