[Laser] A lot but very little

[Chuck Hast]

On 1/2/07, Kerry Banke <kbanke at qualcomm.com> wrote:
>
> Chuck - I'm assuming the response is being limited by the
> receiver.  I have been driving the low cost laser pointers (
> internally only a resistor and laser diode) with a good frequency
> response of at least 500 MHz and I suspect that is still limited by
> my receiver.

This is good to know. I was looking inside the ones I have and they
are all the old style (expensive) with the pot, caps and other components
in there.

> If you want to see what is doable from a frequency
> response standpoint  then I'd go to a 50 ohm system but then you need
> some form of modulation to pass through RFamplifiers. With the system
> you have , you will probably need to reduce the input resistance to
> improve the frequency response but of course that will reduce your
> sensitivity.  For a frequency response  test I drive an LED with a
> signal generator and look at the receiver output vs frequency ( for
> responses in the low  MHz range). For higher frequencies I use the
> laser pointer.  The K3PGP front end is great for low frequency ( 2
> KHz or lower depending on the PIN diode), weak signal work but you
> may be better off with a transimpedance amp design for higher speed data.

In my investigation, I found that PIN diodes used as photoconductive devices
are the way that high speed demodulation is obtained. It looks like the gate
of a FET is driven by the level changes across a PIN diode which is hooked
to +V and a two resistor divider.

I was driving a LED with a square wave source, I was able to observe that
as I went up in freq my RX end started to show ramping on the falling edge
of the signal, so my RX was too slow but on the falling side, the front edge
of the signal was nice and vertical all the way to the frequency where the ramp
intercepted the rise.


-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."