[time-nuts] OT: transfer of 3 GHz via fiber optic

[Didier Juges]

Dr Bruce Griffiths wrote:
> Didier Juges wrote:
>   
>> Dr Bruce Griffiths wrote:
>>   
>>     
>>> Didier Juges wrote:
>>>   
>>>     
>>>       
>>>> Bruce,
>>>>
>>>> I have read about this, noise performance also is not good for analog 
>>>> transmissions, causing very limited dynamic range.
>>>>
>>>> That's probably why they use either FM or digital coding in just about 
>>>> all applications.
>>>>
>>>> I just did not think the jitter would be so bad, even with a laser 
>>>> transmitter.
>>>>
>>>> In my case, we are just trying to send a reference signal, but it must 
>>>> be clean, so unless a clean-up PLL is used, forget about fiber.
>>>>
>>>> Thanks,
>>>>
>>>> Didier
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>>>>   
>>>>     
>>>>       
>>>>         
>>> Didier
>>>
>>> I presume you are directly modulating the laser.
>>> This is hopeless you get chirping and all sorts of other effects that 
>>> are virtually impossible to tame.
>>> An external electrooptic (LiNb03) modulator is the only way that 
>>> actually works and has a halfway decent performance.
>>>
>>> Bruce
>>>
>>>   
>>>     
>>>       
>> Bruce,
>>
>> I am talking about an integrated 4 GB/s transceiver from Avago: 
>> AFCT-57R5. It is a small plug-in module, so they probably directly 
>> modulate the laser. I understand when the laser turns on, there are 
>> probably a bunch of transient effects until the beam is stabilized. An 
>> external modulator would alleviate these problems, but the cost is most 
>> likely out of the question for this market.
>>
>> Both the transmitter and the receiver each have *average* jitter 
>> specifications of about 60 pS, from logic level signals.
>>
>> So even with a better modulator, the receiver still would create way too 
>> much jitter. It is probably a pin diode. I guess the answer would be an 
>> heterodyne receiver?
>>
>> Probably performance would be much better if it were not measured at the 
>> end of 10 kM of monomode fiber, but they don't say. I guess they are 
>> designed for an application where it's 10kM or you don't need it.
>>
>>
>> Didier
>>
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>>   
>>     
> Didier
>
> There is no virtually possibility of direct modulation working 
> sufficiently well.
> If an external modulator is unaffordable, then forget it.
> All successful RF over fiber applications uses external modulators.
>
> Integrated lasers + modulators cost somewhere in the region of $US1000 
> or a perhaps a bit more.
> The microwave InGaAs photodiode and matched amplifier at the other end 
> has a similar cost.
>
> End to end, the phase noise floor of around -130dBc/Hz at L band.
> This may well be good enough.
>
> Bruce
>
>   
Bruce,

Thanks for the rough order of magnitude. I believe the cost you are 
indicating may not be out of line. It will save a remote synthesizer, so 
that would be a significant saving, over simply sending a 10 or 100 MHz 
reference. I will check with my friend and if he is interested, I may 
call you back for more details.

Thanks a lot.

Didier