Discussion:
Fast sampler
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Phil Hobbs
2025-03-05 23:20:47 UTC
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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
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
john larkin
2025-03-06 00:15:14 UTC
Reply
Permalink
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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
Neat. No step-recovery diodes.

The LVDS line receivers are radical parts, for 30 cents.

We also used ATLC2 to get a good match of a cheap fat-pin edge-launch
SMA connector to a multilayer PCB. That was fun.

https://www.dropbox.com/scl/fi/x0m28m9y04771m5puzimi/Edge-Launch-Connector_small.bmp?rlkey=9uded3bkuplmwtvhgvtr8yw9u&raw=1

Loading Image...
Phil Hobbs
2025-03-06 01:00:49 UTC
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Permalink
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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. ;)
Post by john larkin
The LVDS line receivers are radical parts, for 30 cents.
We also used ATLC2 to get a good match of a cheap fat-pin edge-launch
SMA connector to a multilayer PCB. That was fun.
Yeah, you tipped us off to the possibility, so Simon cranked on it.
Deleting the L2 and L3 grounds, and via-stitching a L4 ground through
the whole stack, makes the cheap connectors pretty good.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
Joerg
2025-03-06 01:07:06 UTC
Reply
Permalink
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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.

[...]
--
Regards, Joerg

http://www.analogconsultants.com/
Phil Hobbs
2025-03-06 01:10:08 UTC
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Post by Joerg
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
john larkin
2025-03-06 03:01:41 UTC
Reply
Permalink
On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs
Post by Phil Hobbs
Post by Joerg
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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.

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.
Phil Hobbs
2025-03-06 03:23:20 UTC
Reply
Permalink
Post by john larkin
On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs
Post by Phil Hobbs
Post by Joerg
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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.
Post by john larkin
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.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
Phil Hobbs
2025-03-06 14:53:43 UTC
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Permalink
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs
Post by Phil Hobbs
Post by Joerg
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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.
Post by john larkin
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
--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
john larkin
2025-03-06 15:24:21 UTC
Reply
Permalink
On Thu, 6 Mar 2025 14:53:43 -0000 (UTC), Phil Hobbs
Post by Phil Hobbs
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs
Post by Phil Hobbs
Post by Joerg
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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.
Post by john larkin
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.
Phil Hobbs
2025-03-06 15:54:20 UTC
Reply
Permalink
Post by john larkin
On Thu, 6 Mar 2025 14:53:43 -0000 (UTC), Phil Hobbs
Post by Phil Hobbs
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs
Post by Phil Hobbs
Post by Joerg
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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.
Post by john larkin
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
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
john larkin
2025-03-06 16:06:48 UTC
Reply
Permalink
On Thu, 6 Mar 2025 10:54:20 -0500, Phil Hobbs
Post by Phil Hobbs
Post by john larkin
On Thu, 6 Mar 2025 14:53:43 -0000 (UTC), Phil Hobbs
Post by Phil Hobbs
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs
Post by Phil Hobbs
Post by Joerg
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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.
Post by john larkin
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.
Bill Sloman
2025-03-06 16:29:49 UTC
Reply
Permalink
Post by john larkin
On Thu, 6 Mar 2025 10:54:20 -0500, Phil Hobbs
Post by Phil Hobbs
Post by john larkin
On Thu, 6 Mar 2025 14:53:43 -0000 (UTC), Phil Hobbs
Post by Phil Hobbs
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs
Post by Phil Hobbs
Post by Joerg
Post by Phil Hobbs
Post by john larkin
On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
Post by Phil Hobbs
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.
Post by john larkin
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.
But it's current steering logic and the supply rails stay a lot cleaner
than you see with CMOS switches.

