RF Black Magic- Eliminating Fringing Capacitance

Am 17.11.20 um 17:28 schrieb snipped-for-privacy@highlandsniptechnology.com:

No, it's a screen dump, made on 2020.05.22 at 04:36, definitely from MY virtual machine, with filename, put on flicker for a discussion.

The engineering document is 35 MBytes of .odt files with pics and descriptions that have all the history, including the fails and the whys of the fails.

In avionics, you must document the entire history. You cannot present any results out of nothing. Even if they look good. They don't like it when things fall from the sky. Even results.

Gerhard

One simple Spice model, a small block of text, would tell us everything about a MMIC, except maybe noise. It would tell us about bias, impedances, temperature effects, gain, small and large-signal behavior, voltage swing, everything.

Thousands of s-parameter numbers give us a peek at a few points of small-signal AC response at a few frequencies and usually one bias current. The mmic folks spend tons on developing s and x parameters and refuse to provide a simple Spice model.

I think that s-params are a relic of pre-computer days when people designed with slide rules and graph paper.

There are people who don't comment code either.

Then sack 'em.

Nice edge, 200-350 ps depending on where you measure. It's also unusually well matched to 50 ohms--your average amplifier has a VSWR around 2:1. Settling time, meh. Still a few percent off at ~2 ns.

Folks are often surprised when (after a bit of mental math) I tell them that they'll get to the shot noise limit by just connecting their photodiode directly to the input of a $10 MMIC. Happens fairly often.

Well, that's S21, to be fair. ;)

Blech. Looks like the device is turning completely off. If it were the biasing loop or AC coupling, I'd expect the drooly settling transient to slope the other way. I suppose the bias loop might just be slow.

Cheers

Phil Hobbs

Right. An inverting amp is still an inverting amp if you hang a long cable on it.

Cheers

Phil Hobbs

My latest 1-GHz o/e converter dumps one end of the photodiode into a MMIC and the other end into the DC path, only it uses a 99 cent MMIC.

It worked fine for the longwave version but was crazy slow for the 850 version. Turns out that the longwave photodiode has 20 pF from the pd cathode to the can! We just got some new Appointech diodes that are below 1 pF all around, and Jonathan is trying them.

I was tempted to dump my photodiode current into a resistor to ground, maybe peak that a tad, then voltage amplify it. But a certain book trashed that architecture.

I guess they could have done a real DC bias loop. The slow loop is probably great for RF, but not for pulses.

Does anyone remember h-parameters? They were an attempt to analyze transistors with algebra, before computers and Spice were born.

My always-humble prediction is that time-domain (Spice and such) analysis will eventually kill off s-params, x-params, z-params, load pull testing, all that slide rule/graph paper stuff. We would need a lot of compute power; maybe Mike is working on that. Nvidia?

Well, small-signal analysis will still be linear. You need your amp to be stable with zero input, after all.

H-parameters maintain a tenuous and shadowy existence because a capital beta is just B, so there's no good nomenclature for distinguishing large- and small-signal current gain except H_FE vs h_FE. Otherwise I know of no good reason to use them. Nobody I know of uses impedance or admittance matrices any more. (I actually never did.)

S-parameters are useful because 50-ohms in two leads of any device is generally enough that it won't oscillate, unlike shorts and opens! They also have a natural connection with Smith charts, which are sort of an RF version of Karnaugh maps--you can easily visualize what sort of matching network you're going to need. (Or I used to, back when I was an RF guy long ago.)

Cheers

Phil Hobbs

You've both forgotten about Y-parameters. But in any event, S's are the only ones worth paying attention to today. The rest have fallen by the wayside...

Y-parameters are the admittance parameters I mentioned.

In equivalently relevant news, I used to have a Gump Worsley hockey card. ;)

Cheers

Phil Hobbs