Re: Fast sampler

On Thu, 6 Mar 2025 10:54:20 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2025-03-06 10:24, john larkin wrote:

On Thu, 6 Mar 2025 14:53:43 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
 

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

john larkin <jl@650pot.com> wrote:

On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

On 2025-03-05 20:07, Joerg wrote:

On 3/5/25 5:00 PM, Phil Hobbs wrote:

On 2025-03-05 19:15, john larkin wrote:

On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> 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>
>

>

>
Neat. No step-recovery diodes.

>
Well, 40 years does get you something sometimes. ;)
>

>
And those cheap yet blazingly fast RF transistors, thanks to cell phones
and all. They make nice pulsers. But they are like the princess on the
pea, very low Vce and if you go a smidgen above ... poof.
>
[...]
>

>
They're not that bad, really--their betas are so high that BV_CEO is
lowish, but BV_CBO is 12 volts or more.  Their saturation behavior is
still pretty BJTish, though. ;)
>
Cheers
>
Phil Hobbs

>
I toyed with the idea of using a PHEMT as a series-switch fast
sample-and-hold.

>
They work well for that. A couple of years back, we did a POC for the Navy
that used several SAV551pluses—100 ps is doable. The main problem is that
their voltage gain is lowish, so you don’t get as much speedup as with a
BJT.
>
And of course they’re 10x the price.
>

>
Hey, here's another goofy idea:
>
We used to make fast linear ramps, driving a comparator against a DAC,
as a programmable delay. But we got smarter and just used an RC
charging thing, and mucked the DAC codes with a polynomial to get our
delay.
>
But what if the comparator sees a fast RC on one input and a slow RC
on the other? The exponential curves cancel, and you get a nice slow
linear sampling timebase. If you don't quibble too much.

>
Not sure about that.  For the proto, I used a ramp from an arb to make the
threshold—the sampling loop converged at each point, so I wound up with a
10**7:1 zoom—10 us per picosecond.
>
The fast bit was all over before the slow bit moved perceptibly.
>

>
The other issue is that the prop delay depends on the overdrive. Since
we’re comparing a ramp to a fixed threshold, which that basically means how
far the ramp rises during the time required for the positive feedback to
get going.
>
So we still need an online calibration. Fortunately that isn’t hard—an
open-circuited bit of coax is enough. It doesn’t have to be done often.
>
Cheers
>
Phil Hobbs

 
I did caution about quibbling too much. One issue is that the LVDS
line receivers have a bunch of offset as the common-mode voltage
approaches the positive supply rail. And of course the esd diodes are
nonlinear capacitors. And things always ring a little. Geez, nobody's
perfect.
 
One of my guys did a bunch experiments using an LVDS receiver as the
comparator in a picosecond-resolution delay circuit. We use a 16-bit
DAC and a 4th order polynomial and calibrate the polynomial for every
channel. Our P500 has, I recall, nine of those.
 
https://highlandtechnology.com/Product/P500
 
When I was young and foolish, I used to do time delays with linear
ramps and ECL comparators.
 

>
The line receivers don't seem to have any significant amount of kickout,
either--we can sweep the Rx pulse across the Tx pulse with no apparent
funnies due to interaction.  Have you folks seen any kickout issues?
>
Of course the kickout might be delayed, I suppose.
>
Cheers
>
Phil Hobbs

We didn't test for that. When a comparator fires, all sorts of stuff
happens downstream, that could jostle adjacent channels.

Our comparators typically drive a 1 ns Tiny Logic flipflop as the next
step in the signal chain.

Standard ECL wasn't as fast as 15 cent Tiny parts are now.