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Saturday, 21 October 2017

Can it be real: A DVB-S modulator and transmitter each for under 25 Euro?

Hi,

A DVB-S modulator and transmitter for under 25 Euro?

A DVB-S receiver and demodulator for under 25 Euro?

See this video and find out:


You need:

  1. Raspberry Pi
  2. RTL2832U dongle
That's all!

Does it work great?

No. If you want an amazing modulator for your tests, this is what you need:


Is it amazing?

Hell yes. I learned a lot trying to get it to work!

Cheers,
Vitor


Friday, 20 October 2017

Why not use a RTL2832U dongle for a faster spectrum refresh rate?

Hi,

The RTL2832U dongles are very popular for a cheap entry to SDR (Software Defined Radio).

People who used SDR# (sdrsharp), are amazed by the super fast beyond real time refresh rate of the spectrum - not to mention the high resolution!

Why not use such an SDR dongle as a spectrum analyser? What is the advantage of the SMA/NWT devices?

Well, first off, some quick explanation: the cheap RTL2832U SDR dongles will digitize 2.4MSPS (Mega Samples per Second) and then perform a Fast Fourrier Transformation (FFT) to convert the discrete samples obtained in TIME DOMAIN (signal level in time) into FREQUENCY DOMAIN (signal level in frequency). Because 2.4MSPS are used, this will allow for a span of aproximately 2MHz.

The SMA/NWT devices, however, will capture samples across the frequency range specified, efectively sweeping the band, which is why they are SSA (Sweep Spectrum Analysers) - well sort of, as they are much simpler than "real" spectrum analyser.

Anyway, they manage to get 500 samples in 1-2 seconds. Considering that you can well display a span of 1GHz in 500 pixels, this gives a refresh rate of 1-2 frames per second.

Now, I am surely not the first who thought about using the FFT in 2MHz steps to eventually conver a broader span.

This is what you can do with QSPectrumAnalyzer, a Linux implementation for SDR devices.

Here is the result:


Note that each screen refresh takes about 70 seconds! The reason is fairly simple: a 1GHz range requires 500 2MHz steps for the FFT operation - there goes the fantastic speed...

Here the same signal rendered with the SMA/NWT device using my software:


In this case, the screen refresh takes around 1 second!

My intention is by no means to bash the RTL2832U devices - I love them! I just wanted to explain the difference.

Some notes of interest:

  1. If you look closely at the first image, you will notice that I use Ubuntu with a VirtualBox virtual machine under Windows 10 and it sees the RTL2832U dongle just fine!
  2. Don't buy the cheapest RTL2832U dongles you can find on eBay. Some will suffer from frequent crashes, which will really take any fun of using them.
  3. I just bought the NooElec Smart Premium.It is reasonably priced (around 25 $/£/€) and features a propper design.
  4. The first stop for any RTL-SDR related information is of course: https://www.rtl-sdr.com/
  5. Need some guidance for RTL-SDR under Linux? Look here: https://ranous.wordpress.com/rtl-sdr4linux/
Cheers,
Vitor

Sunday, 15 October 2017

VMA Simple Spectrum Analyser: How to remove the Google Maps warning on the "Log on Maps" function

Hi,

Perhaps you may have noticed that some time ago Google Maps started to complain about the no longer supported browser version when using the "Log on Maps" function.

If you have no complaints - ignore the rest of this message!

This is what pops up (sorry for the Portuguese message - I am using a Portuguese Windows 10 version, but you certainly get the idea):


Though you can click away this message for now, it is annoying and it was worrying me that one day it would no longer work. So I tried to understand what was going on.

The answer is simple: my software uses the browser control provided by Visual Studio. It will emulate by default an old Internet Explorer version and Google Maps correctly flagged this version as not secure anymore.

The fix consists in adding a new entry to the Registry, telling the Browser Control to emulate the latest Microsoft Internet Explorer 11 browser:



  1. Open regedit.exe
  2. Go to: Computer\HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULATION
  3. Add a new DWORD and name it "VMA Simple Spectrum Analyser.exe"
  4. Set its value to "11000" in decimal mode
  5. Restart the the VMA Simple Spectrum Analyser application
That's it!

Atrernatively, you can copy the following green text to Notepad and save it with any name and the *.reg file extention (i.e. "vma.reg"):


Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULATION]
"VMA Simple Spectrum Analyser.exe"=dword:00002af8


Then double-click on the file and allow importing it to the Registry.

The "Log on Maps" function should now run again without any warning message.


