Author Topic: DIY DC-10MHz optical-fiber-isolated scope probe  (Read 381 times)

Online davekni

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DIY DC-10MHz optical-fiber-isolated scope probe
« on: October 08, 2020, 05:23:05 AM »
This is an optically-isolated scope probe for use in probing high-side gate drive and other such non-ground-referenced signals.  Also eventually plan to use a pair of these to measure charge transfer on top of my DRSSTC, from secondary to top-load and from secondary to breakout point.

This probe uses standard 1mm-core plastic optical fiber.  A "T-1 3/4" (5mm-diameter) LED feeds the fiber with light proportional to input signal plus a fixed DC offset.  Photodiode receiver amplifies the light signal and subtracts the fixed offset.  Most newer efficient LEDs have surprisingly linear drive-current to light-output.  Here's a picture of the probe, driver and receiver and fiber.  Each end is powered by a flat 4V lithium-ion cell under the ECB.



Bandwidth without any compensation for LED capacitance is just over 4MHz.  With an added R+C to peak LED current, bandwidth is roughly 10MHz.  (I tried a couple LEDs packaged specifically for 1mm-core 2.2mm OD optical fiber.  They were higher-bandwidth, but much less efficient.  Light coupled into the fiber was much lower, so would have required higher receiver gain and associated higher noise.)  Below are scope traces of a 12V square wave out of a UCC27525 gate-driver chip.  Channel 4 is with a normal 10x scope probe.  Channel 1 is this isolated probe output.  Both scope channels are set to 20MHz bandwidth limit.  Notice that this optical probe output has roughly twice the rise/fall times, so about half the bandwidth.  It also has more delay due to the coil of fiber.







Input divider resistors for this first unit are designed for gate-drive scoping, +-20V.  Output to scope is +-200mV (100x attenuation).  Input impedance is 30k-ohms.  Low enough to avoid needing to tweak input attenuator compensation capacitors, but high enough to not significantly load gate-drive signals.  Here's the driver schematic including R10+C3 peaking to compensate for LED capacitance:



And the receiver using the IF-D91 photociode designed for plastic fiber:



I found two successful ways to couple the 5mm OD LED to 2.2mm OD (1.0mm core) fiber.  One is to drill a 2.2mm hole in the LED deep enough to almost hit the bond wire, then glue one end of the fiber into the hole.  Glue makes reasonable optical matching (similar index of refraction to the LED housing and fiber core).  Here's a close-up of the LED with fiber glued into it:



The other option is to grind/sand down the end of the LED to make a flat surface, again almost to the bond wire.  Didn't have any good way to polish the surface, so instead glued a small piece of mylar to the end to make a shiny surface.  Then used a short piece of 3/16" ID (just under 5mm) rubber tubing to couple the LED to fiber.  I'd found some fiber with an outer jacket that was conveniently 5mm OD.  The 3/16" ID tubing could either join the LED directly to this fiber, or I could remove a short section of the large fiber's outer jacket, join that section to the LED with rubber tubing, then insert normal 2.2mm OD fiber into that piece of jacket.

Finally, on the outside chance that anyone wants to experiment with this design unmodified, here are the zipped gerber files:

* fiber_scope.zip

David Knierim

Offline rikkitikkitavi

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #1 on: October 08, 2020, 08:46:14 AM »
Ingenious!  :)

You have the advantage of transfering the signal galvanically isolated in full without bothering with the CMRR of a differential high voltage divider with all its asociated troubles in high impedance/parastics and high voltage/large transients/sharp edges power circuits yadayada!

Fiber delay is about 5ns/meter 
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Offline Hydron

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #2 on: October 08, 2020, 05:09:46 PM »
Nice work!

You can also get analogue output receivers for plastic fibre (e.g. versalink) and glass (e.g. multimode ST-connector ones).

I have one of the former and a couple of the latter, and did a quick gain vs frequency check quite a while ago on a multimode glass one, see attached (Transmitter was a HFBR1415, Receiver was a HFBR-2414 which doesn't seem to be well documented, but is an analogue output version). I didn't really look at linearity, but the datasheet suggests it will be fairly reasonable.

