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Fun with spherical cows

So I have this gig coming in to build charge amps for a French ion accelerator lab.

The specs are for 1 kHz - 60 MHz, ideally less than 0.3 nV/sqrt(Hz) noise, when hung off a detector using 250-mm diameter plates, spaced by

5 mm, connected with a ~80 mm long, teflon insulated cable.

Fun.

SPICE says that it can be done stably, with realistic strays, using three Mini-Circuits pHEMTs in parallel and a BFU520A NPN cascode.

I have some test boards on order, courtesy of Simon, so in a couple of weeks we'll see if it can actually be made to work.

With things like this, the first goal is to keep them from oscillating someplace up in the gigahertz, and the second is to ake them do what you want.

Parallelling devices with 12-GHz fmax is a good way to make them oscillate. The trick in this instance seems to be source degeneration using Murata's magical BLV03VK600SNLD ferrite bead.

Unlike the vast majority of beads, they're specified by the impedance at

**5 GHz** instead of 100 MHz--these ones are 60 ohm, but you can get 220 ohm ones too (BLV03VK221SNLG).

I spent a bit of time using similar tricks to do a lab amp similar to our LA22 product(*), but with 200 MHz bandwidth instead of 20, and 0.3 nV/sqrt(Hz) noise instead of 1.1 nV. The spherical cows think it can do all that with 1.8 ns edges and no overshoot. We'll see!

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The schematic is a bit busy, as it has to have a lot of strays put in to get anything vaguely meaningful, and I had to scrunch it a bit (connecting blocks using flags rather than wires in some cases) to make it fit in the window. (The actual product schematic will probably be fairly different, but we'll see.)

I have no idea how accurate the pHEMT model is, so I need to build some test boards and find out. Fortunately we can get them monstrous cheap from JLCPCB these days. (**)

Cheers

Phil Hobbs

(*) More details at

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. (**) JLCPCB raised a bunch of Series B money in late 2022, so maybe all those cheap boards are being subsidized by VC money. Enjoy it while it lasts!

Reply to
Phil Hobbs
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Yikes. What's the capacitance? You might almost start with a transformer!

I did a wire chamber amp array for CERN. That's a whole nother story.

I have a 50 MHz triggered LC oscillator based on a SAV541. It likes to oscillate at all sorts of frequencies, like 6 GHz (according to our 6 GHz scope.)

Mini has some new, basically repackaged, versions, which may have less wirebond parasitics.

In my experience, the sources should be hard grounded. The magic beads in the gate seem to help. But the layout really dominates.

Reply to
john larkin

I'm still using ferric chloride but it's getting harder to find these days. There ought to be a better way using lasers to cut the traces by now. You wouldn't need that much power, would you? I'd have thought maybe 4 or 5 Watts would do it for your typical FR4.

Reply to
Cursitor Doom

Why not buy boards? Quick-turn 2 and 4-layer boards are cheap nowadays. You get solder mask, silk, planes, and vias!

Reply to
john larkin

I thought about it, but it's hard to get the ferrite losses low enough to achieve those noise levels. The noise temperature (baseband to 200 MHz) is less than 30 K.

I'm listening intently. Say on!

Yeah, something that resonates at 50 MHz probably has a lot of higher resonances too, not to mention all those PCB traces.

Yes, it does. However, it's quite possible to use 12-GHz f_max pHEMTs in all sorts of circuits, including bootstraps. That low flatband noise makes it worth all sorts of pain to get there.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Interesting. How did the first two layers work?

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Must have been a crap ton of ECL. (That’s ‘merde tonne’ for the SI crowd.)

Cheers

Phil Hobbs

Reply to
Phil Hobbs

I never got to see the installation. The budget didn't include a trip for me to Switzerland.

Switzerland is kind of boring anyhow.

Reply to
john larkin

Not according to John Larkin, whose exquisite judgement also tells us that climate change isn't happening (and wouldn't matter if it did) and that Donald Trump has common sense - which is to say, vulgar greed.

Reply to
Bill Sloman

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