Tagged: receiver

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WSPR Quadrus SDR

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WSPR Quadrus SDR

What is WSPRNet?

In my last post I’ve introduced the WSJT receiver software and mentioned WSPRnet.
Weak Signal Propagation Reporter Network is a group of amateur radio operators using K1JT’s MEPT_JT digital mode to probe radio frequency propagation conditions using very low power (QRP/QRPp) transmissions.

WSPR Quadrus SDR on WSPRNet

Registered user can log in to the site, and their client software will send automatic updates on the currently received radio stations to the database. The connections are visualized on a map. Again, thanks to Andy, HA6NN, we have some pictures about the stations he was receiving with the WSPR Quadrus SDR.

Connecting WSPR Quadrus SDR

I’ve used the virtual audio cable connection in this experiment as well in order to send audio samples from the SRM-3000 SDR software of the Quadrus SDR platform to the WSJT software.


In this post, you see an example of using Quadrus SDR with external software connected through a virtual audio cable. The setup received some DX stations with the DRU-244A SDR hardware, which has enough sensitivity to receive signals from around the word with a simple wire dipole antenna.

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WSJT Quadrus SDR

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WSJT Quadrus SDR

What is WSJT?

WSJT is a special waveform developed for weak signal communication by Joe Taylor, K1JT. It can be used in Earth-Moon-Earth (EME), meteor scatter, and ionospheric scatter scenarios at VHF/UHF; but skywave propagation is supported as well in the HF band. The waveform can fit in the 3 kHz bandwidth, so SSB transceivers can be used.

How to use WSJT with Quadrus SDR?

The easiest way to integrate Quadrus SDR with external signal processing software is by employing a virtual audio cable, which passes audio samples from the SDR directly to the external software. See this post for further details:


DX SWLing with WSJT Quadrus SDR station

Andy, HA6NN has kindly set up a station for a day during the 2014/15 holiday season, and has collected some good data using WSJT and the Quadrus SDR. The image gallery shows signals received from AC2PB, BA4TB, BD8XY, CO2VE, DC6CM, DL7ACA, EA3KY, HS0ZBS, JH1AWZ, K1NOX, K6ESU, LW3DJC, LY2CK, N1NU, N6DM, OH1LWZ, RK6ART, RN1BL, S5500, TF2MSN, UA9CC, VK5DG, XE2FGC, and ZP5yV.

Visualization with WSPRnet

There is a community site, where you can visualize your connections and received stations. Andy has generated some good screenshots using his WSJT Quadrus SDR receiver.


I know he is looking for a contact with R1ANR. I hope he has it on his DXCC list soon…


In this post we’ve described how to connect the WSJT wavefrom with the Quadrus SDR using a virtual audio connection in order to receive weak signals. The WSJT Quadrus SDR combo was able to detect a lot of different DX stations, and proved the reception capability of the DRU-244A SDR hardware.

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SDR pre-selector filter | Direct digital SDR

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What is direct digital SDR?

Software-Defined Radio (SDR) is a type of radio, where the analog signal is converted into the digital domain, and functionality is implemented in the digital domain employing signal processing algorithms. Conversion technology is limited in terms of bandwidth and frequency range, thus the right point for conversion has to be carefully chosen. Conversion can take place at the baseband, Intermediate Frequency (IF), or directly at the Radio Frequency (RF). In case conversion happens at the operating RF (likely after the pre-selector), we can talk about direct digital SDR.

Domain converter frequency parameters

Analog-to-Digital Converters (ADCs) and Digital-to-Analog Converters (DACs) are employed to bridge the analog and digital domains on the radio hardware platform. Converter parameters determine how we can use them in the radio implementations.

Instantaneous bandwidth

One of the most important parameters is the real-time bandwidth or instantaneous bandwidth. It is determined by the sampling frequency of the converter, and according to the Nyquist law, it is equal to the half of the sampling frequency.

Frequency range

The other very important parameter is bandwidth or frequency range of the converter itself. Usually, this is determined by the circuits involved: it starts with the analog components, and includes circuitry within the converter, like the sample-and-hold stage. The Nyquist criteria states that the bandwidth should be equal to the half of the sampling rate in order for a perfect reconstruction in both time and frequency domains. Hence, there is a possibility to generate and sample higher frequency signals too, if we keep the bandwidth inside half of the sampling rate. In other words, we can use upper half bands, called Nyquist bands. If we have a wider spectrum, we have to be sure not to alias or fold from higher Nyquist bands to the baseband. The anti-aliasing filter or SDR pre-selector is used for that propose. If we are talking about ADCs and receivers, the latter terminology is employed.