And what's Standard ECL now? It was Motorola 10k back when I was young,
and Motorola/Philips/Fairchild 100k a few years later. Motorola ECLinPs
took over a about when I stopped using it, about when I started posting
here, some twenty years ago.
--
Bill Sloman, Sydney
Joerg
2025-03-06 20:33:35 UTC
Reply
Permalink
Post by Bill Sloman
Post by john larkin
On Thu, 6 Mar 2025 10:54:20 -0500, Phil Hobbs
[...]
Post by Bill Sloman
Post by john larkin
Post by Phil Hobbs
Post by john larkin
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.
But it's current steering logic and the supply rails stay a lot cleaner
than you see with CMOS switches.
Not a big problem, just place pillows around it, RC filters on the
supply. Drive was never an issue in my case but on rare occasions I have
used cheap signal transformers to isolate that.

The larger concern with supply rails is low frequency noise coming in,
causing phase noise. That is where capacitance multipliers can shine.
Post by Bill Sloman
And what's Standard ECL now? It was Motorola 10k back when I was young,
and Motorola/Philips/Fairchild 100k a few years later. Motorola ECLinPs
took over a about when I stopped using it, about when I started posting
here, some twenty years ago.
AFAIK it's 100E but I have not used any in ages because I always found
them overpriced. I don't like it when two ICs cost more than a crate of
beer :-)
--
Gruesse, Joerg

http://www.analogconsultants.com/
john larkin
2025-03-06 20:51:07 UTC
Reply
Permalink
Post by Joerg
Post by Bill Sloman
Post by john larkin
On Thu, 6 Mar 2025 10:54:20 -0500, Phil Hobbs
[...]
Post by Bill Sloman
Post by john larkin
Post by Phil Hobbs
Post by john larkin
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.
But it's current steering logic and the supply rails stay a lot cleaner
than you see with CMOS switches.
Not a big problem, just place pillows around it, RC filters on the
supply. Drive was never an issue in my case but on rare occasions I have
used cheap signal transformers to isolate that.
The larger concern with supply rails is low frequency noise coming in,
causing phase noise. That is where capacitance multipliers can shine.
I measured one FPGA at around 1 mV per picosecond prop delay, on the 1
volt core supply. Prop delay is about inverse on voltage in cmos.
Post by Joerg
Post by Bill Sloman
And what's Standard ECL now? It was Motorola 10k back when I was young,
and Motorola/Philips/Fairchild 100k a few years later. Motorola ECLinPs
took over a about when I stopped using it, about when I started posting
here, some twenty years ago.
AFAIK it's 100E but I have not used any in ages because I always found
them overpriced. I don't like it when two ICs cost more than a crate of
beer :-)
MC10EPxx, SiGe Eclips Lite.

Really fast and really expensive is Gigacomm, which is actually CML.
The NB7V52M flop is only about $13 in quantity.
Joerg
2025-03-06 22:12:14 UTC
Reply
Permalink
Post by john larkin
Post by Joerg
Post by Bill Sloman
Post by john larkin
On Thu, 6 Mar 2025 10:54:20 -0500, Phil Hobbs
[...]
Post by Bill Sloman
Post by john larkin
Post by Phil Hobbs
Post by john larkin
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.
But it's current steering logic and the supply rails stay a lot cleaner
than you see with CMOS switches.
Not a big problem, just place pillows around it, RC filters on the
supply. Drive was never an issue in my case but on rare occasions I have
used cheap signal transformers to isolate that.
The larger concern with supply rails is low frequency noise coming in,
causing phase noise. That is where capacitance multipliers can shine.
I measured one FPGA at around 1 mV per picosecond prop delay, on the 1
volt core supply. Prop delay is about inverse on voltage in cmos.
In my last TDR psec-jitter would have made the client unhappy. The
sampling window was 100psec but it wasn't supposed to move unless told to.
Post by john larkin
Post by Joerg
Post by Bill Sloman
And what's Standard ECL now? It was Motorola 10k back when I was young,
and Motorola/Philips/Fairchild 100k a few years later. Motorola ECLinPs
took over a about when I stopped using it, about when I started posting
here, some twenty years ago.
AFAIK it's 100E but I have not used any in ages because I always found
them overpriced. I don't like it when two ICs cost more than a crate of
beer :-)
MC10EPxx, SiGe Eclips Lite.
Really fast and really expensive is Gigacomm, which is actually CML.
The NB7V52M flop is only about $13 in quantity.
So far I've only needed "semi-analog", meaning just one bit and then I
did it using RF transistors. It is amazing, you can buy >100GHz fT for
less than 20 cents in qties.