I have some other methods of getting isolated measurements, but these guys are sitting there in-case I need to build a similar isolated probe.

Another thought if linearity proves to be troublesome would be to do something similar to analogue optocoupler circuits - put a second receiver in the feedback loop for the LED driver. If this is fed from a splitter half-way down the fibre (sending back half the signal to the driver) then in theory it should linearise the output (assuming that the receiver characteristics are well matched). This is a lot more hassle and probably unnecessary for most applications (e.g. you just want to see the waveform shape, a few % distortion is fine) but maybe useful for more critical signals?


Red is gain, Blue is phase. Filename suggests the gain is measured as voltage output vs current input (this was done as a quick hack a while ago!) I wouldn't put too much faith in this, but thought it may be of interest.
« Last Edit: October 09, 2020, 10:45:23 AM by Hydron »

Offline klugesmith

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #3 on: October 08, 2020, 06:31:59 PM »
Great work there, Dave.  I like the DIY methods of launching light into fiber.  In 1990's, working with singlemode fiber, I found that some gigabit laser transmitters didn't like the return loss from an open fiber end (in SC connector).  Got a satisfactory termination by rubbing fingertip on oily nose & then pressing it onto the SC ferrule tip.

How stable is your zero-signal point ( transmitted as about half of maximum optical power ? )
over temperature, battery age, and maybe wiggling of optical joints?
I guess it's a non-issue for GDT output signals that are inherently zero-average, and otherwise a simple adjustment of vertical position or offset knob.

Applause for posting schematics, and having a name on every net.   Did I miss the place in sender circuit for connecting the reference side of input signal voltage?

« Last Edit: October 08, 2020, 07:06:33 PM by klugesmith »

Online davekni

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #4 on: October 09, 2020, 12:03:44 AM »
Hydron,

Before starting this project, I'd looked for integrated analog fiber receivers without success.  Since you listed HFBR-2414, I just checked into that a bit.  Mouser lists it as an obsolete part.  Looking through the data sheet, it might be usable, but looks to have three diode-junctions worth of DC voltage temperature coefficient (3 x Vbe) on its output.  Do you have any part numbers for currently-available analog fiber receivers?  Such could be simpler and higher bandwidth than my version.

Klugesmith,

Good point - I'd forgot to label which transmitter node is the input reference.  It is node "vtgnd", the node that the input voltage divider returns to.  There are extra holes/pads on the ECB layout for vtgnd that don't have corresponding schematic symbols.  These can be used for input return and/or for connecting a foil shield around the circuit if that becomes necessary.

The idea for gain/offset is as follows.  The transmitter has a fixed LED current of 11.6mA at 0 input voltage.  That 11.6mA comes from 0.50V across 43 ohms.  The exact current doesn't matter as long as it corresponds to exactly 0.5V.  Whatever amount of light is generated and coupled all the way to the receiver is by-definition 0.5V of input voltage.  (That's 0.5V after attenuator, so 25V with my 50:1 input attenuation.)  At the receiving end there's a fixed -0.25V offset to the scope (scope ground is 0.25V above local receiver ground).  With no light to the receiver, the scope sees -0.25V.  With light from the transmitter, I adjust gain until the scope sees exactly 0.0V (with no input voltage to the transmitter).  Then I know that 0.5V at the transmitter (25V into attenuator) corresponds to 0.25V to the scope, for a total 100:1 attenuation.  Any time I reconnect fibers or the LED temperature changes too much, I can adjust gain to get zero volts out.  Drift hasn't been much of a problem with the TINY bit of experimenting I've done so far.

The one issue with all the above theory is it assumes zero opamp offset.  If there's a next-version, I'll add receiver opamp offset adjustment.  For this version, I had to add a hand-selected resistor on the back-side to adjust for opamp offset.

