Interesting inductor

The first stage (paralleled pHEMTs with a BFU520A cascode and BFU520A follower) has a gain of about 40 and flatband 1-Hz noise of 0.2 nV. That means that the noise of the follower and the second stage is not insignificant.

The second stage is a VCVS active lowpass using an OPA818 at a gain of 10, and the output stage is an OPA695 CFA inverter, to make the overall circuit noninverting and provide a gain adjustment. (TE now makes a low-inductance pot that’s nearly as good as the old Murata PVA2 ones that you use. )

Keeping the supplies simple is important, and so is avoiding ground loops. The box actually makes +7 and -5 by railsplitting a 24V wall wart, and then using regulating cap multipliers. (The second and third stages’ supplies are followers running off the quiet ones, to prevent unwanted feedback.)

Sooo, I want to run the follower on +7/0 if possible, which is where the inductor comes in. It doesn’t save any power, on account of the railsplitter, so I can probably use the -5 rail instead.

There’s no overall feedback in this version, because it’s hard to do without trashing the noise performance and/or stability.

We did something similar for choosing resistor taps in a low noise PGA. Works okay, but is a bit of a pain.

Fun. Analog computers forever!

Cheers

Phil Hobbs

Anything over 1uH has a ferrite core - probably a nickel-zinc ferrite at those sorts of frequencies.

Minimising parallel capacitance is supposed to demand spacing the winding wires by their own diameter, but that doesn't show up on the drawing (and probably wouldn't even if they were doing it).

Definitely interesting.

If that happens, I'll commiserate appropriately. ;)

Nah, relays are amazing. There are low-power muxes that come close, e.g. the TMUX1511 (5 ohms R_on, 2 pF C_off), but nothing that will take any sort of power.

Of course you can do similar things with tubes. ;)

Cheers

Phil Hobbs

Not identical things, just similar. Dragging a grid up to +200V quickly and then leaving it there, with no turn-off charge injection and nearly no capacitive loading, is a job for a tube. (I used an 811A for that BITD--it even had a B battery for the plate and a C battery for the grid bias.) :)

Cheers

Phil Hobbs

My first paying job (50 cents per hour) was at LSUNO, a summer job working with a physicist doing Stark effect microwave spectroscopy. I built two high-voltage square wave generators for him, one with thyratrons and one with giant transmitting tubes. The upper tube was driven by a pulse transformer and as you say, only needed short grid blips.

One thing we used for calibration was OCS, which has a giant Stark effect and is deadly stuff.

I wasn't a registered student (my plan was to go to Tulane, which had an engineering school) and the rule was that only students at LSUNO could be paid. So the dean of physics made a call and they assigned me student number 20,000 on the theory that they'd never get there, which I'm sure they have by now. It's now called UNO on the lakefront in New Orleans.

Gyrator?

We just yesterday had a brainstorm session about that. How can one make a programmable electronic fake inductor?

A real inductor stores energy, and can do things like high voltage flyback. So a fake inductor should store energy, or pretend to. It could be done with a current shunt, a fast ADC, some math in an FPGA, a fast DAC, and a big power amplifier with big power supplies. Too much work.

As you say, nifty. Do you have some means of verifying that Fo claim, Phil? Even a NanoVNA would give a pretty good idea if it's really that high.

Sure, SRF measurements aren’t super subtle.

Cheers

Phil Hobbs

Good show. I'd be interested to know the result.

Yes, I got part way down the road of designing a gyrator to block telemetry signals on a power line comms device. Soon realised it would need lots of power.

Just thinking out loud, and not really a serious suggestion, but would a variac with a fixed inductor on the secondary work as a variable inductor? I guess 500:1 would be impossible.

Not a dumb idea at all. To avoid using a motor to turn it, one or more transformers with binary-weighted windings and relays, maybe.

The inductance of the transformer needs to be large enough, of course.

I’ve occasionally considered using a transformer to make an isolated version of a dpot, but it’s never been quite the right solution, mostly on account of limited inductance.

Cheers

Phil Hobbs

I wonder if you could reduce the power supply needs a bit by switchmoding the incoming current into big storage capacitors so the gyrator does some energy storage and could make flybacks up to some limit?

it's probably easier to just use the output terminals as a variable inductor.