Re: LVDS line receiver with an unencrypted SPICE model?

On 2024-07-06 15:03, john larkin wrote:

On Sat, 6 Jul 2024 11:56:08 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
 

On 2024-07-06 00:46, john larkin wrote:

On Fri, 5 Jul 2024 21:35:29 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

On 2024-07-05 20:48, john larkin wrote:

On Fri, 5 Jul 2024 20:41:01 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

On 2024-07-05 16:30, Phil Hobbs wrote:

On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20
-0400, Phil Hobbs

<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

On 2024-07-05 12:54, john larkin wrote:

On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
<jlarkin_highland_tech> wrote:
>

On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

john larkin <jlarkin_highland_tech> wrote:

On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

boB <boB@K7IQ.com> wrote:

On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

So I have this new gig making a very low cost TDR
system for soil moisture and conductivity, for use
in agriculture.  All very interesting and topical,
what with droughts and low aquifers and all.
>
As one does, I'm planning to use a fast ramp and two
comparators to generate the TX pulse and the
sampling gate.  One will have a fixed comparison
voltage, and the other one's will be set by a DAC, or
possibly by a slower ramp, depending on the BOM vs
performance tradeoff we wind up with.
>
Sooo, naturally I pulled out my fave LVDS line
receiver, the FIN1002. It has nice 250-300 ps edges
and pretty low jitter, comes in SOT23, and and costs
30 cents on LCSC.
>
It even has SPICE models, but of course they're
nasty encrypted HSPICE things.
>
TI's seem to be the same.
>
Anybody got a nice LVDS receiver with a real SPICE
model that mere mortals are allowed to use?
>
Thanks
>
Phil Hobbs

>
>
Does the spec sheet say anything about how it works so
that a model could be made ?  Block diagram or
something ?
>
boB
>
>

>
Too much like work?it?ll be much easier to build the
circuit and measure it. I can simulate most of the fast
stuff, because it?s all discrete.
>
Thanks
>
Phil Hobbs

>
What sorts of risetimes and swings do you want?
>
I haven't used the FIN1002, but we've used several similar LVDS
receivers and all seem to work like decent RRIO comparators. You
can put an RC ramp into one input and a
DAC into the other and make a ps-resolution programmable
delay.
>
I've given up on fast linear ramps... too much work. An RC
with a bit of polynomial DAC correction works great.

>
I’m planning to use some fast gain to sharpen up the edge,
probably a BFP740. It won’t have much chance to oscillate, so
it should be fine.
>
You have to be a bit careful, because the prop delay depends
some on the CM voltage. It’s a bit like the offset voltage of
a RRIO op amp—squirrely things happen within a couple of
V_BEs of one rail.
>

>
The LVDS receivers that we use do behave much better when the
inputs are a volt or so below Vcc.
>
But we just poke in a bunch of DAC voltages, measure the delay
times, and calculate a polynomial. Ramp curvature and cmrr
don't matter much anymore.
>
Take a look at the SY88022 laser driver chip. It's stunning.
Diff in, brutal but clean 25 ps outputs, smooth amplitude
control down to zero, about $7.

>

>
Nice part, thanks.

>
Or slam one or two SAV541 types with the FIN output. They go
from off to about 2 ohms in a few tenths of a volt of gate drive.
The newer packages should have less wirebond inductance than the
originals.
>
Package inductance probably dominates actual fast switching
behavior. Spice models usually ignore that. I suppose that
inductances could be inferred from s-params, but I couldn't do
that.

>
For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground,
you won't go far wrong.  Leo Bodnar has gone upmarket, so I may
need to start using them--I've used his stuff in POC protos, but
not any more.
>
With SiGe BJTs running in normal bias, the real key is keeping the
pad capacitance down.  A good bead right at the base tames them
very nicely at lowish drain current, but if you let the base pad
capacitance get as large as 0.5 pF,  above about 5 mA you're toast
regardless of how good the bead is.
>
The nice thing about them for this job is that they cost $0.16
rather than $2.  (Part of the royalty conversation is BOM
savings.)
>
The business end of the TDR looks like a giant meat fork, with a
slide hammer for getting it in and out of the ground.
(Fortunately the circuit board is at the other end of a cable from
the slide hammer!) It's got some sort of balun inside, which I hope
is a bazooka-style one with ferrite, rather than something more
complicated.
>
You really only care about suppressing the current on the outside
of the shield.  The classic bazooka is a 1/4-wave coaxial sleeve,
shorted to the outer conductor at the generator end, so that it
looks like an open circuit to any outside shield current.
>
It's pretty narrowband, of course, but a ferrite sleeve can do the
job pretty well--all you really care about is that the outside
shield current sees a high impedance, and the phase angle doesn't
matter much.
>
Having control over both the TX and RX means that you can ignore a
lot of stuff that real sampler designs have to worry about.
>
Cheers
>
Phil Hobbs

