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Saturday, 25 February 2017

VMA Simple Spectrum Analyser: SWR Measurement


I finished the first implementation of the SWR measurement.

The proceedure is as follows:

Select the desired frequency range.

Connect a noise source or in case of an NWT4000/NWT6000, do a sweep with the SWR bridge open.

Press the small MEM button under the LIVE trace button. The LIVE trace will be stored in MEM and rendered in orange colour.

Connect the DUT. In this example, I connected an LTE-filter.

Activate the MATH trace and select in MATH definition the "Perform SWR Measurement on MEM Trace".

The SWR curve is drawn and the vertical scale changes from dB to VSW.

Select if you want a scale from 0-5 VSW or 0-50 VSW. It is not automatic as VSW can tend to infinite... 

Open connection: 100% signal loss!

In this example, DUT is a 5dB attenuator.

Same 5dB attenuator, but selected scale from 0 to 5 VSW.

Head on to the download page and get the latest release! Also, do try the other new MATH functions.

But beware: all of this is fairly new and bugs may exist...

UPDATE: Please note that the pictures show the SWR measurement in an unusual way, the Y axis is inverted to what is common. I changed this and corrected the scale, to not show a 0.00 value. If you already downloaded version 2017-02-25, please download the corrected version 2017-02-25a.


Friday, 24 February 2017

Siglent SSA3021X - New firmware P08.02 available... and a big surprise!


Yesterday Siglent published their latest firmware for the SSA3021X spectrum anlyser.

This device is notorious for three reasons:

  1. It is the most affordable spectrum analyser with the best price/performance ratio.
  2. It is hackable and Siglent doesn't seem to care too much.
  3. It not only features lots of great functionality and high refresh rate, it supports SCPI commands over USB and LAN, also.
With this new firmware release, Siglent went ahead and implemented a promised feature: support for SCPI over simple telnet protocol, which means that NI-VISA is no longer required (but still available)!

I had to test it and mind you, telnet with root login is still possible and the hackabilty status remains the same. It is safe to upgrade from P08.01.

With upgrade concerns cleared off, now hold on to your seats and look at the following pictures;

What you are seeing is a quick and dirty hack I just did in half an hour to allow my VMA Simple Spectrum Analyser software to use the Siglent SSA3021X as a spectrum analyser source, instead of the ADF4351 device! No installation of third party software required.

This is of course just a test - implementing support for the SSA3021X will take a lot of work and it is easier to just program a whole new application from scratch. Remember that all SCPI commands to control the spectrum analyser need to be implemented.

But: Siglent opend doors for advanced measurements like the ones I implemented in my software to become available for a propper spectrum analyser at real time refresh rates, The waterfall display is now running smoothly, too. Nice!

And yes, the Siglent only renders 752 points horizontally, which is why there is no trace on the right corner. As I said: quick and dirty hack... But I am already dreaming on doing a whole new "VMA Spectrum Analyser" software to have a platform for all my ideas regarding advanced measurements.


Sunday, 19 February 2017

VMA Simple Spectrum Analyser: New automatic Channel Power Measurement!


After implementing the Automatic Bandwidth Measurement, I thought "why not implement the Channel Power Measurement, too?".

Well, here it is:

Very simple to use: if you activate just one of the three Bandwidth Measurements (at -3dB, -6dB or -60dB), the resulting channel power is calculated and displayed.

To make this easier to analyse, the area contemplated for this measurement is painted in purple colour.

The channel power displayed is the summed power of this area, according to the formula:


I did some measurements with the Siglent SSA3021X and the Deviser S7200, to get an idea of the Channel Power Meter Measurement precision.

Here are the results:

Picture 1: Channel Power Measurement carried out with the VMA Simple Spectrum Analyser. I get at -6dB bandwidth a channel power of -47.8 dBm.

Picture 2: Same measurement carried out with the Siglent SSA3021X. Channel power is measured at -52.08dBm.

Picture 3: Using the Deviser S7200 TV Analyzer, this picture shows the spectrum, which looks exactly like the ones obtained in the previous two pictures.

Picture 4: The measurement screen of the Deviser S7200 shows a channel power of -48.3dBm.

