Tagged: sdr radio

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:

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:



Sensitive SDR receiver for secret transmissions

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




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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Coherent multi-channel SDR receiver

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[Coherent multi-channel SDR receiver with coherently sampling SDR hardware development platform]

Coherently sampling hardware architecture

The DRU-244A digitizer hardware contains 4 Analog-to-Digital Converter (ADC) chips connected to a common sampling clock source. The output of the ADCs is routed to the on-board Field-Programmable Gate Array (FPGA) through Digital Signal Processors (DSPs), which contain four Digital Down Converters (DDCs). The decimated samples are routed to the host PC via the PCI bus. On of the main features of the DRU-244A hardware is that all of the sampling and decimation is phase coherent and synchronous. The sampling clocks and even the start of the DDCs are synchronized employing trigger signals implemented in hardware. The samples can be kept in sync within the SRM-3000 SDR receiver software by explicitly turning on this feature on the control panel.

Proving the coherent operation with off-line processing

Signal connections

In this experiment, we’ve used a passive or resistive power splitter to provide the signals for each of the RF inputs of the digitizer card. As a first step, each RF input was connected with cables of the same length to the splitter. Later 8 m of RG-223 coaxial cable was inserted in one of the signal paths to add phase shift to one of the channels.

Testing coherent SDR channels with zero deg splitter
Testing coherent SDR channels with zero deg splitter
Testing coherent SDR channels with delay in one input
Testing coherent SDR channels with delay on one input

During the tests we’ve used three test frequencies 4.5 MHz, 9.4 MHz, and 16.1 MHz. The receiver was tuned to the given signal in USB operation mode and generated a ~1 kHz sine wave at the audio output.

Recording coherent channels

One can easily record a channel’s audio output in the SRM-3000 SDR radio software to an (almost) standard wave file, which then may be processed off-line using other tools. In the following examples, we’ve used the Matlab environment to display the time domain wave form and to calculate the power spectra and phase information of signals. As mentioned above, in order to make coherent SDR channel recordings, the user has to explicitly turn on the synchronous recording mode on the user interface.

Switch coherent recording in SRM SDR receiver
Turning on coherent recording in the SRM SDR receiver

We’ve made recordings for only the first channels of each DDC block. As a reminder, the DRU-244A SDR receiver platform actually contains four dedicated hardware DDCs in each signal processor. It has one wide band signal input, so, it makes sense to record only one of the output channels, as the phase delay will be the same for the rest of the channels of the same DSP.

The internal architecture of the coherent SDR hardware
The internal architecture of the coherent SDR hardware

Initial calibration results

The output was recorded to a wave file, and subsequently read into Matlab to display the time domain. Not surprisingly, we see the four (noisy) sine waves with no phase difference among them.

Coherent SDR channels with zero deg at 16.1MHz
Coherent SDR channels with zero deg at 16.1 MHz
Coherent SDR channels with zero deg at 9.4MHz
Coherent SDR channels with zero deg at 9.4 MHz
Coherent SDR channels with zero deg at 4.5MHz
Coherent SDR channels with zero deg at 4.5 MHz

Signals with phase delays

As the next step, we’ve inserted an 8 m RG-223 coax cable into one of the signal paths. The phase delay of the cable is frequency dependent. The calculated phase delays follow for the frequencies at hand:
4.5 MHz – 65.44 deg
9.4 MHz – 136.71 deg
16.1 MHz – 234.15 deg
* Zo = 50 ohm, C=101 pF/m, Z0=SQRT(L/C), t=SQRT(L*C)
* PH=360*F[Hz]*L[m]*t[s], t=5.05 ns/m

Coherent SDR channels with 234 deg at 16.1MHz
Coherent SDR channels with 234 deg at 16.1 MHz
Coherent SDR channels with 136 deg at 9.4MHz
Coherent SDR channels with 136 deg at 9.4 MHz
Coherent SDR channels with 65 deg at 4.5MHz
Coherent SDR channels with 65 deg at 4.5 MHz

Frequency domain phase delay processing

It is very hard to observe phase delay in time domain, thus, we’ve employed frequency domain calculations as well for the delay. The complex spectrum of the input signal was calculated with FFT. It contained the amplitude and the phase of each signal. We can get the phase difference between the different signal paths by subtracting the calculated phases.
As we can see on the figures, the phase differences give the same value as the previously calculated estimates.

