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 > -- JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France
