Tagged: drm

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Have your own HAM SWL radio station!

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Gadget of the past? HAM SWL radio station!

In most cases we write posts about unique designs and ides, which are useful for advanced level HAM SWL radios, e.g., this or that posts on software defined radio (SDR) technology. But what about the beginners? Well this post is for them! Ready, steady, HAM SWL Radio!

Some weeks ago I was surfing the internet looking for some new ideas for a special issue of Quadrus SDR. It was easy to find a creative idea for my problem, and to prove to my family that HF broadcasting is alive. We can access high quality radio service there based on new, digital operation mode (i.e., DRM), which has a very useful community for SDR fans. When I started to deal with HAM SWL radio, there were no opportunities such as the internet. It was ooch… 35+ years ago. We – the members of the community – talked to each other only using the radio at the club station or using our self-made radios at home, but it was an amazing experience that I never forget. I had my own HAM radio station at home – built with my own hands from scratch – with CW capability for the 80m ham radio band. Lovely, isn’t it? (and I was 17 years old when I made this…)

front internal top front

I have two sons. They are not interested in making, but they are professional in using gadgets. Like every teenager nowadays. Maybe my father said the same thing about my HAM radio equipment. Gadget.

Do or do not, there is no try

So, what about beginners? I found a very good post about how to start this kind of a hobby. Hobby? No! This is a way of life. I could not summarize this better then Gregory L. Charvat:

“The only way to get started is to build something. Start small, check out the QRP community, try making a single-conversion receiver, and move up to something with a crystal IF filter. Borrow and scale circuits from books such as these:

Or leverage complete ICs and modules like those from Mini-Circuits.  There is nothing like making that first long distance contact (DX) on radio gear you created from scratch.”

You can read the whole article on Hackaday.

But if you are more of a computer geek, you can switch to software implemented radio and start with less complex and less expensive SDRs or a professional one like Quadrus SDR from our webshop. Even I experienced that old fashioned radio moment Gregory mentioned above, when I first received a DRM station from Mubay, which was a very nice feeling for my radio infected heart. You can see the report on this reception here in the Quadrus SDR blog:
http://spectrafold.com/quadrus/radio-software/sdr-drm-receiver/

Further reading on DRM, the new digital HF broadcast technology:
http://spectrafold.com/quadrus/go/whitepaper-drm-broadcasts/

And don’t forget to share your success stories or questions regarding SDR issues with us on our Facebook pageTwitter page, or G+ community page. Be social; whatever is your preferred platform, we are there !

Bertalan, HA6QU

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Quadrus SDR for DRM receiver in education

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AM and DRM broadcasts in the HF bands

Traditionally, AM modulation is used in the LW/MW/SW bands for broadcasting purposes. It is very easy to identify these the AM broadcast stations based on their Dual Side Band (DSB) shape in the spectrum. The spectrum and waterfall displays of the Quadrus SDR show such a modulation on the following pictures.

am spectr
am water

However, digital waveforms, i.e., DRM, have started to populate the HF bands, which can provide high quality content. The modulation format is optimized to the propagation behavior, and is based on the multi-carrier scheme. It is also very easy to recognize them in the band using the Quadrus SDR for DRM, because these stations have a distinct rectangular shape in the spectrum.

drm04

DRM in the telecommunication curriculum of universities

As DRM represents a significant part of broadcasting systems nowadays, most universities around the world have included this standard, or parts of it, into their curriculum on telecommunications. It is also an important part of the telecommunications program at the Budapest University of Technology and Economics as well, where we have recently introduced the Quadrus SDR for DRM by showcasing its DRM reception capability.

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drm05 drm02

 

 

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Sensitive SDR receiver

Sensitive SDR receiver for sensitive information

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[Sensitive SDR receiver for sensitive information]

Sensitive SDR receiver based on the DRU-244A digitizer SDR hardware platform

The DRU-244A digitizer board was designed and implemented for building radio receivers. It contains a 16 bit resolution low-noise Analog-to-Digital Converter (ADC). It has a low-noise input preamplifier and an input attenuator in front of the ADC. The low-noise preamp provides input sensitivity as low as -111 dBm in SSB reception mode with 2.1 kHz bandwidth and 10 dB SNR. With external preamplifier it could be as low as -122 dB. If you would like to see more detailed test results on SDR receiver sensitivity click here:
http://spectrafold.com/quadrus/digitizer-hardware/sdr-receiver-sensitivity-test

Practical results for receiving weak signals

I’ve already made some practical tests with my very simple inverted-V shaped wire dipole antenna. It was used in the country side, so there was less human made noise than in the crowded city or urban area.

