[Ronja] new receiver design

[Gullik Webjörn]

On Tue, 2010-03-30 at 02:03 +0300, mohamad al-jamal wrote:
> the design is performing very nicely ... distance without lenses was
> roughly 1.5m .  i'am still experimenting though ... as for the
> sensitivity and noise , i intend to increase the feedback resistor to
> 400K or so , it will form a pole almost at the same point inferno
> does , but with better sensitivity and noise performance ( at least
> theoretically !!)  .. the signal at the output of the TIA is a
> sawtooth with 450mV p-p ... i don't know how to measure the noise
> properly !! but without an optical signal,we get a random noise
> (without oscillations) with about 80mV p-p .  

My simplistic view 450/80 = 15 dB. If you use a good oscilloscope, 
measure the signal, then increase gain and measure vertical "thickness"
of the trace, this works best on a square wave.

>  the strange thing we found is that the smaller R3 is , the better the
> performance gets !! any explanation !?!? as for the B.W , we used a
> function generator as a signal source , and the we ranged the
> frequency from 1MHz up to 13MHz or so , and nothing went wrong ..  
> 
Measure voltage drop across R3 on various signal conditions. Maybe
you find you are saturating the pin, causing large voltage drop, 
decreasing bias on PIN, causing less gain.

What about a small R3, cap to ground, small L (ind), cap to ground??
Since the other node, PIN cathode goes to OP input / feedback resistor,
a low R3 will not increase risk of excessive current in PIN.

Regards,

Gullik