gimmick

Den fredag den 20. september 2013 21.57.32 UTC+2 skrev John Larkin:

I seem to remember something like 1pf/inch though that probably depends on what kind of wire you use

make it too big and untwist and spread the ends I'd think it's doable

-Lasse

The twirled ones like these are hard to get below 2-3pF:

Yours falls into the range of "two tiny sticks of wire" bent more together or more apart. That's what the wife of a friend did for many years, at a high tech RF electronics place. What I find helpful with those is a small piece of wood with some mass behind it. So I can gently "bang" them into alignment versus bending. Because bending tends to creep back in value to where it was before.

I was shooting for 0.3pf here.

So, maybe two pieces of solid wire sort of nearby. Or one piece leaning over a copper pour.

I guess the rough model could be one of those parallel-wire transmission line things, like Appcad.

I could spec it up a bit, snip length about right, then bend up-down from there.

This in one-off, breadboard. I wouldn't do this in production.

Just twist one up and measure?

But aren't gimmicks one way? The few times I had to neutralize an IF (think a 455kHz IF using transformers and CK762's :-), I just snipped off ends of the twist until I got it stable.

I can't locate the photo right now, but I once built a 60MHz IF strip where I made low-profile trimmers by mounting only the rotors on the PCB (with a half circle pattern :) ...Jim Thompson

The bent wire thing dates back to early TV production where the IF was aligned by guys with tools to bend the wire while they watched the spectrum analyzer. ...Jim Thompson

For a cap to ground it's really easy. Stick in a piece of wire, snip off until you get just under the min capacitance required by 20% or so, modify a wire cutter with a matching stand-off for production. Your CAD system could bark at this because it does not like capacitors with only one terminal. Mine lets me define them though.

Nah, just an ordinary cap. Unless you are operating at tens of giggeehoitz.

Why not? The RF guys do this all the time, even in super-expensive hi-rel gear. Can't disclose the companies but I know.

I remember when I landed my first surplus deal as a kid. Here I came home with a couple of really stout-looking drab-green transceivers, the kind you see on M.A.S.H. I opened up the first one to start my parts scavenging and came upon a large area with what looked like beat up strips of tin can material. Turns out that is how they aligned for proper channel matching. I had hoped for some mil-spec highfalutin gold-plated variable caps on glass or ceramic but ... nope.

In hindsight that was pretty smart and I just learned that recently on an older scope. It behaved erratic and I found that the center slip contacts of several trim caps had started to corrode. That simply can't happen with gimmick caps. So whenever I find enough time I'll open it up again and replace all those with gimmick caps. Simple and cheap is often also the best.

No, there is also the twirl-untwirl method. Of course, you can't do that too often or one to the wires will fatigue and snap. You also need a super-duper professional gimmick twirler. In my case a slotted piece of thin wooden stick, the "handle" side painted in British racing green.

Strips of tin can works as well. Learned that from the innards of an old military radio. I used Danish butter cookie cans because I liked to eat the contents.

Sounds tricky at 0.4 pF. Maybe I'll do this:

At what frequency? ...Jim Thompson

I could show you what's in the third can, but then I'd have to kill you.

From 2004...

Modify according bias conditions needed. ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

Not really. Even some of the early UHF TV tuners have them.

With a hot opamp that can go unstable on you because the more you turn the pot to the upper end the more capacitive load you have at IN-. That is one of the reasons why some TIA designs are really close to going berserk (C-load on IN- but almost nothing in the FB path). I'd try to keep it all in the FB path, no potmeter.

Looks like the perfect application for a couple of wires and a tweak stick. If you wanted to automate the calibration procedure I'd just use a reverse biased diode as a cap. Since IN- is at ground potential that should be easy.

I always liked the red ones with the Copenhagen mermaid on there.

Ah, the camouflage can.

It happens that John Larkin formulated :

I always used a pair of wires made from copper tin plated bus wire

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at

ld

the little mermaid ;)

anyway if it a one off how about two of these:

-Lasse

She weighs almost 400lbs :-)

She also has a big sister, and since the sister lives in North America she can of course not be naked:

It's only an 80 MHz amp and the pot will be sorta near mid-span. It's just going to be a breadboard. In production I'd use the right cap, or if I can't get that, a couple of caps in series. We actually stock caps from 0.2 pF up, but I'd like to be able to tweak this without a lot of soldering.

In San Francisco, it's Girl in a Parka.