Tagged: sdr


Low-frequency reception with Quadrus

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What is low-frequency reception?

Well, it depends on your primary activity area. For regular High Frequency (HF) guys the lower end of the HF band is around 1.8 MHz or 160 m wavelength. Below the HF or ShortWave (SW) band we can find the Medium Wave (MW), the LongWave (LW), or even the very longwave bands too. These are usually called Low Freqeuncy (LF), Very Low Freqeuncy (VLF), Extra Low Frequency (ELF).  Additional information can be found here: http://en.wikipedia.org/wiki/Frequency_allocation

Low-frequency reception limit of Quadrus SDR

The DRU-244A SDR digitizer hardware of the Quadrus SDR platform has AC coupled inputs. Thus LF reception on the platform is limited by the transformer at the input stage. According to the datasheet, the lower frequency -3 dB band edge is at 60 kHz.

Practical tests

I’ve used a simple inverted-V shaped 2×20 m G5RV antenna and bypassed the usual High-Pass Filter (HPF) in the HF receivers.

First, I looked at lower frequencies than MW broadcast bands, and I’ve found some interesting signals in the 300-400 kHz band.

mw01 mw02
mw03 mw04

There are a couple of Double SideBand (DSB) AM transmitters with strong carriers modulated by simple Morse code. These can be heard with a simple AM receiver as well. It is very simple to copy them, and you can visually decode the call sign from the waterfall diagram. Later, I’ve learned on one of the forums that these are Non-Directional Beacons NDBs for navigation proposes. More info here: https://en.wikipedia.org/wiki/Non-directional_beacon

Low-frequency reception of LW broadcasts

The lower part of the band, around 150-250 kHz, contains again AM modulated LW broadcast stations.

mw05 mw06 mw07 mw08

Low-frequency reception the DFC77 transmitter

Below 100 kHz we’ve found some interesting signals. I was only familiar with the DCF77.

mw09 mw10

This is a time-frequency standard from Hamburg. http://en.wikipedia.org/wiki/DCF77 It can be received very well with the wire antenna, and decoding the message is also possible with a special decoder program SpectrumLab by Wolf, DL4YHF.



The LF reception capability of the Quadrus SDR platform was introduced. NDBs were received in the 300-400 kHz band. The LW AM broadcast stations in the 150-250 kHz band as well. The well known DCF77 reference transmitter was received and decoded at 77 kHz and some signals were detected around 50 kHz. LF reception needs some modification on the analog input stage of the receiver hardware.


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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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CW Skimmer – Quadrus SDR

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What is a CW Skimmer?

A CW Skimmer is multi-channel CW decoder and analyzer by Alex, VE3NEA. You can read about this great tool and his other works on his website: http://www.dxatlas.com/CwSkimmer/

A great tutorial is available from Pete, N4ZR as well: http://www.dxatlas.com/CwSkimmer/Files/Skimmerintro.pdf

Go ahead, and look at the links if you are not familiar with the topic.

How to connect the CW Skimmer and the Quadrus SDR ?

The CW Skimmer is accepting signals from radio receivers. The traditional hardware radio can be connected through a sound card audio input to the software, and the CW Skimmer software can remote control the receiver on CAT. The bandwidth, in this case, is limited to the usual 3 kHz of typical HF radios.

However, the CW Skimmer software can handle a couple of SDRs too, and can accept higher bandwidth signals from them. Some of the SDRs on the market can also be controlled from software trough their APIs.

Today the CW Skimmer – Quadrus SDR connection is not supported directly by the CW Skimmer. Thus, we have to use the simple audio interface connection. This limits the analysis bandwidth to 3 kHz. Because the SRM-3000 SDR receiver is software too, we can use a virtual audio cable to connect to the CW Skimmer. I’ve used the VB-Cable A&B from VB-Audio Software for my tests. A single connection is free for non-commercial use.

