Other than frequency translation, AM is basically a linear process. So it will show THD, noise, etc.
BTW, my DVB-T dongle has a MCX connector. I'm not really sure how serious these manufacturers are in controlling impedance. Maybe they are 75 ohm, and maybe not. There are 75 ohm BNCs, as determined by the center pin. I have more audio than RF experience, but much of the testing is the same. In audio, if you had your test source phased locked to the the A/D, you SINAD can be computed direction from the FFT bins. Signal is one bin, noise and distortion is everything else. [There is probably some RMS addition of the bins, which you would have to look up.] Once the source and sampler are not phased locked, then windowing comes into play and things get more complicated. It isn't all that different for your AM demodulated signal. In the case of the DVB-T, they are not phased locked, so you will have to count a certain number of bins as signals and the rest as noise. For PSK or any type of digital demod, examining the eye or constellation would make more sense. But I see nothing wrong with doing all these evaluations with just AM, since it is a decent figure of merit test. It has been my experience just using these dongles as NFM or FM radios, that they are not particularly good at low frequencies (in the 100MHz range). I only use them for 1090MHz mode-s. On Tue, 26 Aug 2014 07:44:19 +0200 friedtj <[email protected]> wrote: > I have just reproduced all measurements making sure to compensate for > frequency offset between the synthesizer and the local oscillators of > the DVB-T, and added a table summarizing the measurements (peak power > and noise floor of the RF and FM-demodulated signals). > > Could you please comment on the FM demodulation sudden drop ? This is > indeed what I observe but I fail understanding the cause. I have > access to most modulation schemes: could you please advise on which > would be most relevant ? I have selected FM because it is the > modulation scheme I am aiming at in the case of NOAA POES reception, > although the 1500 MHz measurements are targeted towards GPS reception > so PSK would be more relevant indeed. > > Thanks, JM > > > > In order to quantify my abilty to receive weather satellite > > > images using DVB-T running with GNURadio, I have setup a very > > > simple experiment as follows: > > > * tune a frequency synthesizer to 137 MHz, FM modulation, 5 kHz > > > deviation and 2400 Hz FM frequency > > > * frequency synthesizer output power setup to -91 or -101 dBm > > > (same setting for all receivers, 2 sets of measurements) > > > * connect the output of the synthesizer to the input of the DVB-T > > > (in all cases, a new BNC connector was solder instead of the > > > original 75-ohm antenna connector), > > > * plot the RF signal FFT and the FFT of a WBFM demodulator block > > > as found in GNURadio (always same settings: 30 dB IF gain and 30 > > > dB RF gain) > > > * assess the receiver quality by the signal to noise ratio of the > > > FM output. > > > > I'd suggest using something other than FM if you can find a good > > reference. Looking at the output of an FM demod is going to mask > > most of the differences you're looking for, and you're much more > > likely to get a very sharp cutoff between having good output, and > > not having an intelligible output at all. > > > > David > > > >
