Hi Based on a 30 db carrier to noise floor at the sat:
Going from -120 dbc/Hz to -60 dbc/Hz will have << 0.1 db impact. Going from -60 dbc / Hz to -30 dbc/Hz would be in the 3 db range. Simply put, the signal is way down in the noise by the time it gets to the sat. There is very little gain from a low noise transmit signal. Keep in mind that if you get into modulation rates that are << 1 Hz, getting to -30 dbc / Hz phase noise may indeed be challenging. What matters is the noise “in band” with your signal. Say your transmit signal occupies a 10 Hz (+/-5 Hz) bandwidth centered at the carrier. The system mostly cares about the phase noise in the 1 to 5 Hz range. If you are transmitting at 2.4 GHz and multiplying up from 10 MHz, you get a noise increase of 20 log(N) = 20* log( 2400/10) = 47.6 db. To hit a -60 dbc/Hz “goal” the 10 MHz device would need to be around -107.6 dbc/Hz. Also the synthesizer would have to add no noise to the output. That synthesizer would be running a pretty good OCXO to hit -107 at 1 Hz offset. Designing a “zero noise” synthesizer …. ummm …. errr … good luck :) Lots of fun !! Bob > On Apr 24, 2020, at 12:52 PM, Karen Tadevosyan via time-nuts > <[email protected]> wrote: > > Hi Bob, > > Your calculation is close to real with QO-100. > With ERP = +55 dBm we have about 30 dB S/N on RX side. > If so when we changing the TX LO's PN level from 120 dBc/Hz to 90 dBc/Hz we > lose only 3 dB in terms of RX S/N? > Not sure if you can even notice ... > > Karen, ra3apw > >> >> Hi >> >> Assuming you know the power level you will be delivering to the sat and > the >> noise figure of the >> receiver on the sat, the calculation is just as presented earlier. If the > antenna >> on the sat has gain, >> that also gets into this and that. >> >> Using some made up numbers, since I do not have the real ones for this >> exact system: >> >> You start out with +50 dbm ERP from your antenna. >> You have a link loss of 190 db. >> The sat antenna has 1 db of gain. >> >> You are delivering 50 - 190 + 1 = -139 dbm to the sat. >> >> If the noise figure of the sat reciever is 3 db, then it?s noise floor is > -174 + 3 = >> -171 dbm >> >> That puts a limit on the uplink signal at 171 - 139 = -32 dbc. >> >> If the noise on your transmitted signal is 32 db down, you will degrade > the >> SNR at the sat by >> 3 db. ( = they add as power not voltage). >> >> You will need to do some research with Mr Google to come up with the real >> numbers to plug >> into the calculations. They are different for each system design ( your >> antenna, your erp ?.). >> >> Bob >> > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