When I was a kid I had to shell out around $3 for an AF116 Ge-transistor
that had an fT of 75MHz. In 1970's Dollars, which really hurt. Digital
wasn't any better. I needed a 1kbit RAM for a project and that set me
back about 10 bucks. It still works.
--
Regards, Joerg

http://www.analogconsultants.com/
john larkin
2025-03-06 23:04:12 UTC
Reply
Permalink
Post by Joerg
Post by john larkin
Post by Joerg
Post by Bill Sloman
Post by john larkin
On Thu, 6 Mar 2025 10:54:20 -0500, Phil Hobbs
[...]
Post by Bill Sloman
Post by john larkin
Post by Phil Hobbs
Post by john larkin
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.
But it's current steering logic and the supply rails stay a lot cleaner
than you see with CMOS switches.
Not a big problem, just place pillows around it, RC filters on the
supply. Drive was never an issue in my case but on rare occasions I have
used cheap signal transformers to isolate that.
The larger concern with supply rails is low frequency noise coming in,
causing phase noise. That is where capacitance multipliers can shine.
I measured one FPGA at around 1 mV per picosecond prop delay, on the 1
volt core supply. Prop delay is about inverse on voltage in cmos.
In my last TDR psec-jitter would have made the client unhappy. The
sampling window was 100psec but it wasn't supposed to move unless told to.
Post by john larkin
Post by Joerg
Post by Bill Sloman
And what's Standard ECL now? It was Motorola 10k back when I was young,
and Motorola/Philips/Fairchild 100k a few years later. Motorola ECLinPs
took over a about when I stopped using it, about when I started posting
here, some twenty years ago.
AFAIK it's 100E but I have not used any in ages because I always found
them overpriced. I don't like it when two ICs cost more than a crate of
beer :-)
MC10EPxx, SiGe Eclips Lite.
Really fast and really expensive is Gigacomm, which is actually CML.
The NB7V52M flop is only about $13 in quantity.
So far I've only needed "semi-analog", meaning just one bit and then I
did it using RF transistors. It is amazing, you can buy >100GHz fT for
less than 20 cents in qties.
When I was a kid I had to shell out around $3 for an AF116 Ge-transistor
that had an fT of 75MHz. In 1970's Dollars, which really hurt. Digital
wasn't any better. I needed a 1kbit RAM for a project and that set me
back about 10 bucks. It still works.
My first transistor was a Raytheon CK722 germanium. I think Ft was
measured in KHz. It cost $7, about a month's allowance, or dinner for
two at a decent restaurant.

I got tubes for free.
Joerg
2025-03-06 23:38:19 UTC
Reply
Permalink
[...]
Post by john larkin
Post by Joerg
Post by john larkin
Post by Joerg
Post by Bill Sloman
And what's Standard ECL now? It was Motorola 10k back when I was young,
and Motorola/Philips/Fairchild 100k a few years later. Motorola ECLinPs
took over a about when I stopped using it, about when I started posting
here, some twenty years ago.
AFAIK it's 100E but I have not used any in ages because I always found
them overpriced. I don't like it when two ICs cost more than a crate of
beer :-)
MC10EPxx, SiGe Eclips Lite.
Really fast and really expensive is Gigacomm, which is actually CML.
The NB7V52M flop is only about $13 in quantity.
So far I've only needed "semi-analog", meaning just one bit and then I
did it using RF transistors. It is amazing, you can buy >100GHz fT for
less than 20 cents in qties.
When I was a kid I had to shell out around $3 for an AF116 Ge-transistor
that had an fT of 75MHz. In 1970's Dollars, which really hurt. Digital
wasn't any better. I needed a 1kbit RAM for a project and that set me
back about 10 bucks. It still works.
My first transistor was a Raytheon CK722 germanium. I think Ft was
measured in KHz. It cost $7, about a month's allowance, or dinner for
two at a decent restaurant.
Now you've revealed your age bracket :-)

