Re: Fast sampler

On Tue, 11 Mar 2025 14:12:51 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2025-03-09 19:22, Tom Del Rosso wrote:

Phil Hobbs wrote:

Hi, All,
>
Late last year we did a fast sampler/TDR with nice clean 60 ps edges.
>
We're gearing up to actually sell them, so I did a short technical
writeup on the design, which may be of interest.
>
<https://electrooptical.net/News/a-high-performance-time-domain-reflectometer>
>
Cheers
>
Phil Hobbs

 
Why is a LASER ruler that can measure distances in air with 2mm accuracy
$16,
 
and an OTDR for measuring fiber $600?
 

>
Well, the OTDR needs a fiber-coupled laser, for one thing, and a decent
TIA.  Also as John says, it's a time-domain instrument.  A laser ruler
can just be a diode laser with a monitor photodiode, a collimating lens,
a simple TIA and a micro with built-in ADC.
>
You put a small (1-3 mA) current ramp on the laser, and look at the beat
signal coming out of the monitor photodiode.  The frequency gives you
the round-trip delay.  This sort of laser feedback measurement can be
pretty good if the diode stays reasonably single-mode.
>
I have no idea how the $16 ones do it, but if I were building one,
that's the first thing I'd try.
>
Cheers
>
Phil Hobbs

I wonder if you even need a photodiode.

Reminds me of the 3-tube proximity fuse, where the plate current of an
RF oscillator was modlated by doppler from a target airplane. Or the
ground.