Amplitude and phase difference spectrum of coherent SDR channels 4.5MHz Amplitude and phase difference spectrum of coherent SDR channels at 4.5 MHz Amplitude and phase difference spectrum of coherent SDR channels 4.5MHz (zoom) Amplitude and phase difference spectrum of coherent SDR channels at 4.5 MHz (zoom)
Amplitude and phase difference spectrum of coherent SDR channels 9.4MHz (zoom) Amplitude and phase difference spectrum of coherent SDR channels at 9.4 MHz (zoom) Amplitude and phase difference spectrum of coherent SDR channels 16.1MHz (zoom) Amplitude and phase difference spectrum of coherent SDR channels at 16.1 MHz (zoom)


The Quadrus SDR receiver platform – including the DRU-244A digitizer card and the SRM-3000 SDR receiver software – are ready to provide phase coherent signals. This platform feature makes it possible to use it in interferometric direction finding and digital beam forming applications. It is possible to record signals as standard windows wave files for off-line processing.

Downloads related to this content: Phase coherent SDR cahnnel records with read script (Matlab) Share Quadrus SDR


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:
Or you can use other similar audio analyzer program from Daqarta where you can download a 30 days trial of the latest version:

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.

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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sat receiver

Direct digital SDR receiver for satellites

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[Direct digital SDR receiver for satellites]

Direct digital SDR receiver principle

As I’ve introduced it in an earlier post, the DRU-244A phase coherent SDR receiver digitizer card has no significant input pre-selection. Thus, it can implement direct digital SDR reception. The bandwidth of the input network makes it possible to digitize the signals in the upper Nyquist bands (which is referred to as under sampling). The platform may be used as a Direct Digital Radio (DDR) receiver for the VHF and UHF bands. We just need to add some input gain to compensate for the slope in input sensitivity at higher frequencies. The SRM-3000 Software-Defined Radio (SDR) application is prepared for this type of operation, and can tune to the equivalent frequency in the baseband.

Preparation of the VHF/UHF direct digital SDR receiver

I’ve already made some sensitivity tests in the 70 cm HAM radio band. See earlier post: Direct-digital-uhf-sdr-radio-receiver-dru-244. I’ve waited for an opportunity to test it on a real target, which ended up being the MASAT-1 - the first Hungarian cube sat. Yesterday, I had a chance to visit the ground control station of the university, where the folks have a tracking antenna with 20 dB gain. I’ve connect my direct digital SDR receiver to the split antenna signal.
To prepare I’ve tested two input pre-selector filters around the 437.345 MHz downlink frequency. One was a Mini-Circuits HPF-LPF cascade, the other was a ceramic filter for the 433 MHz ISM band for radio remote controllers. Both showed 1.5 dB insertion loss, which seems be acceptable; there is no significant input noise figure reduction, and hence significant loss of sensitivity.

LPF-HPF filter response for direct digital SDR receiver
LPF-HPF filter response for direct digital SDR receiver
LPF+HPF for direct digital SDR receiver
LPF-HPF pre-selection for the direct digital SDR receiver
BPF filter response for direct digital SDR receiver
BPF filter response for direct digital SDR receiver
BPF for direct digital SDR receiver
Monolit BPF for direct digital SDR receiver pre selection

I’ve also prepared a Mini-Circuits connectorized block LNA with 20 dB gain and <1 dB noise figure. This seemed to sufficiently improve sensitivity, and thus provided reception capability for direct digital SDR receiver.

Visiting the satellite control ground station

I’ve checked the satellite tracking information on-line, and showed up at the station at the right time to set up the rig. The station operator told me that they had a high-selectivity coax resonator filter installed before their 20 dB low noise preamp, so my pre-selector filter proved unnecessary. We had set up a computer display with incoming packets from some other stations, which helped us checking the reception in the area. We had nothing else to do, so we just waited for the satellite signal to appear on the display. I’ve utilized the +/- 12.5 kHz bandwidth to cover the doppler shift.

IMG_0396 IMG_0397

Receiving the satellite with the SDR receiver

At the predicted time, we’ve observed the first signals at the high side of the display. The observed doppler shift was more than +10 kHz.

rec01 rec02

Later, as the satellite got closer to us, the doppler shift got smaller. I’ve slowed down the waterfall display; this way we could see the doppler shift during the whole transmission.

rec04 rec05

On the last picture, we can see the uplink command packet right below the zero frequency. Our receiver seems to have been tuned a couple kHz below the exact frequency. However, the DRU-244A SDR receiver platform has an external 10 MHz reference input, so next time a GPS clock reference can be employed to keep the frequencies more accurate.

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Free introductory paper on Software-Defined Radio (SDR)

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Our original paper on SDR development is now freely available on the QUADRUS SDR platform site. The paper introduces our high-speed, high-bandwidth domain converter solutions, and demonstrates their capability through practical examples, like our wideband SDR search-and-intercept receiver and our SDR radar. The paper was first published in the proceedings of the advanced communication seminar by the NATO Research and Technology Organization (NATO RTO) – now known as the NATO Science and Technology Organization under the NATO Collaboration Support Office.

