SMA noise source/Simple spectrum external tracking source DV 12V/0.3A

On eBay, chinese sellers are offering what they call a “SMA noise source/Simple spectrum external tracking source DV 12V/0.3A”. I bought one to test it and here are the first results...

I purchased such a device, as I was looking for a long time to have a noise source. These are handy to test and measure RF filters, as well as, to detect problems in a CATV/SAT distribution network.

Basically you compare what the spectrum looks like before hooking up the filters or at the entry point of the distribution network and then compare it with the spectrum after inserting the filter or at any point within the distribution network.

A good noise source will be totally flat, i.e. deliver the exact same signal level across the whole supported frequency range. This is what makes them expensive!

If you are a professional installer, you may want to consider the purchase of a professional noise source/noise generator, like the ones provided by Rover Instruments:

CNG 90 STC

http://www.roverinstruments.com/prodotti.php?idprod=165

CNG 70 USB

http://www.roverinstruments.com/prodotti.php?idprod=214

For an amateur, these devices come at a cost, which often is not very reasonable, if no money is to me made with the equipment.

So the question is: how good does the Chinese low cost noise source perform, at a price under 20 Euro (with shipping included)?

Here are a few pictures and comments on my initial tests.

Picture of the Terrestrial/CATV Spectrum (5 MHz – 900 MHz), without the noise source:

As you can see, without noise source there is a base noise at less than 20 dBuV.

With the noise source connected, the signal goes to na average of 45 dBuV, without using any attenuator. Notice that there is some loss due to the use of connectors and adapters. The signal is moderately constant with a variation of about 10dBuV across the whole range.

I tested the noise source in the range of 950 MHz – 2150 MHz, also, and the variation is a little more accentuated but still pretty good for such a cheap device.

What can the noise source be used for? For example to test and evaluate filters.

The image shows the result of using an LTE filter. Notice the drop at 780 MHz.

So, overall, my opinion is:

• The noise source does not provide a very constant signal level across the frequency range, since the variations are of +/-10dBuV
• There are constant spikes of +/- 5dBuV
• Still, for less than 20 Euro you can perfectly use it to test filters or problems within the distribution network.
• You need a spectrum analyser do make any use of the noise generator
• I would not recommend it for calibration purposes (as advertised on eBay for the “USB 138M-4.4G SMA signal source, signal generator, simple spectrum analyser 1kHz”, which I have not tested, yet)
• The spectrum analyser should have a functionality that allows superimposing a stored spectrum image to the current spectrum image, in order to easily see what is happening in the distribution network or after inserting a filter. Most professional spectrum analysers offer such functionality

Final result: great device for 20 Euro, ideal for amateurs, but not comparable with a professional noise generator.