Tagged: filter

New ADS-B Filter with Built in Bias Tee Available

Adam who is the manufacturer of the popular LNA4ALL low noise amplifier (LNA) that is commonly used with the RTL-SDR has come out with a new product for ADS-B enthusiasts. The product is an ADS-B filter with a built in bias tee for providing phantom power. Adam previously sold an older version of the ADS-B filter that came without the bias tee.

The bias tee allows you to inject DC power into the coaxial cable in order to easily power an LNA (like the LNA4ALL) or other device that is placed near the antenna. The antenna could be far away from a power source, such as on your roof or up a mast. It ensures DC power reaches the LNA, but at the same time does not enter the RTL-SDR dongle, as DC current on the antenna input could destroy the RTL-SDR. For best performance it is recommended to use an LNA near the antenna, especially if you have a long run of coaxial cable between the antenna and RTL-SDR.

The filter uses Low Temperature Co-fired Ceramics (LTCC) type components as opposed to the seemingly more commonly used SAW and microstrip filters. Adam writes that each type of filter has its tradeoffs, but he believes the LTCC filter is the best for this application.

Comparison between different filter types.
Comparison between different filter types.

The insertion loss of the filter in the pass band is about 2.4 dB and the filter will significantly attenuate broadcast band FM, TV stations, WiFi and 1.8 GHz+ cell phones. However, it does not do so well with 950 MHz cell towers and possible radar on 1.2-1.3 GHz as the LTCC filter is not as sharp as a SAW filter. In Adams own tests he shows that the addition of the filter improves ADS-B decoding performance by about 20%, but the improvement you see will vary greatly with your RF environment.

The filter is currently selling for 20 Euros + 5 Euros shipping (~$28 USD).

ADS-B LTCC Filter with Bias Tee
ADS-B LTCC Filter with Bias Tee

Building a 145 MHz Low Loss Helical Bandpass Filter

Over on YouTube user Mile Kokotov has uploaded a video showing his home made low loss helical bandpass filter for 145 MHz, but also tunable from 110 MHz to 160 MHz. Bandpass filters are useful for the RTL-SDR as often strong out of band signals can cause overload, causing poor reception. A bandpass filter blocks all signals outside of its pass band. A helical bandpass filter is simply a coiled wire enclosed in a conductive container that can be tuned with a variable capacitor made out of two plates.

In his video Mile shows the inner construction of his helical filter, explains the parts and shows what calculations he used for construction.

145 MHz Low Loss Bandpass Helical Filter by Mile Kokotov

Characterizing RF Filters with a Noise Source and RTL-SDR Dongle

Over on YouTube RTL-SDR experimenter Adam Alicajic has uploaded a video showing how it is possible to use the RTL-SDR as a tool to measure the frequency response of an RF filter. To do this he uses a noise source circuit which produces wide band white noise connected to an LNA4ALL, connected to the RF filter and finally connected to the RTL-SDR. Then using the Touchstone spectrum analyzer software he does a 300 MHz bandwidth sweep over a section of the spectrum which shows the response of the filter.

The noise source can be built from a simple diode based circuit as shown in a previous post, or if you have the Ham-it-up upconverter you can buy the parts for the noise generator part of the circuit.

In his video he shows the frequency response of a 145 MHz helix filter, a coax notch filter and a 1090 MHz home brew bandpass filter.

http://www.youtube.com/watch?v=X_gd2gWyGi4
http://www.youtube.com/watch?v=UvdaURc01Ts

More filters and DVB-T dongle

Testing a FM Broadcast Bandstop Filter

Over on YouTube user Cameron Conover has been testing a simple FM broadcast bandstop filter with his HackRF. The same filter can just as easily be used with the RTL-SDR to remove broadcast FM interference and images. Cameron uses a MCM Electronics 88 – 108 MHz FM Trap which can be found very cheaply on Amazon or Ebay for around $15 USD. His video shows that the FM trap works very well and significantly reduces out of band FM interference.

HackRF One with an FM BCB filter

Effect of a Filter on RTL-SDR ADS-B Reception

Over on YouTube user Adam Alicajic has posted a video showing the effect of a filter tuned for 1090 MHz used on ADS-B reception. Adam switches the filter in an out showing the difference in the number of received ADS-B frames. With the filter enabled he is able to receive around 1200 messages per second and without only around 800 messages per second.

A filter (aka preselector) can help to reduce out of band interference from strong signals.

DVB-T dongle receiving the ADS-B using the filter

RTL-SDR HF Preselector Project

Over on YouTube user Theo Faber has uploaded a video showing his adjustable multiband HF preselector project for his RTL-SDR dongle. The preselector covers the entire HF band. Theo designed and built his own homemade upconverter for this project as well.

Preselectors on the RTL-SDR can vastly improve reception quality. Without preselection, noise and strong images of other nearby stations can cause strong interference.

My project ‘pre selector’ for wideband radios and in particular the popular tv-sticks than can be used for SDR reception. The sticks are very cheap and the software free available on the internet. SDR Sharp in this case, is operating stable under XP and W7. Sticks are sensitive, however the input level can only vary +- 20dB. The sticks are easely overloaded and will produce cross and intermediation. So you need in front of the stick / converter band pass or tunable filters. Hopefully this inspired you to build your own set of filters, to fully benefit from the perfect performing of this affordable SDR.
for more info contact me on: t.faber87__AT__upcmail.nl
kind regards and 73’s

SDR RTL dongle and pre selector with HF converter

Measuring the Frequency Response of a Bandpass Filter with the RTL-SDR

Lacking both an expensive signal generator and spectrum analyzer, Hans used his RTL-SDR together with a simple diode based wideband noise generator to measure the frequency response of a 137 MHz bandpass filter.

To do this he used the RTL-SDR Scanner software which allows you to create a composite spectrum over a frequency range wider than the maximum 3.2 MHz of bandwidth the RTL-SDR provides. The wideband noise generator was cleverly constructed out of a diode operating in it’s reverse breakdown mode.

Apart from the obvious excessive spurs, this method worked quite well and the shape of the filter is clearly visible.

Bandpass Filter Response
Bandpass Filter Response
Wideband diode noise source
Wideband diode noise source

Homemade ADS-B Filter

Reddit user BigReid has posted on the Reddit rtl-sdr forums his homemade bandpass filter with a pass range of 1-1.2GHz for improving adsb reception. A bandpass filter blocks any interference from signal frequencies outside of the pass range. This might be useful for adsb if you experience a lot of interference from out of band signals at 1090MHz.

His filter is a hairpin filter, which can easily be etched onto a PCB board. You can find the PCB schematic files linked on the post.

ADSB Hairpin Bandpass Filter