I salvaged some German OC-series Ge-transistors out of discarded gear.

https://www.cedist.com/products/transistor-oc45-valvo-germanium-so-2-glass-case-pnp

You could scrape off the black paint and clear glass showed up, with the
bare transistor inside. That way I had free opto-transistors that could
be used for really cool stuff. LDRs were very expensive over there so
that helped a lot.
Post by john larkin
I got tubes for free.
Same here. In Germany we had regular bulk waste days where people placed
their non-working TV sets and similar large items at the curb for
pickup. I carted a lot of that home, all on the baggage rack of my
bicycle. Some TVs I repaired, others I used for scavenging.
Unfortunately the German ones had a suicide chassis (hot) and the tube
filaments were all in series. 300mA but the voltages varied widely. So I
had to rewind transformers for the filaments. Finding old radios brought
easier tubes with 6.3V filaments but that also meant a fierce scavenger
competition. Who ever got up earlier or had a faster bike won.
--
Regards, Joerg

http://www.analogconsultants.com/
Joerg
2025-03-06 01:04:04 UTC
Reply
Permalink
Post by Phil Hobbs
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>
Very nice. Mine was more like Uncle Scrooge's version with every penny
being turned around, only one diode :-)

However, that darn diode is 0102 size and I had a hard time handling it
with my not so young eyes.
--
Regards, Joerg

http://www.analogconsultants.com/
Bill Sloman
2025-03-06 01:48:34 UTC
Reply
Permalink
Post by Phil Hobbs
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>
Cute. When I got stuck with driving a diode bridge, I used a
transmission line transformer to get a perfectly balanced drive.

As Kibble and Rayner point out, carefully wound (and that means a
non-progressive winding) 1:1 transformers can get to one part per
billion balance. Other ratios tend not to let themselves get pushed
beyond one part in ten million.
--
Bill Sloman, Sydney
Tom Del Rosso
2025-03-09 23:22:19 UTC
Reply
Permalink
Post by Phil Hobbs
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?
--
Defund the Thought Police
Bill Sloman
2025-03-10 02:38:23 UTC
Reply
Permalink
Post by Tom Del Rosso
Post by Phil Hobbs
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?
Air is a rather less complex transmission medium than optical fibre, but
the most likely explanation is that you can sell a lot more laser rulers
than you can sell tools for measuring optical fibres, so you can afford
the masks for an ASIC for the former, but not the latter.

Cunning designers have been known to use ASICs in applications for which
they weren't designed, but if the application is too specific, it won't
work.
--
Bill Sloman, Sydney
john larkin
2025-03-10 15:22:59 UTC
Reply
Permalink
On Sun, 9 Mar 2025 19:22:19 -0400, "Tom Del Rosso"
Post by Tom Del Rosso
Post by Phil Hobbs
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?
Most rangefinders use sine modulation and measure phase shift, which
doesn't involve picosecond timing or expensive wideband signal
processing. And they report one number, not a graph. OTDRs are much
more complex than rangefinders.