I'd tried cheap laser-pointer diodes for the transmitter.  Much higher optical power into the fiber, but two issues.  One is less stable light output vs. temperature.  The other (related) issue is a large thermal-transient in generated power after a step change in input current.  There's a tiny bit of that issue with the LED.  Actually I don't know for sure that it's thermal-related.  Just that the few microsecond time-constant is reminiscent of thermal transients often seen in analog power amplifiers, such as video drive for old CRT monitors.  If you look very closely at the overview plot of this fiber probe's response, the step goes quickly to ~98%, then takes a few microseconds to go the remaining ~2%.
David Knierim

Offline flyglas

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #5 on: October 09, 2020, 05:16:19 PM »
This thread is very inspiring. I like the concept to measure small voltages without common mode influence.

I have searched for commercial units and other DIY projects. During my search I found a similar project:
 https://hackaday.io/project/12231-fiber-optic-isolated-voltage-probe

The approach is similary, but they use other circuits to realise the transmitter (fet input stage) and a transimpedance amplifier as light to voltage conversion.
Layout and schematic is available.


Offline Hydron

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #6 on: October 13, 2020, 11:27:14 PM »
I had a quick look, and the analogue-output receivers I have are the following:
2x HFBR-2526, POF:
https://docs.broadcom.com/doc/AV02-1502EN

1x HFBR-2416, glass fibre:
https://docs.broadcom.com/doc/AV02-0176EN

2x HFBR-2414, glass fibre, as discussed before this doesn't seem to be a current product or have much info available.

All were purchased from CPC's bargain section at greatly reduced prices, most likely due to CPC stopping stocking the parts.

Online davekni

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #7 on: October 14, 2020, 12:40:06 AM »
Those two receiver parts appear to still be reasonably available.  Both have very loose DC offset specifications, 0.8 to 2.6V with no light input.  They have the same internal simplified schematic which suggests 3 x Vbe temperature coefficient.  They are much higher bandwidth than what I built.  Would be useful if the offset can be managed.  Thank you for listing the part numbers.
David Knierim

Offline klugesmith

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #8 on: October 14, 2020, 05:10:27 AM »
Hey, the higher speed Tx/Rx set in Hydron's link is recommended for 20 to 160 Mbaud.  Speed limitations according to distance and type of fiber are just from fiber dispersion and optical power budgeting.

How about transmitting a frequency modulated square wave, say 70 +/- 10 MHz?
Or a much smaller deviation, for smaller signal bandwidth.
Not sure about voltage-to-frequency IC's for the Tx end, but the Rx end might employ one of many low-cost FM or FSK receiver building block IC's. 

This is not intended to promote FM as better than simple analog optical power modulation with baseband signal.

Offline Hydron

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #9 on: October 14, 2020, 09:53:14 AM »
Those two receiver parts appear to still be reasonably available.  Both have very loose DC offset specifications, 0.8 to 2.6V with no light input.  They have the same internal simplified schematic which suggests 3 x Vbe temperature coefficient.  They are much higher bandwidth than what I built.  Would be useful if the offset can be managed.  Thank you for listing the part numbers.
I think the anticipated use for the analogue output parts is with an AC coupling capacitor, so the DC offset is not a critical specification. When I'm a bit less busy I can probably do a quick check to see how much they actually vary with no optical input, though it would be at best over a modest indoor temperature range (i could blow some hot air over them I guess). In use you'd have to adjust gain/offset to account for loss in the fibre etc anyway (and try not to move the fibre once setup either!), so a fixed offset is less concerning than a heavily temperature dependent one.

You may find the ST connector (glass fibre) analogue output parts in old networking kit, though I believe that Mads had some issues desoldering them without damage (albeit without a vacuum desoldering iron). The ST stuff is easy to get pre-made patch leads for too (and for absurdly low prices). For POF you can use cheap optical audio cables (the thin ones, I think they're 2.2 or 2mm diameter) and put connectors on yourself, but duplex is more annoying as the duplex POF isn't readily available.

Online davekni

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #10 on: October 14, 2020, 07:44:42 PM »
"How about transmitting a frequency modulated square wave, say 70 +/- 10 MHz?"

