[time-nuts] time-nuts Digest, Vol 189, Issue 37

[Bob kb8tq]

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 <time-nuts at lists.febo.com> 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
>> 
> 
> 
> 
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