Please forgive the long question. I have split it into 2 parts.
I am very confused about how harmonic suppression is measured in RF system (in my case, ham radios). I have picked up a couple of the Baofeng radios: The UV-5R+ and the GT-3, and I am keen to understand how bad the harmonics are whist using them.
I have read the following 2 blog articles that do measurements on these devices: http://hamgear.wordpress.com/2013/01/30/baofeng-uv-5r-spectrum-analysis-revisited/ http://hamgear.wordpress.com/2014/01/24/review-baofeng-gt-3/
Also, referring to the EU standard EN-301-166-1 Section 7.4.3:
"The power of any spurious emission, occurring more than 2,5 × CSP from the centre of the channel on which the transmitter is intended to operate, shall not exceed [0,25 µW (-36,0 dBm)]"
When doing these experiments, you configure a certain attenuation on the Spectrum Analyser. However, by applying different levels of attenuation, doesn't that mean that the measured harmonics will vary?? Does this mean that to obtain the absolute dBm value coming out of the radio, you need to add the attenuation figure onto any measure figures? For example, if you are applying 20dB attenuation, and you measure -50dBm on your analyser, does this mean that your radio is actually putting out -30dBm?? Or have I got this all wrong?? Also, what does reference dB mean? Is that simply the top line on the graph?
I don't have a spectrum analyser, but I do have a very cheap RTL-SDR dongle (DVB-T stick hacked to act as a wideband receiver). Using the program SDR-Sharp, this gives about 50dB dynamic range (i.e. the amount I can see on the FFT display). The FFT display in SDR-Shaper has a y-axis of dB (not dBm, unfortunately). With the absence of any calibration equipment, I had to make do with relative measurements, rather than absolute values...
Using this dongle, an experiment I did, was set my radio to a certain VHF frequency (144.500Mhz), and by way of adjusting the SDR gain and moving the radio closer/further away from the USB dongle, made sure that the main frequency was just touching 0dB on the SDR-Sharp FFT display. I then went on to measure the harmonics, up to the 7th one.
The values I got from the UV-5R+ radio, from the 1st harmonic (main frequency) up to the 7th harmonic were as follows (in dB): 0, -55, -47, -45, -48, -52, -54
Have I measured anything useful here? And can I relate these figures to the EU standard at all?
Any help is appreciated.