About the -3dB bandwidth of TIA

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Dear All, Would you please answer the following question?

 If the small signal bandwidth(-3dB) (Cin=0.5pF)of TIA designed for
1.25Gbps application is only 700MHz,is this TIA OK for 1.25G PIN-TIA
application?If not,what is the problem caused by narrow bandwidth?


Thanks and best Regards, Jack


Re: About the -3dB bandwidth of TIA


Jack schrieb:

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What is the optical modulation format? If it is NRZ (non-return to zero)
than the most energy in the power density spectrum of your signal is
contained up to 0.6 or 0.7 times the bit symbol rate. And -3dB BW does
not mean that any frequencies above 700 MHz are sharply cut-off, either.
So 700 MHz is just OK for 1.25Gbps NRZ without significant penalty on
eye-opening.

More bandwidth would reduce the rounding of the 0-1 edges somewhat but
would mean more noise power which is proportional to the TIA bandwidth.
So 700 MHz maybe indeed a good choice. Be sure that your photo diode has
only the specified 0.5 pF capacitance or you will see a penalty for
reduced bandwidth and reduced eye-opening.

If your modulation format is RZ than 700 MHz would be to small for 1.25
Gbps.

Regards
Rainer


Re: About the -3dB bandwidth of TIA


Hi Rainer,

Thanks for your kind reply!

The TIA is designed for GPON/GEPON ONU and 1.25G SFP applications,would
you please explain the concern of TIA bandwidth for the
applications,respectively?

Thanks and Best Regards,

Jack


R.E. wrote:
Quoted text here. Click to load it


Re: About the -3dB bandwidth of TIA


Hi, Jack:

I'm not the expert for GPON, but as far as I know its a optical NRZ
format. But 700 MHz TIA bandwidth really seems to be too small. A common
choise is TIA-BW = 0.75 * bitrate. A smaller bandwidth will reduce
signal power and edge-steepness of the pulses, a much higher bandwidth
will generate more noise. Both will increase bit error rate. See also
the application note from Maxim:
http://pdfserv.maxim-ic.com/en/an/2hfan901-rev.pdf

For example, if you look on the Maxim chipsets, the MAX 3266 designed
for 1.25 Gbps has 920 MHz TIA BW.

I'm not related with Maxim in any way. But I know their chip sets and as
an academic teacher I'm quite happy with the detailed specifications and
application notes they provide. Good for use when training students.

Did you ask your manufacturer of the 700 MHz TIA if it is really good
for 1.25 Gpbs? What did they say?

Regards
Rainer


Jack schrieb:

Quoted text here. Click to load it


Re: About the -3dB bandwidth of TIA


Hi, Jack:

I'm not the expert for GPON, but as far as I know its a optical NRZ
format. But 700 MHz TIA bandwidth really seems to be too small. A common
choise is TIA-BW = 0.75 * bitrate. A smaller bandwidth will reduce
signal power and edge-steepness of the pulses, a much higher bandwidth
will generate more noise. Both will increase bit error rate. See also
the application note from Maxim:
http://pdfserv.maxim-ic.com/en/an/2hfan901-rev.pdf

For example, if you look on the Maxim chipsets, the MAX 3266 designed
for 1.25 Gbps has 920 MHz TIA BW.

I'm not related with Maxim in any way. But I know their chip sets and as
an academic teacher I'm quite happy with the detailed specifications and
application notes they provide. Good for use when training students.

Did you ask your manufacturer of the 700 MHz TIA if it is really good
for 1.25 Gpbs? What did they say?

Regards
Rainer

Jack schrieb:
Quoted text here. Click to load it


Re: About the -3dB bandwidth of TIA


R.E. wrote on 22-Jun-06 06:30 :
Quoted text here. Click to load it

That's obvious.

[...]
Quoted text here. Click to load it

If "Jack" is still reading this...

"0.75 * Baud" is related to transmitted signal power of an NRZ data
stream.  If you were to integrate the power spectrum of NRZ PRBS you
would find that 94% of total power is contained in a bandwidth that is
roughly 75% of the line rate.

Go ogle this:  http://www.maxim-ic.com/appnotes.cfm/an_pk/870

One reason to worry about that effect is SNR.  With wider bandwidth you
can preserve more of a transmitted pulse shape, but you also get more
noise. So the goal is to restrict bandwidth enough to reduce noise below
a tolerable level -- thus, preserve sensitivity at a TIA -- and avoid
unacceptable pulse distortion.  Hence, choose a cutoff frequency of
~0.75 x Baud.

If more than one element in a serially-connected, mostly-linear link has
bandwidth of 0.75 x Baud, then overall bandwidth will be <= 0.64 Baud.
That effect is "bandwidth shrinkage":

Google:  Results 1 - 10 of about 1,170,000 for bandwidth shrinkage.
(0.21 seconds)

One goal here is to ensure that all link elements but one have bandwidth
 >= 0.75 Baud.  My favorite choice for those other elements is at least
4x higher bandwidth.

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