Frequency parameters of the DRU-244A SDR hardware

We’ve used 80 MHz as sampling frequency for our hardware platform, so, the instantaneous bandwidth is 40 MHz. We can tune to radio channels within this band using on-board hardware DDCs. The input bandwidth of the ADC itself is 650 MHz. This is the -3 dB point of the input stage, and it has no brick wall slope.

bandwidth response

This means that we can use not only the 0-40 MHz first Nyquist band, but upper bands, like 160-180 MHz, too using an SDR per-selector filter. However, the bandwidth is degraded, because we have to use some other input analog circuits, like input low-noise preamplifiers and leveling attenuators. Still, it is possible to receive with good results up to 500 MHz. See this post about satellite signal reception at 435 MHz:
For more information, please see AN-835 application note from Analog Devices:

Designing SDR pre-selector filter

You can find a lot of different filter design tool kits on the net, which will approximate your requirements, and determine the right components for different realizations. I think, the best practice, – which I’ve used in the last decades – is to cascade a separate high-pass and  a low-pass filter if the relative bandwidth is high. On the other hand, the band-pass approach will work for narrow band (<10%) filters. I always like to use standard components. E12 or E24 1% components will do good job for anti-aliasing and pre-selection filter implementations. Usually, the capacitors are the easier part, inductors may have to be manually wound and tuned.

Bandpass filter for VHF bands

Using the Dyonusos filter design software, I’ve designed a band-pass SDR pre-selector filters utilizing the capacitive coupled resonator structure, which is my favorite. The relative bandwidth is higher than 10%. During the approximation phase, I like to see ~40 dB attenuation at the Nyquist band corner. However, only 30 dB could be achieved by the high-pass filter at the lower band edge frequency if the insertion bandwidth was kept at 20 MHz. You can see the calculated filter response, the filter values, and the measured response after having very careful fine tuned the inductors in the circuits. Seems easy enough, but you need some practice to reach such results with a 5th order resonator filter. For beginners interested in designing and implementing filters, let me suggest to start with 3rd order structures and standard complements as close as possible to the calculated component values.


SDR pre-selector BPF 160-200 SDR pre-selector BPF 120-160


SDR pre-selector BPF 120-160 SDR pre-selector BPF 160-200


SDR pre-selector BPF 160-200 SDR pre-selector BPF 160-200


SDR pre-selector BPF 160-200 SDR pre-selector BPF 120-160

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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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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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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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Wideband SDR reception in the SRM SDR software receiver

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[Wideband SDR available bandwidth settings]

The SRM-3000 radio receiver SDR software supports the following six built-in bandwidths: +/- 6.25, 12.5, 25, 50, 100, 200 kHz. These are Single Side Band (SSB) specifications. So, for example, the widest (200 kHz) setup actually enables the reception of 400 kHz wide transmissions, as all the signals are complex. This is seen on the spectrum/waterfall display, which has a frequency range going from -200 kHz to +200 kHz in this case. The wideband SDR software radio receiver SRM automatically sets the Digital Down Converts (DDCs) on the DRU digitizer to the desired Nyquist frequency. But this DDC output sample rate is NOT equal to the bandwidth specification above! In fact, the sample rate is much higher, thus the system can deliver unattended performance within the useful bandwidth. So, for example, in case of the 200 kHz bandwidth setup, the actual DDC output sample rate is 398 kSPS – almost double. This overhead allows to have a magnitude drop of less than 1 dB within the useful ±200 kHz bandwidth.

200 kHz is wide enough to display and receive WB FM broadcast transmissions. As the input frequency range of the DRU-244A digitizer hardware can go up to 320 MHz, we just need to tune the radio receiver to the desired station.

FM_broadcast_if_spectrum400 FM_broadcast_if_waterfall

Thanks to the high SNR the large amplitude 19 kHz stereo pilot is easily identifiable in the audio spectrum. The audio processing runs at 48 kSPS, thus the difference channel is not processed for the time being. Full stereo decoding is an upcoming feature.


Recall that the display only contains the 1 dB band of the DDC output, which runs in the example at 398 kSPS if IQ pairs are counted as one sample, or at 796 kSPS if counted as two. This is a significant burden on the subsequent DSP processing, which is entirely performed by the x86 Intel GPP in the PC. The processor workload is very high, so you need a powerful machine (i5 or preferably i7) to process one or more wideband radio channels. On the other hand, WB FM broadcasts may be processed in the 100 or 50 kHz bandwidth mode as well due to the robust nature of FM modulation.

FM_broadcast_if_spectrum200 FM_broadcast_if_spectrum100a

The wideband radio receiver is not only for demodulating WB FM broadcasts. The receiver may be tuned to any band of interest. E.g., the following images show the 20 m HAM radio band; the center of the ±50KHz window is at 14.050MHz. In this case, the demodulator offset functionality may be used to select and demodulate the narrowband signal of interest within the available bandwidth. This involves mixing, band filtering, and CW/USB demodulation – all performed on the PC.

20m_band_if_spectrum 20m_band_if_waterfall

Finally, a note on safe device handling. Direct digital radio reception at wide input bandwidths (i.e., 320 MHz for the DRU-244A digitizer board) necessitates the use of input preselection filters, in order to avoid overload conditions for the sensitive receiver input circuitry. To design and simulate such filters, you may use the excellent free filter design program DIONYSUS by ComNav Engineering.

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