Regards,
Vitor

Friday, 13 October 2017

Some considerations about the limitations of the ADF4350/ADF4351/MAX2870

Regarding the SMA and NWT devices, I often receive these questions:

  • Why is there a "notch" in the spectrum?
  • Why does the spectrum look different than expected?
  • I compared the spectrum of the NWT4000 with the spectrum of my professional spectrum analyser and it looks much different... Why?
  • ...
First of all a disclaimer: I did not develop the hardware and I don't sell the hardware! Because I did not like the included software I wrote my own.

For example, this is how my local FM radio band looks like, when using my "SMA Simple Spectrum Analyzer" with ADF4351:


Here is the same spectrum rendered on my Siglent SSA3021X:


The frequency range is exactly the same (88MHz - 108MHz), but the difference is due to the fact that the Siglent SSA3021X is rendering the spectrum with a RBW of 30kHz, while the ADF4351 probably uses a RBW of more than 300kHz.

To explain the concept, here is the a signal captured with different RBW values:

RBW = 1 MHz

RBW = 100kHz

RBW = 30kHz

The Siglent SSA3021X is a "real" Sweeped Spectrum Analyser (hence the "SSA" in the name) and it allows to set the RBW as required. Imagine the RBW to be a window: the sample you grab is the average of what you see through the window. The narrower the windows, the higher the resolution, but the sweep will take longer.

In case of the SMA/NWT devices, we don't have a user configurable RBW. You can only determine a start frequency, a frequency step and the amount of samples you want to capture. The device will then sweep the frequencies - but at a constant RBW.

This GIF tries to show how the detector sweeps the RF spectrum:


The detector will AVERAGE the power level within its window. Unfortunatly, the way the device and its IC's were designed, it has a "blind spot" in the centre frequency of the detector. This is the culprit of the notches on signals with a narrow bandwidth.


When the detector is exactly in the middle of the signal, the blind spot will mask a significant part of the signal, which causes the sample to actually have a smaller power level than the neighbouring samples.

Note that the “blind spot” is not in the centre of the frequency span you are seeing on the screen, but in the middle of the detector used to measure the power level in each sample!

When the narrow band signal is narrower than the RBW of the SMA/NWT device, the samples that completely contain the signal will measure a smaller power level, than the samples where the signal is not completely contained inside the detector window.


Here some extra information on spectrum analysis:

FFT - Fast Fourrier Transformation

In this case, the device will digitize a given amount of samples in TIME DOMAIN. This is much like a digital oscilloscope: you digitize for example 2000 points in one second.

Imagine the signal is a sine at 100MHz (in other words, the period of the signal is 1/100000000 seconds). Capturing 2000 points will allow you to calculate the sine function matching the samples through FFT math. You can then render the function in FREQUENCY DOMAIN, as you know the amplitude (signal level) and frequency.

This is how SDR# works for the RTL2830 based DVB-T dongles.

Advantage: very fast spectrum rendering
Disadvantage: limited span and you need a fast processor to do the maths!

SSA - Sweeped Spectrum Analysis

Here you take the input signal and convert it down to your BAND PASS FILTER. In a good device, the appearture of the filter is configurable (i.e. 10kHz, 100kHz, 1MHz). The filter always operates at a fixed frequency span, so instead of sweeping the filter across the frequency span, the frequency is downconverted accordingly (in the animation of my blog entry, the detector is fixed and it is the frequency ranges that moves).
Next comes the detector: it measures the signal power of the signal leaving the band pass filter.

So the very simplified schematic would be:

INPUT > FREQUENCY CONVERTER > BAND PASS FILTER > DETECTOR

Please note that my explanations are very simplified!

Interested in a deeper insight? Make sure you read this great document from Keysight, a manufacturer of premium spectrum analysers:


Final word:

The SMA/NWT are great devices, considering their prices - no doubts about it. One just has to accept the limitations...

Regards,
Vitor

Thursday, 12 October 2017

VMA Simple Spectrum Analyser - Reflectometer Measurement

Hello,

In the past I wrote two blog entries about a reflectometer measurement with the SMA device:

http://vma-satellite.blogspot.pt/2016/08/reflectometer-measurements-with-sma.html
http://vma-satellite.blogspot.pt/2016/08/reflectometer-measurements-part-2.html

Since then I wanted to implement this as an automatic measurement function within the VMA Simple Spectrum Analyser software and, well, here it is!



Cycle through the measurement screens until you reach Measurements - Reflectometer:


Activate the Reflectometer and select the used Cable Type. The Distance to fault will show the a value in meter: this is the length of the unterminated cable or the distance to the fault in a terminated cable.

