Technical Updares:MEASURING FILTER CHARACTERISTICS AND ANTENNA VSWR WITH AN RTL-SDR AND NOISE SOURCE

By using an RTL-SDR dongle together with a low cost noise source it is possible to measure the response of an RF filter. Also, with an additional piece of hardware called a directional coupler the standing wave ratio (SWR) of antennas can also be measured. Measuring the response of a filter can be very useful for those designing their own, or for those who just want to check the performance and characteristics of a filter they have purchased. The SWR of an antenna determines where the antenna is resonant and is important for tuning it for the frequency you are interested in listening to. These tutorials are based heavily on information learned from Adam Alicajic’s (9A4QV), videos which can be found at . Adam is the creator of the LNA4ALL and several other RTL-SDR compatible products. Recently Tim Havens also posted some experiments with characterizing home made filters on his blog.

The BG7TBL noise source is a wideband noise source that can provide strong noise over the entire frequency range of the RTL-SDR. It requires power from a 12V source which can be obtained from a common plug in power supply. It also uses an SMA female connector, so you may need some adapters to connect it to your filter under test (adapters can be found cheaply on Ebay). Finally a quick warning: be careful when handling the circuit board after it has been powered for some time as some of the components can get very hot. Note that if the Ebay store runs out of these there is also a seller on Aliexpress with some available, just type “noise source” in the search bar.

If you have a ham-it-up upconverter and are good at soldering small surface mount components you might instead consider purchasing the noise source kit add on. Here is a video showing how to build and test the ham-it-up noise source.

The software we use is as follows:

rtl_power (keenerds build)

RTL-SDR Panorama (or any other rtl_power GUI or RTL-SDR spectrum analyzer) – (Optional)

We will use rtl_power with the RTL-SDR Panorama GUI to record and graph a frequency sweep over the effective range of the filter. If you are experienced with command line use of rtl_power you may also use it without the GUI. To set up RTL-SDR Panorama place the rtlpan.exe file into the same folder as rtl_power, which can be obtained from the official RTL-SDR release, or from one of keenerds builds (GitHub for Linux). We recommend using keenerds builds as his modified drivers seem to retune much faster making scans of large bandwidths significantly faster.

To characterize a filter simply connect the noise source to the input of the filter and the RTL-SDR to the output. Then open RTL-SDR Panorama and set the frequency range to the expected range of the filter and set the resolution to 1M. Also set the gain to 0 and enable Auto dB. Next click start and after a few seconds a graph of your filter response should be generated.

The image below shows the response of a cheap FM bandstop filter, such as the one available from MCM electronics plotted between 50 MHz and 150 MHz.

The image below shows the response of a 1090 MHz bandpass filter plotted between 100 MHz and 1.7 GHz

After viewing the response in RTL-SDR Panorama we recommend plotting the same graph against a baseline graph of just the noise source connected directly to the RTL-SDR. This can be done using Excel or any other similar plotting software. Each time RTL-SDR Panorama finishes a frequency sweep, the data is stored in a CSV file called scan.csv. It will be located in the same folder as rtlpan.exe. First do a baseline sweep with no filters connected and then stop RTL-SDR Panorama by pressing the stop button. Note that after you press the stop button the final sweep will still be running, so wait until it finishes first. Next rename scan.csv to baseline.csv. Now do a sweep with the filter connected – this will create a new scan.csv file. Next open baseline.csv in Excel. You should notice that the third column C contains the frequency values and the last column G contains the power data. Use the excel plotting tools to create a scatter plot in baseline.csv with frequency on the x-axis and power on the y-axis. Finally, open the scan.csv which contains data for the scan with the filter. Copy the last column containing the power values (column G), and paste it into baseline.csv next to the baseline power values. Add a plot of the filtered power values to the same plot as the baseline plot and compare. Next you can calculate the attenuation by simply subtracting the baseline column from the filter power column.............

For more details click on the following link.

Source :- http://www.rtl-sdr.com/rtl-sdr-tutorial-measuring-filter-characteristics-and-antenna-vswr-with-an-rtl-sdr-and-noise-source/

Credit :- Facebook account of Shri. Alokesh Gupta