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Sunday, 21 February 2016

Measuring an LTE filter with a Rover Instruments HD Touch STC

In my last post, I tested the cheap SMA Noise Source, which is sold on eBay for less than 20 Euro.

To better show the applications of a noise source (sometimes called noise generator), today I used the Rover Instruments HD Touch STC meter - a top of the class professional meter.

The left picture shows the CATV band with just the noise source, while the right picture shows the spectrum with the LTE filter inserted:

With the HD Touch it is possible to use a stored spectrum picture as a back picture. It will be shown as a blue contour.

The photo shows the stored spectrum (using just the noise source) superimposed by the live spectrum, but this time a LTE filter is applied between the meter and the noise source. This filter blocks frequencies from 790MHz upwards, as used by LTE (mobile internet). The LTE signal is infamous for interfering with DVB-T/T2 transponders at close frequency ranges.


As you can see from the picture, my LTE filter behaves exactly as advertised, blocking the whole signal starting at exactly 790 Mhz.

Finally, a picture of the SAT frequency range (The HD Touch supports the full range from 930 MHz - 2250 MHz):

Please ignore the fact that the frequencies are shown with the L.O. frequency added (just subtract 10600 MHz).

As can be seen, the generated noise signal is not exactly flat, but still usable. On a spectrum analyzer with memory function or the possibility to overlay a stored spectrum (like the HD Touch) it can still be used to measure attenuations due to faults in the distirbution network.

Friday, 19 February 2016

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




CNG 70 USB




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.