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Winner of the "Project Idea" contest

February 21st, 2012 Leave a comment Go to comments

STM32F4DISCOVERY Board

Yesterday our “Project Idea” contest ended and we have spent the day reading all the project ideas from you.
Some of them were more advanced than others but we have received a great number of project ideas and would like to thank you all for participating.

So now we are ready to present the winning Project Idea:


Project Title: Use of a Micro controller in a 50MHz to 1GHz spectrum analyser project.

This project idea was sent to us by Roberto Savo, an RF Electronics Engineer from South Africa.



Here is his submitted project description:

A spectrum analyser is a useful tool for the Radio Frequency (RF) design egineer enthusiast. However, the cost of such equipment is far beyond the average individual. In fact, an entry level spectrum analyser that spans 50MHz to 1GHz is typically in the region of $1000.

The objective of this project is to design a low cost spectrum analyser that will provide satisfactory frequency measurements so that the RF engineer can “see” the RF signals

The circuit will comprise of three main blocks

Block 1:
An array of RF voltage controlled oscillators (VCO’s). A number of VCO’s are banked together to span the frequency range 50MHz to 1GHz. For example, VCO 1 will cover the range 50MHz to 150MHz with a voltage sweep of 0 to 5V. VCO 2 will cover 150MHz to 300MHz with a voltage sweep of 0 to 5V. VCO 3 will cover 300MHz to 450MHz with a voltage sweep of 0 to 5V etc all the way up to 1GHz

Block 2:
A balanced mixer will take in the frequency that is generated by a VCO and mix it with an incoming RF signal. The output of the mixer is fed into a 10MHz filter and rectified to give a DC voltage. For example, if the incoming RF signal Fin= 100 MHz, then the VCO will be at Fvco =90MHz. The mixer will give an output frequency Fout = Fin – Fvco = 10MHz. This 10MHz frequency is rectified and its DC voltage is measured. If Fin is indeed there, then Fout will generate a voltage in the rectifier which is proportional to Fin.

Block 3:
This is where the micro controller comes in. The micro controller will be resposible for (i)creating the sweep voltage for the VCO’s, (ii) switching between VCO’s and (iii) sampling the rectified voltage at the output of the 10MHz filter that was described in block 2 above.

Note that the voltage applied to a particular VCO bank can be correlated with a data sheet to verify what frequency is being generated. This can be done by means of a lookup table.

It would know which VCO that it is sweeping with. It would know what output voltage is being applied to the VCO. Using a look up table, the micro would look up the associated frequency. The voltage at Fout is monitored to see if a signal is indeed present.

Finally, if a voltage is present at Fout, it implies that Fin is present where Fin = Fout + Fvco. The level of the voltage at Fout will give an indication of the RF power level of Fin.

In summary, the objective here is to build a low cost spectrum analyser so that the RF engineer can “see” the fruits of his design without spending an arm and a leg on expensive

At first we were a bit skeptical whether it was possible to do this project with a limited budget of around $200. But after reading a bit on the topic we discovered that it would be possible to create a bare-minimum spectrum analyzer within that budget.

We like this project beacuse it contains both a great amount of hardware RF development but also a decent amount of microprocessor development. It might sound as a complicated project, and it sure is, but hopefully Roberto will be able to complete this project with his new STM32F4 Development. We will also try to help him complete the project or we might even try to do it on our own.

We at TKJ Electronics are looking forward to get some pictures, datasheets, schematics etc. of your project and soon we might also be seeing some images.

Once again thank you to all the participants.
We enjoyed making this contest and there will definitely be more in the future.

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  1. Darren
    April 13th, 2012 at 03:30 | #1

    You can you a cheap $30 DVB-T USB tuner card as a spectrum analyser on the PC. I wonder if the discovery board would have enough processing power to capture, run an FFT, and dump the results to an LCD.

    http://sdr.osmocom.org/trac/wiki/rtl-sdr

  2. April 13th, 2012 at 21:52 | #2

    @Darren
    With the help of the extra DSP math core instruction the FFT calculations will be speeded up a lot, so I think it will be possible.
    Thank you for sharing the link with us. If I get time and get hold of a such DVB-T tuner card I will definitely try it out.

    Best Regards
    Thomas Jespersen

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