And the markets are different too.
Joerg
2025-03-11 19:20:30 UTC
Reply
Permalink
Post by john larkin
On Sun, 9 Mar 2025 19:22:19 -0400, "Tom Del Rosso"
Post by Tom Del Rosso
Post by Phil Hobbs
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?
Most rangefinders use sine modulation and measure phase shift, which
doesn't involve picosecond timing or expensive wideband signal
processing. And they report one number, not a graph. OTDRs are much
more complex than rangefinders.
And the markets are different too.
Also, if a range finder fails to display an accurate result it's usually
not a big deal. So it doesn't have to be hi-rel or anything. If a TDR
like the one I designed fails that can make the evening news. Nobody
wants to be on the evening news in a context like that.
--
Regards, Joerg

http://www.analogconsultants.com/
Phil Hobbs
2025-03-11 18:12:51 UTC
Reply
Permalink
Post by Tom Del Rosso
Post by Phil Hobbs
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
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
john larkin
2025-03-11 19:35:43 UTC
Reply
Permalink
On Tue, 11 Mar 2025 14:12:51 -0400, Phil Hobbs
Post by Phil Hobbs
Post by Tom Del Rosso
Post by Phil Hobbs
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.
Phil Hobbs
2025-03-12 00:03:02 UTC
Reply
Permalink
Post by john larkin
On Tue, 11 Mar 2025 14:12:51 -0400, Phil Hobbs
Post by Phil Hobbs
Post by Tom Del Rosso
Post by Phil Hobbs
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.
It’s similar. Laser feedback measurements are flaky as can be, but they
have heterodyne gain and selectivity, which rejects ambient light to a very
high degree.

I’ve done coherent lidars whose SNR stayed the same even looking directly
into the sun. It’s all about having enough LO power and rejecting the laser
noise adequately.

The monitor PD makes it fairly easy, at least for undemanding tasks such as
detecting a wall. ;)

The laser may mode-hop in some range of current, but with sawtooth
modulation you can just ignore the intervals where it misbehaves.

We have a very nice laser controller product, the LC120, which automates a
lot of stuff like that.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
KevinJ93
2025-03-12 02:02:20 UTC
Reply
Permalink
Post by Phil Hobbs
Post by Tom Del Rosso
Post by Phil Hobbs
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
Short distance laser rangefinders can be implemented with the integrated
time-of-flight sensors now being used in cell phones.

https://www.st.com/en/imaging-and-photonics-solutions/vl53l0x.html#overview

$5 single quantity from Digikey.

kw
Bill Sloman
2025-03-12 02:29:07 UTC
Reply
Permalink
Post by Phil Hobbs
Post by Tom Del Rosso
Post by Phil Hobbs
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.
Keeping a laser diode single mode seems to mean that you keeps its
junctions temperature stable to better than a degree (and they do
self-heat in operation.

My 1996 millidegree thermostat was designed to stabilised the
temperature of the laser's target, but the machine worked better if the
laser was operating single mode, and stayed in that single mode, so my
thermostat got duplicated to stabilise the temperature of the laser
diode as well. The stable temperature was set up during final test to
give a temperature half-way between that particular laser's single-mode
switching temperatures, but that just meant writing a single word into
memory.
--
Bill Sloman, Sydney
Bill Sloman
2025-03-12 03:31:38 UTC
Reply
Permalink
Post by Phil Hobbs
Post by Tom Del Rosso
Post by Phil Hobbs
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>
Post by Phil Hobbs
Post by Tom Del Rosso
Post by Phil Hobbs
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.
Post by Phil Hobbs
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.
Post by Phil Hobbs
I have no idea how the $16 ones do it, but if I were building one,
that's the first thing I'd try.

Keeping a laser diode single mode seems to mean that you need to keep
its junctions temperature stable to better than a degree (and they do
self-heat in operation).

My 1996 millidegree thermostat was designed to stabilise the temperature
of the laser's target, but the machine worked better if the laser was
operating single mode, and stayed in that single mode, so my thermostat
later got duplicated to stabilise the temperature of the laser diode as
well. The stable temperature was set up during final test to give a
temperature half-way between that particular laser's single-mode
switching temperatures, but that just meant writing a single word into
memory.

You could detect when the laser diode changed modes,so you just ramped
the laser temperature over a couple of degrees (which was enough to get
you two mode hops) and you fixed a temperature halfway between the mode
hops).
--
Bill Sloman, Sydney
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