Yes, I've thought about modulating a digital signal, with pulse-width or frequency or sigma-delta.  Digital links are much more available and to higher frequencies, 1Gbaud and up.  Might try such some day.  This was simpler.

"In use you'd have to adjust gain/offset to account for loss in the fibre etc anyway (and try not to move the fibre once setup either!), so a fixed offset is less concerning than a heavily temperature dependent one."

Yes, if offset is stable enough, it can be adjusted out.  The advantage of stable offset, as in my simple opamp receiver, is that only a single adjustment for gain is needed to compensate for fiber connection losses.  With accurate base-line LED power at the transmitter, that single gain adjustment can be made using zero signal input at the transmitting end.  That way, for measuring charge on top of my DRSSTC, I don't need to connect a reference signal generator to calibrate, then remove it without disturbing the fiber connections.  I can set everything up, then make a simple gain adjustment at the receiving end before running the coil.  (That will be next spring or summer.  Put my coil away for the winter.)
David Knierim

Offline Mads Barnkob

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #11 on: October 18, 2020, 06:50:25 AM »
Very nice project and thank you so much for sharing this!

Could the input divider have a voltage range selector between two sets of resistors without ruining the fixed offset? Could be a cheap alternative to differential probes on the inverter output, something that many people do not own.

Is there a reason beyond too-expensive or did-not-have-the-part-at-hand for using the IF-D95T receiver, but not the IF-E96 transmitter?

For you topload charge measurements I assume you set up resistors or CTs with a output in the same region of the gate drive voltage it was designed for? Leads back to my first question.
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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #12 on: October 19, 2020, 05:12:05 AM »
"Could the input divider have a voltage range selector between two sets of resistors without ruining the fixed offset? Could be a cheap alternative to differential probes on the inverter output, something that many people do not own."

Yes, switched input attenuation is certainly possible, and might be reasonable.  I didn't do that because it can be tricky to compensate high divide ratios.  100:1 is likely fine without tweaking compensation, given the low 10MHz bandwidth.  By 1000:1 (for 100V/div), even 0.1pF of stray capacitance across the input resistor needs compensating.  Gets harder if there's significant capacitance to the middle of the input resistor.  So, for now, my plan is to build another board with different input resistance if I need different attenuation.  Then I can learn more about compensation without having switch capacitance to complicate issues.  Scope probes usually contain tiny ceramic circuits with resistance elements built (deposited) onto the ceramic with shaped electrodes on the opposite side to distribute capacitance along the resistance.

"Is there a reason beyond too-expensive or did-not-have-the-part-at-hand for using the IF-D95T receiver, but not the IF-E96 transmitter?"

Yes, I bought a couple IF-E96 transmitters.  They are higher bandwidth, but send much less light down the fiber.  LED technology has advanced rapidly, and IF-E96 appears to use old low-efficiency LEDs.  The trade-off is needing more amplification at the receiving end.  If I stayed with the same TLV3542 (100MHz gain-bandwidth), then higher receiver gain lowers bandwidth more than IF-E96 increases it.  Could use a higher-bandwidth opamp or more stages, but wanted simplicity and low noise.

"For you topload charge measurements I assume you set up resistors or CTs with a output in the same region of the gate drive voltage it was designed for? Leads back to my first question."

My plan for topload charge measurement is to use capacitors instead of resistors for sensing, so voltage will be proportional to charge rather than current.  Planning 20 paralleled C0G 100nF SMD caps to keep inductance low.  2uF total should keep the voltage within my +-20V range.  Measuring charge avoids issues with integrating scope traces with DC offset, and avoids issues of high peak breakout point current during ground-strikes.  Charge is great for topload voltage.  Down-side is that deriving current requires differentiating the signal, so amplifying high-frequency noise.  That's one reason I want to keep noise low, so use a bright LED.
David Knierim

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Re: DIY DC-10MHz optical-fiber-isolated scope probe
« Reply #12 on: October 19, 2020, 05:12:05 AM »

 


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