Category: RTL-SDR

Listening to SCA with HDSDR, SDR# and an RTL-SDR

In the USA and Canada a subcarrier called SCA (Subsidary Communications Authority) is used to add additional services to a broadcast FM signal. Some examples of the extra services provided are live financial stock telemetry, audio books for the blind, specialized audio radio programs for doctors etc and background music for supermarkets and stores. These SCA signals are modulated into standard broadcast FM radio signals, but require a special radio to receive them. Subcarrier signals can easily be spotted in the audio/baseband waterfall and spectrum plots available in most SDR software.

Over on the new RTL-SDR DX blog, the author (Jay Moore) has uploaded an article showing how to use an RTL-SDR dongle to listen to audio SCA signals. The process involves using HDSDR to receive the broadcast FM signal, then using Virtual Audio Cable to pipe the audio into SDR#, where it is then possible to tune to the audio SCA signal. The same process could also be used to receive different subcarriers used in other countries such as Finland where a subcarrier is used to transmit DARC encoded bus stop sign telemetry.

SCA audio received via a combination of HDSDR and SDR#
SCA audio received via a combination of HDSDR and SDR#
Decoding SCA with HDSDR and SDR#

RTL-SDR Panadapter Using Hardware Radio Receiver IF Stages

Over on YouTube user Jay Moore has uploaded a video explaining how to connect an RTL-SDR dongle to the IF stage of a hardware radio in order to create a panadapter. In the video Jay briefly explains how a radio with an IF stage works and then shows how he tapped into his Sansui 2000 hardware radio’s IF stage directly from the circuit board. The IF stage then connects to a ham-it-up upconverter which connects to the RTL-SDR.

By connecting the IF stage of a hardware radio to the RTL-SDR it is possible to use the hardware radio as the receiver while using the RTL-SDR to still maintain the benefits of a spectrum display. Most purpose built hardware radios will have better reception than the RTL-SDR.

RTL-SDR on receiver IF stages

Updates to the Power Line Noise Detector “Driveby” System

A few days ago we posted about Tim Havens “driveby” project, which is a RTL-SDR based system he developed to help create heatmaps of power line noise in his neighborhood. Upon doing further research, Tim discovered that the main source of power line interference was appearing at a frequency of 44.252 MHz. Then by using his driveby system to create a heatmap of his area at this frequency and also by using a hand held radio he was able to pinpoint a massive source of power line noise.

The noise appears to be coming from a faulty and unsafe power pole near a local school. He writes that the power poles primary ground line has been severed and that a wire from the AC line is simply dangling in the air, ready to be grabbed by school children.

Heatmap showing sources of powerline interference
Heatmap showing sources of powerline interference
The faulty power pole
The faulty power pole with dangling AC line
Severed grounding line
Severed grounding line

CubicSDR: New open source, cross platform SDR software

Over on YouTube a video showing off a preview version of a new cross platform and open source SDR software program has recently been uploaded. The software is named CubicSDR and aims to support all major operating systems including Linux, Windows and OSX. The code can be downloaded from its GitHub page at https://github.com/cjcliffe/CubicSDR.

CubicSDR currently supports the RTL-SDR, but in the future hopes to support the HackRF and other SDR’s. They also hope to eventually integrate several demodulation filters, make managing multiple SDRs easy and allow for scripting of custom demodulators.

CubicSDR v0.01 Early Preview

Testing an LNA on receiving a weak signal with the RTL-SDR

Over on YouTube Adam Alicajic the designer of the LNA4ALL low noise amplifier has uploaded a video showing the effect of an LNA on reception of a weak signal. He shows an example of how a very weak signal cannot be received by the RTL-SDR even when the gain is set to maximum unless an LNA is connected.

Adam has posted this video in regards to some statements saying that an LNA will only increase the noise floor and cannot bring signals out of the noise floor. There is a discussion about this on this Reddit thread.