Decoding calls in the 20 m band

Yesterday, I had a chance to use one of the remote stations with TeamViewer. It is located in the country side with a simple inverted-V shaped G5RV wire antenna. I found a spot around 14.025 MHz in the CW band. There were a lot of stations in the 3 kHz bandwidth. The outcome of only a couple of minutes of recording can be seen below with the call signs collected by the CW Skimmer.


Further improvements with server version CW Skimmer

There is another software from Alex called CW Skimmer Server. This application can handle up to 192 kHz bandwidth in seven ham radio bands. The SDRs can be connected with a 3rd party driver. We are looking for interested programmers to provide a driver to CW Skimmer Server based on the TCP/IP remote control interface of the SRM-3000 SDR software receiver.

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Using Quadrus SDR with a laptop

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Why a laptop?

Even until fairly recently, the resources offered by an average laptop were insufficient to run wide-band, multi-channel SDR applications. Thus, the original SDR hardware was designed with more capable desktop computers in mind. However, with increasing laptop performances, it is now finally possible to run even the more challenging applications. The obvious advantages are flexibility and mobility, and by now they are omnipresent in our everyday lives.

Connecting Quadrus SDR to a laptop

The Quadrus SDR platform’s phase-coherent SDR hardware digitizer board is a standard PCI slot card. This form factor does not allow us to connect it directly to a laptop. Fortunately, we have the possibility to use an external PCI slot extender, and place the DRU-244A card into one of the external slots. There are several products in the market, they differ mainly in the number of slots and connections. One of the most well known suppliers is Magma, who offers different solutions, like the one slot PCI extension. They also offer products with different interfaces to the laptop: ExpressCard 34mm and 54mm versions, and CardBus/PCMCIA card with 1 m or 1.5 m cable length.

1slotB_xl_0 1SlotPCI_connection

Beyond this well known and proven supplier, we’ve just found another very cost-effective external PCI solution. Polotek offers a solution based on the ExpressCard interface. It essentially contains one PCIe and one USB interface. Their idea is very simple: use the PCIe connection with a high-speed extender cable and add a PCIe-PCI brige chip on the external slot card. Their other approach is to use a standard USB3 cable manufactured in high volume. However, the connection itself is not following the USB3 protocol, they simply utilize the high-speed differential wire pair within the cable to connect the PCIe slot to the extender card, which has the PCIe-PCI bridge.

polotek2 polotek

Testing the DRU-244A phase-cohernet SDR hardware digitizer with a laptop

You can place low volume orders at several places:
I’ve ordered from Aliexpress, and received the package with the components as shown on the web.

dru ext1 dru ext2

Setting up the hardware and installing the DRU driver was trivial. The single issue, I’ve noticed, is that the Plug-and-Play functionality is somehow not working properly in all cases. Sometimes I’ve lost connection to the card after some sleep or screen saving actions. In these cases, I just removed and reconnected the ExpressCard and re-initiated the Plug-and-Play cycle. I had no chance to test it with any other computer than my Dell power notebook with an i7 processor.

dru driver machine

driver1 driver2 driver3



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SDR hardware manufacturing batch arrived

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Design and preparation of manufacturing

The DRU-244A digitizer SDR hardware went trough some design polishing and preparation for mass production without close interaction with the original design team.


Verification and testing

Before releasing the SDR hardware, the manufacturing plant is responsible for the full verification and testing of all functionality. They need to program the clock chip that provides the different sampling clocks and other miscellaneous clocks of the architecture as well. After that, the initial EEPROM content of the PCI interface should be loaded. If the card is working fine with external power supply at this point, it can be placed into a PC for further testing using its test software. In this phase, the RF parameters are tested.

dru-sample-app dru-sample-fequ

Beta testing with selected users

After all the cards were tested in the factory, we’ve immediately shipped some of them to our beta testers. They have the latest version of the SRM-3000 receiver SDR software available to use with the card. We are looking for the initial responses from them, and appreciate any suggestions for further improvements.


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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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