Sensitive SDR receiver as long range DRM SDR

If you are not a professional radio enthusiast with a vast knowledge of exact broadcasting carrier frequencies, one of the easiest ways to find some DRM radio stations is to just look for its unique spectrum in the HF band. The DRM spectrum is a very typical, noise like, wide band signal. This is exactly what I did; I simply connected the antenna to my receiver through a 30 MHz low-pass input pre-selection filter, and started visually looking for DRM radio signals in the spectrum. I was really surprised when one of the first signal received turned out to be be coming from India and the next one was from South Africa. Not bad. The SDR receiver seems to be real sensitive if I am able to decode these signals in the middle of Europe.

drm01 drm02

If you are interested in how the SDR receiver was connected to the Dream DRM decoder software, please read these posts:

http://spectrafold.com/quadrus/radio-software/virtual-audio-connection/

http://spectrafold.com/quadrus/radio-software/sdr-drm-receiver/

Sensitive SDR receiver for secret transmissions

One of my other favorite signals in the HF spectrum is the UVB-76 Buzzer on 4555KHz.

https://en.wikipedia.org/wiki/UVB-76

http://priyom.org/military-stations/russia/the-buzzer.aspx

http://www.wired.co.uk/magazine/archive/2011/11/features/enigma?page=all

As the frequency is well known, I could simply dial it in to the SDR receiver software, and immediately see the transmission on my screen and hear the famous “Buzzer” tone on the speaker with very high Signal-to-Noise Ratio (SNR).

Receiving the UVB-76 Buzzer on 4625KHz with sensitive SDR receiver

There are a lot of mysterious signals in the HF spectrum. The sensitivity of our receiver – especially with an external, low-noise per-amplifier and a good antenna – makes it possible to listen to even the most remote signals.

listener Contact014

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sig

SDR receiver sensitivity test

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Receiver sensitivity specifications

One of the most important features of a radio is its ability to receive low level signals, in other words, its sensitivity. We have lot of different definitions for receiver sensitivity. Some excellent descriptions can be found on radio-electronics.com. For linear modulation formats, like AM, SSB, and CW, the Signal-to-Noise Ratio (SNR) or one of its variants, e.g., the signal plus noise to noise ratio ( =(S+N)/N ), are most commonly used.

In the first case, we can measure the signal level and the noise level separately. This may be done with a spectrum analyzer employing a simple sine wave test signal. In the second case, we resort to measuring the signal and the noise together, because can’t separate the noise from the signal. For this we utilize a wide band power meter or a Root Mean Square (RMS) voltmeter.

If the difference between the signal and the noise level is greater than 10 dB, the above defined two ratios are practically equal. When we look at the specs of different receivers, sometimes it is hard to immediately compare the performance of different models, because they are specified differently. For example, the SNR may be defined with different bandwidths in mind. More specifically, 10 dB or 12 dB SNR values represent vastly different receiver sensitivity based on whether it is defined for 500 Hz, 2.1 kHz, or 2.4 kHz bandwidths.

Practical receiver sensitivity test of the DRU-244A-based SDR

Receiver sensitivity test setup

The test setup is very simple. We need to use a calibrated test generator to feed -80 dBm and lower signal levels into the input of the receiver, while we measure the audio output level with an RMS voltmeter.

Receiver sensitivity measurement procedure

Switch on and tune the receiver to the test frequency (F) with a given bandwidth (BW). First, we disconnect the signal source and measure the output noise level. Secondly, we connect the RF signal source, and increase the signal starting from a very low level, until we have an audio output voltage with a given level. The signal level on the generator (P) shows the receiver sensitivity for a given bandwidth and the SNR level. Instead of traditional voltage meter, like the venerable HP-400, you can use a sound card-based scope and audio analyzer. Usually, it has built in SNR measurement capability. For my last measurement, I used the Multi Instrument software by Virtual Instrument Technology.
You can download the 21 day free trail from this page:
http://www.virtins.com/downloads.shtml
Or you can use other similar audio analyzer program from Daqarta where you can download a 30 days trial of the latest version:
http://daqarta.com/dqdown.htm

We already have digitized samples in the SDR radio, so, it is possible to skip the DAC/ADC sound card conversion, and with the Virtual Audio driver we can send the samples directly from the SDR radio software to the measurement software. I’ve used this audio driver to connect the SDR receiver to the DRM decoder in one of my last post.
http://spectrafold.com/quadrus/radio_software/receving-drm-broadcast-sdr-radio-receiver/

SDR receiver sensitivity test results

I’ve tested the DRU-244A at F = 10.1 MHz, BW = 2.1 kHz, and S+N/N = 10 dB with and without a pre-amplifier. During my tests, I’ve used a ZX60-P103 amplifier from MiniCircuits with fixed 23 dB gain and less than 3 dB noise figure. It is specified from 50 MHz, however, it can be used down to 2 MHz.

