Automatic Gain Control in HF

On a sunny day (Mon, 31 Jan 2011 00:34:02 +0100) it happened M.Emmanuel wrote in :

A dual gate MOSFET, with the control voltage on the second gate, works great for small signals. Use a normal diode detector after amplification for the feedback, compare it with some transistor agaist a reference. This all assumes some tuned circuits and gain.

Lots of possibilities come to mind, with various levels of difficulty.

Why do you want to use discrete devices? Variable gain amplifiers and multipliers are easy to apply, but can be rough to duplicate with discretes.

Do you really need to go down to 0Hz? That brings along all sorts of problems, from not being able to AC-couple your signals, to having difficulty measuring signal amplitude at low frequencies.

Tell us what you're doing, and maybe we can help more.

Which dual-gate MOSFET would you recommend? I would like to use very standard and easy to find devices, available in both through hole and SMT -if possible-.

Is there a dual gate MOSFET device as common as 2N3904 and BFR92 can be for Bipolar NPNs?

Dual gate MOSFETs aren't quite as rare as hen's teeth, but they approach that. Note too that Jan said "this assumes some tuned circuits and gain", while you said "0 to 25MHz", which implies no tuned circuits.

Check out the "Gain Tweak.." thread. There have been several AGC type suggestions. CDA?

George H.

If you don't truly need to go all the way down to DC, how about an AGC similar to the one used (among other places) in the AGC (Apollo Guidance Computer)?

The AGC consists of CR3, CR4 and associated components.

It was good enough to send men to the moon, right?

They made those trips in Model Ts.

Vest way is an analogue Multiplier, a second one that squares the signal, (to get some measure of dc) a pure integrator and feedback. More costly but do-able.

Hardy

Have you ever been in an early Ford? I have, in a Model A. Darn good car, quite comfy. The only strange thing is that it's louder on the inside than on the outside.

ce

I'm a bit confused is a model T a good car or a bad car.

I've got this old tractor, the 'throttle' lever on the steering column doesn't connect directly to the carborator throttle. It 'basically' set's the rpm of the tractor, as the load changes the tractor adjusts the throttle to compensate. I still don't understand the system of links, springs, weight's, and centrifical sensors* that connect the two. But an AGC that's lasted since ~'47.

George H.

Engine design was pretty intricate back then. They could not just sit down and re-program some DSP. I remember when the owner of a Alfa Romeo shop in Italy looked under the hood of an Alfa Giulietta Sports, scratched his head and said "Only Giuseppe can adjust those".

That's a rather modern one. A friend had one these:

One-cylinder 5 to 10 liter engine depending on model, would run on just about anything that was somewhat liquid. Starting those can be "interesting":

A little fire is lit and normally that's done using a little burner that came with the tractor. Then the steering wheel is pulled and used as a crank:

I had a friend in Ohio that manufactured restoration parts for Model 'A' & Model 'T' Fords. I was around a lot of restored, and unrestored early Fords. His quality was so high that Ford allowed him to have their logo on his shipping boxes. He had the nicest metal shop outside the steel mill in that town, and I had free access to any machine that wasn't in use.

Still, the space program to date is in the experimental/Model 'T' stages. They are crude, compared to what you would need, not to have to spend months or years on maintenance between missions.

The Model 'T' was the first car built on an assebly line. it was affordable and ran fairly well, but the design was crude with things like wood floor boards, and a wood cased ignition coil. They also had a hand crank to start the engine. They sold for about $400.

This could be very hard or almost trivial, depending on the accuracy requirement and allowable distortion level. If you post some more detail, you'll get better answers.

Cheers

Phil Hobbs

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Neat, Can anyone but Gunter get it going? The removable steering wheel is a hoot. My Fergy is a four cylinder, but I've also got this ~50's John Deere Backhoe. (It came with the house.) It's a "Johnny popper" two cylinder and makes a lovely "pop, pop, pop, ....." sound.

George H.

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A bit of legerdemain (hearsay; but i heard it from an acquaintance who owned a Model "A") is that the floorboards were parts of the packing = crate the vehicle was shipped to the customer or dealer.

I've heard that it was from the crates that vendors used, to ship parts to the Ford factory.

And no one thought about touting how "environmentally friendly" such a process was way back then! :-)

And the remainder was used to make and sell charcoal:

We use it a lot but I guess it's not from Model A floorboard scraps anymore :-)