The cable type is important, as the propagation speed of the electric signal depends on the medium. In vaccum, the speed would be the sped of light, but in a medium with a foam dielectric, it may only reach 82% of that speed!

The manufacturer of the cable should normally indicate the propagation speed of the cable. This parameter is called VoP (Velocity of Propagation) or v/c (ratio of the propagation speed in the cable and the speed of light).

What if you don't know the VoP of your cable? Well, you can measure it yourself. Just take a cable of the same type and measure it, in order to know its length in meter. Click the "Calculate v/c" button and let the software determnine the VoP of your cable!

The software looks for a file called "Reflectometer VoP.ini", which contains pre-configured cable types. You can edit this file, so that your cables are listed. The default cable type is always the first in this list. Of course, if you determine the VoP as described above, you can select to have it added to the list automatically.


What do you need to carry out reflectometer measurements?

Apart from the obvious (SMA/NWT device and the VMA Simple Spectrum Analyser software), you need a "T" connector and in case of the SMA devices with only one ADF4350/ADF4351/MAX2870, you need a noise generator. NWT users can use the OUT connector.

Interested in further reading about reflectometer measurements?

Here are some links:


Regards,
Vitor



Saturday, 7 October 2017

VMA Simple Spectrum Analyser - Finally corrected the MEM trace resolution!

Hi,

Finally I had time to think about how to correct something that has been bothering me for a long time: when using the MEM trace (clicking the one of the small buttons under the TRACE buttons), the MEM trace would store the respective trace, but with less resolution.

As a consequence, the MATH trace would operate with this lesser resolution MEM trace, too.

It worked, but with less precision than possible and it looked kind of weird.

Well, I have fixed that and you can now benefit from increased MEM and MATH resolution:


You don't need to change anything or operate the software in any different way. Just keep using it as always and benefit from improved resolution!

By the way, did you notice that you can now customize the colours in the main spectrum window? Head on to the SETUP tab and click the "Set Spectrum Colours" button in the "Customization" field.

All configured colours will be stored in the Registry.

Regards,
Vitor

Sunday, 1 October 2017

VMA Simple Spectrum Analyser - New version with support to render pre-definied transponder names

Hi,

I just uploaded a new version that will show transponder names.

When a file named “transponder names.txt” is present in the same folder as the executable and if the checkbox to "Show transponder names" is enabled in the Settings tab, then the spectrum will show the transponder names.

The format of the “transponder names.txt” file is very simple:

Frequency, Name

Example:

754.000 MHz, Porto DVB-T
97.7 MHz, Radio Comercial
104.1 MHz, RFM
105.3 MHz, TSF

Please note that the frequencies have to be entered in MHz.

You can put as many items in the file as you want and the software will render the ones that are within the frequency range. Here the above four entries in action:

This image shows my local FM radio stations.

This images shows the local DVB-T transponder, using the same "transponder names.txt" file. Note that only the names of transponders within the selected frequency range are shown.

There is no further “intelligence” built in! The names are shown in absence of signal, too! And if you show the whole supported frequency range, all set transponder names will be shown, which may look confusing. Yet, I think this is the correct way to do it.

 Don't forget to activate or deactivate this new function as needed in the Setup tab. This setting will be stored in the Registry.

You can use the Recognize Transponder funcion to create the template for the "transponder names.txt" file.

Here I am getting the transponders of the NOS CATV digital transponders:


In order to increase the quality of the recognition algorithm, I reduzed the frequency range in order to first scan 250MHz-500MHz and then 500MHz-750MHz. I exported both transponder lists separately and then merged them together in Notepad++:


This gave me a full digital transponder list in seconds - none of my field meter is able to do such a quick scan!

I could now add a comma and a description for each transponder frequency, for instance the name of the main channels contained in each transponder.


If you add the frequency, but don't add the comma and the name, the software will just show the freqeuncy value:


Again, if you create a big transponder names list and render the whole freqeuncy range, things will look a bot chaotic:


In this case it is better to turn of the display of transponder names in the Settings tab.

Other changes of this new version include:
  • Bug fix: the transponder list is now saved correctly with three decimal places. It can be used as a template for the "transponder names.txt" file
  • Improvement: all pop-up windows now appear in the middle of the main application form
  • Improvement: frequencies could already be entered with the keyboard, but now you can accept the form with the m/M (MHz) or k/K (KHz) keys. In case of the percentage input form, you can use a/A (Accept)
Cheers,
Vitor