  1. The overall difference in readings of around 4dB results from different factors like:
    • Varying signal power due to live transmission.
    • Different connectors/adapters.
    • Intrinsic errors.
    • Measurements carried out at different RBW: the first picture shows my measurement at 100 samples - if I increase it to 1000 samples I get a slighly higher channel power reading, due to the higher resolution.
  2. The VMA Simple Spectrum Analyser allows fairly accurate channel power measurements at an incredible price, considering how much the ADF4350/ADF4351/MAX2870 devices cost.


VMA Simple Spectrum Analyser: Support for MAX2870 added - allows use of NWT6000 devices


Just to let you guys know I have added support for the MAX2870 synthesizer, which is used for instance in the NWT6000 range of devices, supporting a frequency range of 23.5MHz up to a whopping 6GHz!

I used the opportunity and fixed an annoying bug with the auto amplitude algorithm. It now uses the data of the finished sweep, instead of using the values up to the current sweep position.


Monday, 13 February 2017

DTT - Digital Terrestrial Television in Portugal: How to monitor the signal for little money...

Today I was looking at the spectrum with the VMA Simple Spectrum Analyser software and satisfaction took over...

Here is a nice measurement of the Portuguese TDT ("Televisão Digital Terrestre"):

In one single screen, the following information is automatically measured:

  • Transponder frequency (754MHz)
  • Transponder bandwidth at -6dB (8MHz)
  • Signal to noise ratio (16.4dB)
  • Signal power (62.9dB)
  • Real time spectrum
  • Waterfall diagram
Head on to the download page and try it out yourself!

All you need is an ADF4350/ADF4351 based spectrum analyser, which costs around 60 Euro on eBay.


Sunday, 12 February 2017

VMA Simpe Spectrum Analyser: New function! Automatic Bandwidth Measurement


I just added a new function to the VMA Simple Spectrum Analyser: Automatic Bandwidth Measurement.

This function will measure the bandwidth at -3dBm, -6dBm and/or -60dBm.

Picture 1: Automatic bandwidth measurement at -6dBm, determining the correct 8MHz bandwidth of my local DVB-T transponder.
Picture 2: Simultaneous automatic bandwidth measurement at -3dBm, -6dBm and -60dBm.

For this to work properly, you need to setup the spectrum so that only the desired signal is visible. Switch to the "Measurements - Bandwidth" tab under the spectrum view and select the required bandwidth measuremtents.

There is a problem: because the ADF4350/ADF4351 will actually produce clipping on the centre of small bandwidth signals, my algorithm needs to know the minimum frequency distance it should seek. If you see the bandwidth measurement drawing the end frequency line in the centre frequency of the signal, reduce the "Min. seek distance" field.

This is still beta - feedback is welcome!


Thursday, 9 February 2017

VMA Simple Spectrum Analyser: Important bug fix


Shame on me!

When I first started playing with my first SMA Simple Spectrum Analyser device, I noticed that all individual measurements are sent from the device as 4 Bytes.

On the original NWT series, the first two Bytes form a 10 Bit value, which is repeated on the second two Bytes. With all my tests using my ADF4350/ADF4351 based device, the second and fourth Byte was always zero.

This led me to believe that the ADF4350/ADF4351 based device could only produce an 8 Bit measurement.

And so I programmed the software accordingly...

Only yesterday a new user (thanks!) complained about a strange clipping on Signals with more than  -40dBm. And he told me that with WinNWT4 those Signals are displayed correctly.

My first reaction: impossible.

Today, after some discussion with said user I realized a huge mistake I made: the ADF4350/ADF4351 can of course do 10 Bit resolution. I just never fed a signal with more than -50 dBm!

Arghhh! How stupid of me. Always playing with TV, CATV and SAT signals, I never tried to feed a signal from a signal generator with -30 dBm, -20 dBm. heck 0 dBm (it is safe to go up to +5 dBm).

This is how it looks like:

Picture 1: Clipping due to my software only evaluating the first 8 Bit instead of the whole 10 Bit

Picture 2: WinNWT4 showing the signal correctly

As a result, I sat down and corrected all routines, adding the missing two bits.

It does work now:

Picture 3: Same signal shown correctly using 10 Bit resolution of the ADF4350/ADF4351

 Please download the latest version!