>
You can make a TDR that's coyote ugly, as long as it's fast, and
deconvolve a software FIR filter to beautify it. And speed it up a
bit maybe.
>
>

https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1
>

>

>
Are you, of all people, suggesting that we use that nasty evil ill-posed
ill-conditioned _deconvolution_ thing you used to beat me up about?
>
Nah, must be my ear wax. ;)

>
If I can come up with a good online cal scheme, that's potentially a
helpful method.  I really don't want to have to be Agoston Agoston. ;)
>
But you can make things like blowby, nonlinear capacitance, poor
sampling efficiency due to the diode R * sampling C time constant being
too slow, and so on, go away.
>
This is done by (1) doing the second-stage sampling when nothing is
going on at the input.  That fixes the blowby problem.
>
Then (2), you feed back the second-stage sampled value to the sampling
cap, which is the normal sampling loop approach.
>
Finally (3), you take 20 samples per delay value,  so that the sampling
loop converges, resulting in zero volts across the sampling bridge at
the sampling instant.  That gets rid of the nonlinear capacitance problem.
>
Cheers
>
Phil Hobbs

>
What sort of rise time are you after?
>
There are goofy possibilities.
>

>
For TDRing dirt, I find it hard to believe we'll need better than 100 ps.

>

>
Something sort of like this might work.
>
https://www.dropbox.com/scl/fi/yq7an2udjygxrd2bf0rdl/DirtLine_TDR.jpg?rlkey=jyu9gb74y6dciml307jpad8bw&raw=1
>
>

Fun.  I like it in general--it should have a nice clean Tx edge, for one
thing, and the ON resistance of the pHEMT is a lot better than a
Schottky diode for the same capacitance.  That might eliminate the need
to use multiple sampling pulses to get the sampling loop to converge.
(Which would be good, because you can't easily stack samples with a
track/hold architecture.)
>
ESD protection is one possible issue, and the hold capacitor loads down
the line throughout the pulse and return, so it has to be very small.
>
So far I'm envisioning a two-diode series sampler, with a fast sampling
pulse made by taking a fast edge from a BFP840 and differentiating it
with a shorted Tx line, maybe with a bit of capacitive help.
>
As long as the final voltages across the diodes don't change from sample
to sample, the nonlinear capacitance funnies pretty well go away.  There
might be some schmutz due to reflections off the bridge in the OFF state
and other discontinuities rattling round.  It'll be hard to get rid of
the reflection from where the tines hit the soil surface, for instance.
>
As I say, fun.
>
Cheers
>
Phil Hobbs

 How about this?
 https://www.dropbox.com/scl/fi/lo60weugmmmzkg1w5aykd/Dirt_Line_2.jpg?rlkey=96nrdt0deouoxa85sv2bogdqx&raw=1
 It has some nice behavior around the diode capacitance feed-thru. At
sample time T2, nodes D and L both slam to ground. Before then, they
track.
 This slow sampler drives C1 hard, unlike a faster feedback sampler
that can only poke a bit of incremental charge into the cap each shot.
 

That one's nice too.  How hard it drives the sampling cap depends on the video resistance of the diodes.  BAT1502s seem to work okay.
At the moment I'm trying to do it with just BFP840s and diodes, to make sure I understand all the details.
I'm someplace between you and Joerg on the ideal functionality-vs-BOM cost continuum--some years ago ChesterW and I did a 100-ps class TDR sampler for a fuel gauge that sells for $25.  My bit was well under $2, including the fast pulse generator, 1-diode first stage sampler, fast buffer amp, second stage sampler, and slow output buffer.
It worked great, for a gas gauge--in fact CW and Co. Muntzed it a bit
and got a few cents of cost out, in exchange for turning it into a 200-ps sampler, which was better than good enough.
The present one needs good waveform fidelity because we're basically measuring the shape of the drooly return pulse and converting that to the water and conductivity measurement.
The spherical cows think that a FIN1002 driving a BFP840 can easily make a < 20 ps edge.  I may disagree with that, but at least it's encouraging. ;)
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