DVB-T dongle + LNA = Myth or Truth

Measuring Frequency Deviation of an FM Transmitter with an RTL-SDR

Over on YouTube user KP4MD has uploaded a video showing how she uses an RTL-SDR together with SDR#, a program called Visual Analyzer and an AEA PK-232 Terminal Node Controller to measure the frequency deviation of a Yaesu FT-8800R Transceiver. She writes:

The SDR# receiver is tuned to 145.050 MHz and the bandwidth set to 20 kHz.

The deviation level of the 1200 Hz tone is increased until a null appeared on the carrier frequency.

This is called a Bessel Zero and occurs at various predicted modulation indices (2.4, 5.52, 8.66, etc).

The Modulation Index is defined as the peak frequency deviation divided by the modulation frequency.

This Bessel Zero occurred at a modulation index of 2.4 corresponding to a frequency deviation of ±2.88 kHz (2.4 x 1.2 kHz).

The oscilloscope indicates that a peak to peak amplitude of 54.3% corresponds to ±2.88 kHz deviation.

The 1200 Hz tone modulation is increased to yield a peak to peak amplitude of 66%.

This corresponds to the desired ±3.5 kHz frequency deviation.

Frequency Deviation Measurement with an RTL-SDR Dongle

We now sell RTL-SDR’s with the R820T2 Tuner and 2x Telescopic Antenna + R820T2 Tests

We now sell R820T2 RTL-SDRs on Amazon.com (currently for US customers only sorry!) and are currently running a $2 off promotional sale which will expire January 31, or until the first batch of stock runs out. Compared to the other choices our RTL-SDR Blog branded units come with several improvements which we list below.

  • Use of the R820T2 tuner which has been shown to have slightly better noise performance and give better SNR compared to the standard R820T chip.
  • Use of improved component tolerances which help the circuit to operate at its optimum.
  • Use of a surface mount 28.8 MHz oscillator instead of the “can” type. We believe this will reduce the PPM offsets to below 30 in most dongles, but note we can not guarantee this.
  • Improved “full braid” coax cable on the antenna base which has significantly lower loss compared to the coax used on other brand RTL-SDR stock antennas.
  • Comes with 2 x telescopic antennas. 1 x 9.5 cm to 31.5 cm telescopic antenna and 1 x 20 cm to 1.5 m telescopic antenna. Great for beginners to receive a wider range of frequencies without buying extra antennas.
  • No IR LED. The IR LED is useless for SDR operation and the long legs on the LED may pick up interference.

We currently have two options for sale that are shown below. The dongle only unit is perfect as a replacement dongle or for those who just want to try out the R820T2 chip. The unit with the two telescopic antennas is great for beginners who don’t have any good antennas already.

We also have limited quantities of some MCX male to various female adapter sets for sale which work out to be much cheaper than when buying them individually. Buying a set will have you ready for almost any antenna connection you need. The pigtail adapters come with 20cm of RG316 cable and the straight adapters don’t use any cabling.

Pigtail Adapters SetMCX -> Various Female Pigtail Adapter Set – $19.99

Straight MCX Adapters SetClick here to buy a MCX -> Various Female Straight Adapter Set – $16.99

Currently because of the way Amazon works, we can only ship to US customers, but we may ship overseas in the future. Shipping from Amazon is fast and free if you spend over $35 or are a Prime member. Returns from faults are also easy and welcome. If you are overseas and can’t buy from us, the alternatives for R820T2’s are the Nooelec R820T2 (US shipper), the Cosycave R820T2 (ships from Channel Islands, UK) and there are also some Chinese R820T2 (Chinese shipper) models available on ebay.

We also offer unofficial support over on our forums. If you do buy from us we hope that you will consider leaving a product review on the Amazon page as that will really help us out as small time Amazon sellers.

As an added bonus, we will also have our e-book on sale from January 16 to January 23 at $6.99 USD, reduced from $9.99 USD.

We also performed some simple performance tests on the R820T2 which we show below.

R820T2 Tests

The first test was a noise floor test. We used rtl_power and ran a noise test with maximum gain and a 50 Ohm terminator connected for 15 minutes over the entire receivable frequency band. We averaged the results over three different R820T dongles and three R820T2 dongles to remove dongle to dongle variances. The results show that noise floor on the R820T2 is around 2-3 dB lower at most frequencies.

