[Laser] TX Circuit for high speed data.

[F1AVYopto at aol.com]

>I have been working with the K3PGP TX  circuit to run high speed data. I have
>found that when my data speed goes  much above 150Khz the data starts
>getting rounded off, and by the time it  I get it to 200Khz the signal 
driving
>the gate on the IRF-510 starts  looking pretty ugly. In order to drive the 
IRF
>510 I am driving directly  from the data output, which is buffered. Once I 
get
>rid of the two 1K  resistors in line between the input and the gate I am able
>to run the  data rate all the way up to 10Mhz with no problem. I have not put
>a laser  diode on the circuit  yet, I have been using LED's to test since I  
do
>not want to put a laser in there and destroy it. I am assuming that  the 
circ-
>uit was really designed for the low speed testing that is  generally being 
done
>on the NLOS activity. Is there any problem with  removing the 1K resistors
>and driving the gate directly with the TTL  buffered data output? I have kept
>the diodes and the zener in line just  jumped over the 1K resistors.
>Any observations on this?  


Chuck
I think the problem is due to the 100 K resistor effect  that give a high 
impedance to the global input gate circuit and so reduces the  response speed 
with the input capacitances.
The two diodes with the zener and  the 1K resistors set the gate voltage 
limits if the input signal is too  high.
The coupling between your signal generator and the laser driver must be  a 
low impedance one.
The best is may be to reduce the 100 K resistor to a  lower value (5K or 
less) to  improve the response time without changing  other components mandatory 
for MOSFET and laser diode protection.
If your  signal generator or your digital output for the modulation has a low 
output  level you can however reduce the 1K resistors to a lower value. 
73 Yves  F1AVY