Re: LVDS line receiver with an unencrypted SPICE model?

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
--
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