The following pictures show the different steps of the SDR receiver sensitivity measurement for SSB, CW, AM, and FM signals.

SSB (2.1 kHz) and CW (400 Hz)
sdr receiver sensitivity noise sdr receiver sensitivity noise 2
sdr receiver sensitivity noise o sdr receiver sensitivity noise 2 o
sdr receiver sensitivity signal sdr receiver sensitivity signal 2
sdr receiver sensitivity signal o sdr receiver sensitivity signal 2 o
sdr receiver sensitivity ssb sdr receiver sensitivity cw

AM and FM with signal display on the SDR receiver, noise and signal out, and the generator.
sdr receiver sensitivity signal 3 sdr receiver sensitivity signal 4
sdr receiver sensitivity signal 3a sdr receiver sensitivity signal 4a
sdr receiver sensitivity signal 3c

sdr receiver sensitivity noise 3 sdr receiver sensitivity noise 4
sdr receiver sensitivity signal 3o sdr receiver sensitivity signal 3o
sdr receiver sensitivity AM sdr receiver sensitivity FM

Receiver sensitivity results and conclusion

As you can see from the receiver sensitivity measurement results, the sensitivity is
SSB -111 dBm at 10 dB S+N/N with 2.1 kHz bandwidth
CW  -119 dBm at 10 dB S+N/N with 400 Hz bandwidth
AM -105 dBm at 10 dB S+N/N with 30% modulation
FM -108 dBm at 10 dB S+N/N with 3 kHz deviation

The sensitivity can be improved with some external low noise preamplification and additional external gain to reach -122 dBm sensitivity in SSB operation mode.

sdr receiver sensitivity noise sdr receiver sensitivity signal and noise
sdr receiver sensitivity noise sdr receiver sensitivity signal and noise
sdr receiver sensitivity 2

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Receiving DRM broadcast with SDR radio receiver

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Digital broadcast on HF bands

Due to significant progress in Digital Signal Processing (DSP) technology, it is now possible to use spectrum efficient digital transmissions for high quality audio broadcast in HF bands. The current and already stable standard is called Digital Radio Mondial (DRM). This transmission is using the same 10 kHz bandwidth as a traditional AM broadcast station, although with a multi-carrier modulation format, and delivers content as source coded streams. The spectrum of such transmissions is typically noise-like and rectangular, as you can see on the screen of an SRM-3000 SDR radio receiver.

DRM signal on SDR radio receiver

Demodulation and decoding of DRM signals

We need a special DSP tool to demodulate such waveforms with several sub-carriers and QAM modulation of each carrier. Fortunately, we have an open source community project delivering such a software, called Dream DRM receiver.  You can download the latest version of their software from its SourceForge repository. The digital stream is provided by the demodulator. However, we need to use the decoder to generate an audio sample and other meta info associated with the transmission, i.e., station name, program characteristics, etc. The DRM stations use AAC+ codec. It is built in the Dream software stack as a dynamic library.

Connecting the SDR radio receiver station components

We can use the DRU-244A digitizer card with SRM-3000 sdr radio receiver software to receive the signals from the air. The Dream software is used to demodulate the stream and to decode the audio signal. Both of them are able to feed signals to and consume signals from a sound device. A jumper cable can be used to connect the two applications. However, that’s not recommended for analog conversion and back. A virtual audio cable may be used instead, which directly forwards samples from one app to the other. I’ve used the VB-Audio Virtual Cable for my test runs. See their web site for more information.

vb-audio

Enjoy DRM programs form world wide providers

Finally, we need to find some DRM signals on the air, or look after program guides on the net for given stations. I simply search the spectrum. It was easy to recognize some DRM signals using the SRM-3000 SDR radio receiver. One of the first transmissions I was able to find was from All India Radio, which is pretty good DX from Europe with a simple long wire antenna. We have a enough receiver station sensitivity with the DRU card and the antenna for such a DX reception.

drm01 drm02

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QUADRUS SDR radio receiver development platform

Quadrus SDR Blog

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Latest relevant discussion | QUADRUS SDR blog

You could find some latest relevant discussion in the QUADRUS SDR blog about the technology and its application.