[time-nuts] On choosing reasonable synthesizer PN requirements

[Bob kb8tq]

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

> On Apr 24, 2020, at 9:37 AM, Karen Tadevosyan via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hello Bill,
> 
> 
> 
> Thank you for the clarification. It is especially pleasant that our opinions
> are 100% the same.
> 
> However, I would like to find some tool for calculating the balance of the
> radio link in order to understand exactly the reasonable requirements for
> synthesizer's PN. 
> 
> The issue of stability is now gone - I use a good OCXO with the well-known
> Allan deviation values
> (http://www.ra3apw.ru/wp-content/uploads/2020/04/OCXO_Allan_dev_photo-768x48
> 3.jpg &
> http://www.ra3apw.ru/wp-content/uploads/2019/10/OCXO_AllanDeviation_1-768x37
> 4.jpg ) and sufficient stability is confirmed by the successful work in FT8
> mode via QO-100. 
> 
> 
> 
> Regards,
> Karen ra3apw
> 
> 
> 
>> Hello Karen,
> 
>> As a general rule of thumb, if uplink LO phase noise power ends up being 
> 
>> at least 15-20dB below the expected kTB noise received at the satellite 
> 
>> transponder, its contribution is not really significant. With -98dBc-Hz, 
> 
>> on Tx, you should be in good shape because, to simplify things a bit,  
> 
>> Tx SNR at the transmitter far exceeds the SNR at the QO-100 receive 
> 
>> antenna.  Thermal noise from the antenna and receiver LNA will dominate 
> 
>> the received signal noise at the satellite.  It's more important to keep 
> 
>> your Tx frequency stable.  A GPSDO or Rb reference will be useful here.  
> 
>> It's also important to keep the DL 10.45GHz receive on frequency as well 
> 
>> (even more important, given the freq. multiplication up to 10GHz).
> 
>> 
> 
>> On the DL side, antenna temperature and LNA noise should be the dominant 
> 
>> signal degradation factors, so good antenna/LNA G/T is all-important.
> 
>> 
> 
>> Cheers,
> 
>> Bill
> 
> 
> 
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