R820T2_NoiseFloor

Next we tested the SNR with the gain set to zero using a HackRF as the signal source. The results show that the R820T2 is about 2-5 dB more sensitive depending on the frequency. Also, compared to the R820T, the sensitivity seems to be significantly better at 1.5 GHz to 1.8 GHz as all tested R820T units could not even detect the test signal above 1.5 GHz without increasing the gain.

R820T2_SNR

Oliver Jowett HF Driver Test

The R820T2 should have better performance at HF frequencies when using the experimental Oliver Jowett drivers. We tested an R820T and R820T2 on broadcast AM reception. At broadcast AM frequencies the R820T starts with a very high noise floor after starting it for the first time, but after about 5 minutes seems to settle down to a lower noise floor shown in the right image below. In comparison the R820T2 starts at a low noise floor almost immediately. We are unsure why there is a settling time in the first place. Even after the settling time the R820T2 had better reception and SNR as shown in the comparison image below. Both dongles were set to the second highest gain setting.

R820T2vR820T_BAM

At 15 MHz international broadcast AM can be clearly heard with Oliver’s drivers. The R820T2 gets clear reception with a very low gain setting, whilst the R820T can obtain similar SNR with a higher gain setting. Though with a higher gain setting used on the R820T more noise seems to appear as can be seen in the comparison image below.
R820T2vsR820T_14MHz

Low Loss Coax

We also tested the low loss coax cable used in our RTL-SDR Blog branded antenna bases and found that it had approximately 3 dB less loss compared to the standard cable when used at most frequencies above 100 MHz. The test used a 1M length of each coax, with the HackRF as the signal generator. The direct connection test used a straight MCX->SMA Male adapter to directly connect the HackRF and RTL-SDR together.

Low_Loss_Coax

Measuring Power Line Noise in the Neighbourhood with the RTL-SDR “Driveby” System

Over the last few months Tim Havens has been developing a portable RTL-SDR based scanner that he intends to use for geo-tagged noise floor readings. He hopes to use such a system to map sources of RF noise related to power lines so that he can contact his power company to resolve them.

Tim’s system uses a powerful Odroid XU3 which is a Linux based mini embedded computer that sells for $179 USD. The Odroid XU3 has dual quad core ARM CPUs which is enough power to run rtl_power with 5 RTL-SDR dongles simultaneously. Rtl_power is an RTL-SDR tool which allows you to scan and record the power levels in the frequency spectrum. By using 5 dongles he is able to scan the 49 MHz, 50 MHz, 144 MHz, 222 MHz and 432 MHz bands simultaneously.

From a previous post Tim writes:

The idea behind this project is to be able to MAP real-world Geo-tagged noise floor readings.  This can be used for the primary purpose I intended for this application (mapping of problematic sources of RF Noise related to power lines in the area so that I can approach the local power company to resolve them) or any other sort of RF signal MAPPING.  Such as cellphone/cellsite coverage or FM broadcast coverage (and dead spots) among other things.

RTL dongles are CHEAP, and reliable, although not 100% stable (they drift a bit for the first 5 minutes of warm up) they can be used to measure changes in the RF Noise Floor (once warmed up).  While they don’t really seem to be able to be calibrated to anything less than -87db all we’re really looking for are relative changes to the noise floor while driving around a particular location (there is probably some complex math that could applied to these measurements that could be calibrated).  So for this project these inexpensive receivers are really just fine.

While rtl_power is scanning, the Odroid uses a GPS receiver to tag the timestamped noise readings with GPS coordinates. Then by driving around with the system and combining the GPS coordinates with the noise floor readings from rtl_power he is able to create a heatmap showing exactly where in his neighbourhood noise levels peak, indicating a power line RF noise problem to be fixed by the power company.

Some more information about the hardware build of his system can be found on a previous post.

Powerline Noise Mapped with RTL-SDRs and GPS
Powerline Noise Heat Mapped with RTL-SDRs and GPS Logging
The Insides of the Driveby System
The insides of the driveby system

Tim also has uploaded a video to YouTube showing his system running a stationary test demonstrating the hardware and some of his custom software before everything was boxed up.

Driveby App stationary test