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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.


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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RF input response for a -120dBm input signal level

Direct digital UHF SDR radio receiver with DRU-244

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Direct digital receiver

A direct digital receiver has the Analog-to-Digital Converter (ADC) directly connected to the incoming RF signal without any frequency translation. This is contrary to the superheterodyne or direct conversion methods, which translate the RF signal to an Intermediate Frequency (IF) or the BaseBand (BB) respectively before digitizing. The direct digital receiver concept can be regarded as an example of the ultimate software defined radio.

Instantaneous versus input bandwidth

Instantaneous bandwidth is the frequency band that is continuously processable by the digitizer device. This bandwidth is determined by the sampling frequency; half of the sampling frequency is often called the Nyquist-frequency. The maximum bandwidth of the processable signal should be less than this Nyquist-frequency in accordance with the Nyquist-Shannon sampling theory. However, the input bandwidth is determined mainly by the the analog front-end and the sample-and-hold circuit in the ADC. If the limit imposed by these circuits is higher than the Nyquist-frequency, we have a chance to sample higher frequency signals as well. This is usually called under sampling or sub-Nyuist sampling.

Input bandwidth of the DRU-244

I have an older version of the DRU-244 digitizer board. The input bandwidth was not specified, but it should be up to a few hundred MHz. Maybe even as high as 144 MHz or 432 MHz. I’ve connected the 432 MHz output of the signal generator to the input of the SRM SDR radio receiver, and I’ve tuned to the same frequency to get a good look at the signal. I’ve observed that the level is more than 20 dB less than in the HF band. So, I need at least 20 dB preamplification to maintain the sensitivity in the UHF band.

Sensitivity testing in the 432 MHz (70 cm) band

First, I’ve checked the sensitivity with my FT-897 transceiver. I’ve connected a signal with a -120 dBm output power, which had a well audible sound level employing the Singel SideBand (SSB) demodulator.


Next, I’ve connected two MiniCircuits ZX60-6013E amplifiers in cascade providing 30 dB amplification with a reasonable noise figure.


I’ve checked the input noise level of the receiver without the connected preamp.


Then, I’ve checked again with the preamp. The noise floor increased by a couple dBs.


This was a good indication that the system sensitivity was determined by the preamp as opposed to the digitizer in the receiver.

I’ve connect the -120 dBm signal to the input, which was unfortunately less audible after the SSB demodulation then with the FT-897.


The Signal-to-Noise Ratio (SNR) at AF level was not sufficient.


437 MHz falls into the 11th Nyquist band of the converter. My idea was that all of the preamp output noise ended up getting aliased into the baseband, and consequently reduced the SNR. So, I’ve tried a Band-Pass Filter (BPF) at the preamp output, before the digitizer input. It helped a lot, and the SNR increased substantially.


I had a really good, clear audio signal without any noise. The next picture shows the audio output spectrum of the direct digital SDR radio receiver with DRU-244 running the SRM radio receiver software for the -120 dBm input signal at 437 MHz.




Usually, modern ADCs have significant input bandwidth, and allow sampling in higher Nyquist bands. This way direct digital VHF/UHF radio receivers can be built with simple architectures. However, input signal degradation should be mitigated with input preamplifiers. Although, we loose some dynamic range, this is an acceptable price for a very simple receiver architecture.

PS: Why 437-450MHz? This is the down-link of the MASAT-1 satellite. So, I guess, now you know my next plan… :-)

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Audio output from the SRM SDR receiver

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Audio output

The SRM-3000 SDR receiver can handle up to 16 independent receiver channels, but usually, we only have one sound card output in the PC. The radio receiver software defines one channel as monitored, and sends its audio output to the default audio out device. This way, you can listen to an active channel with your speakers or headphone.

Processing the audio output

Usually, after the filtering and demodulation, we want some additional post-processing of the audio output. The post-processing software can run on the same or on a separate computer. In the latter case, we can directly connect the audio out to the microphone input of the other PC using a simple jumper cable with two standard 3.5 mm jack plugs. In case we use the same computer for post-processing, we can use a virtual audio cable. This is a software that defines virtual output and input devices. It can be set as the default audio device, and thereby act as a gateway: the SRM SDR receiver can send its audio output to it, which the post-processing software can directly receive.


Testing with different post processors and decoders

First, I used an external recording software to save audio output for later processing. I’ve employed Audacity, which is a free, open-source audio editor and recorder software for a variety of platforms.

srmin auda

After that, I’ve successfully tested the connection with the well-known Spectrum Lab post-processor developed by DL4YHF.


Finally, I’ve tested the demodulation possibility for the audio output by utilizing the Code300 software package from Hoka Electronic. The audio signals contained a real FSK transmission, and it was successfully demodulated and processed.




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