Tagged: srm sdr receievr


SRM GUI tips and tricks series – RF record and playback

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Hi Everyone,

We at Spectrafold thought it would be helpful for the community if we provided some tips on how to use SRM – even the simpler functions. This is the first piece of this “tips and tricks” or “best practices” series. We are going to look into record and playback this time. Playback and recording will be essential for both amateur and professional SRM use.

For testing purposes I am always using the latest stable release of our software (release 20150415 at this point), which you may get from our support site: http://spectrafold.com/quadrus/support/

RF Record and playback

Looking into the RF record and playback functionality, one will quickly realize that most of the time we deal with .DSRS binary files (essentially saved samples), which are unique to SRM as an IF file type. I will cover audio recording in a separate post.


We have recorded and shared some IF spectra in a prior blogpost. Feel free to download any of them -  I have chosen 14100+-100KHz-20140316-111835-0984.DSRS, because it has a 200 kHz bandwidth.

You may open and load such a file in SRM by selecting FILE as an input method, then choosing the appropriate file from your hard drive or other location. Start playing it by clicking on ‘Start’.


The user may freely change a number of functions while listening, for example:



You may choose from the following demodulation types:

  1. AM – amplitude demodulation
  2. USB – upper sideband (single sideband) demodulation
  3. LSB – lower sideband (single sideband) demodulation
  4. ISB – independent sideband (or Kahn method) demodulation
  5. FM – frequency demodulation
  6. CW – continuous wave demodulation
  7. IQ – ‘I’nphase ‘Q’uadrature demodulation

If you are using the IF spectrogram (or ‘Waterfall’ as it’s colloquially called), you may want to understand the use of Reference signal strength and the AutoMax/AutoMin functions. You may re-shape the appearance of your waterfall with these, which is very useful to find weaker signals and to separate them from noise more effectively.


Please note that recorded files will be played back continuously and restart unlimited times.


SRM will record into the same DSRS files, which we have discussed at Playback. Firstly, I would recommend to set up a proper folder to save into, which may be done on a per channel basis.


Then you choose the spectrum type in Control -> Recording as shown.

SRM_tips_01_setting_IF_capture_settings_01Recording will start as soon as any source starts feeding data to SRM – just hit the start button. In my case, seen below, I have been generating a known signal with the Internal Generator, to make sure I get the exact same result back.


Saving a spectrum is quite storage intensive: a 1 minute long recording will be approximately 30 MiB with 100 kHz bandwidth. Also, note that due to longer buffering times, your file will appear somewhat later after recording. In my case, it was some 30 seconds after recording stopped. You may close SRM to ensure your file is saved.

The files follow a naming convention:


{filetype IF/AF_channel number-/YYYYMMDD/-/HHMMSS/-/code/}


Feel free to download the SRM-3000 SDR software receiver from the support page and some recorded IF files with different bandwidths from the 20 m and the 15 m HAM radio bands. Using the recordings feels like having actual receiver hardware under your SDR.

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BD8XY_IF_Rec_150113114300-830x990 - feature

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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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 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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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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Recorded signals for the SRM SDR radio receiver

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I had a chance to try and test the DRU-244A digitizer board and the SRM-3000 HF SDR receiver utilizing said hardware platform. I took some recordings in the 20 m (14 MHz) and 15 m (21 MHz) bands, which can be used to test the SDR receiver software offline. The SRM-3000 allows the direct recording of the incoming IF signal into a binary output file.

The file stores the IQ samples as generated by the DDC hardware with the selected sampling rate (bandwidth). Unfortunately, the file itself does not store the bandwidth information, so I’ve encoded that into the file name. During playback, you should set the same bandwidth on the Option/Settings panel.

The two higher bandwidths (200 kHz and 400 kHz) are only available for the four channel  (or less) operation mode. This may be set manually, by editing the config file.

After you’ve set up the appropriate bandwidth for your recording, select the “FILE” input on the control panel. A standard file open dialog will appear, where you may select a *.DSRS file for playback.

After starting the receiver, you will see the IF spectrum, and you can place your demodulator anywhere within the IF spectrum. To increase the volume, set the AGC max gain to 100-120 dB (instead of the default 60 dB) on the Option/Setting panel, or use the manual gain to set the volume. The file is played back continuously in loop back mode.

I’ve made some 50 kHz and higher, i.e., 100 kHz and 200 kHz, bandwidth records.

14050+-50KHz-20140315-142257-0798 14200+-50KHz-20140316-103448-0796
14100+-100KHz-20140316-111835-0984 14200+-200KHz-20140316-114122-0718
21050+-50KHz-20140315-160832-0881 21200+-50KHz-20140316-105210-0046

The recorded signals are available here:


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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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