Category: intro and description


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:

Further reading on DRM, the new digital HF broadcast technology:

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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SDR FM Pre-selector filter

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SDR FM Pre-selector filter and direct digital SDR

In one of my earlier posts I’ve already described the direct digital SDR architecture and the possibility to receive in higher Nyquist bands.

If we are to use this possibility, however, we have to carefully avoid spectrum overlaps, for which we need an analog SDR FM pre-selector filter. Let me show you how I’ve designed and implemented one.

SDR FM pre-selector filter design

SDR FM Filter design

FM boadcast is in the 88-108 MHz band in most countries, which is in the 80-120 MHz 3rd Nyquist zone of our 40 MHz converter. The relative bandwidth is ~20% (20/98=0.24), which suggests to use an High-Pass Filter (HPF) – Low-Pass Filter (LPF) structure.

We can approximate the filter using the Dyonusos filter design software. I’ve manually fine tuned the autogenerated results, so that the band edge insertion loss falls in the 1-3 dB range while the inductor values are close to standard values. The capacitors can be implemented with parallel connected standard values. I’ve employed the same method for both the HPF and LPF filters, and decided to use 82 nH and one 100 nH inductor.

HPF90-res LPF110-res

HPF90-sch LPF110-sch
Filter implementation

After the design, I’ve collected the components, and utilized a previous PCB design to solder the components together.

FM boadcast filter

Measurement results

Before assembling the complete band-pass SDR FM filter, I’ve measured the high-pass and low-pass parts separately.

hpf90-meas lpf110-meas

Finally, I’ve connected them, thereby forming a Band-Pass Filter (BPF), and I’ve tested the close-in pass-band performance and the higher frequency response as well. The latter is important for rejection of miscellaneous signals, like DVB.

bpf-meas1 bpf-meas2

As expected, the amplitude response of the assembled structure is not so good, and we see more insertion loss, and also other degradation due to the inaccurate component values.


I’ve introduced a simple method to design and implement an SDR FM pre-selector filter. It is designed with a filter design software with some manual fine tuning and adjustments to achieve the right performance in the corner frequencies and for amplitude response. During this process, one has to take into account the standard chip inductor values, e.g., from the E6/12 component line. Non-standard capacitor values can be constructed with parallel connected smaller values. Due to the limited quality factor of the inductors, the insertion loss in only moderate. Due to the high tolerance of the components, the pass-band response is high. With tunable high-Q inductors we could reach better values in terms of insertion loss and response.

Happy SDR receiving!

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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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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

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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