am transmitter - vlsi project

Hi, I need to design an AM transmitter as a vlsi project (it needs to be located on smart dust later on). Does anyone have any good suggestions were to start from? I am familiar with vlsi, but don't really know how to implement such a big idea, into a vlsi simulation and circuit design. Looking on trasmitter's schemes didn't help, 'cause that's all resistors, amplifiers and capacitors. How do I translate it into vlsi design? Be very glad to any guidance... Thanks.

Reply to
hananl
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You would need to provide more info: Frequency, power level, supply power budget, what's it modulated with, allowed distortion, regulatory requirements (harmonics etc.)...

Look at what your available chip technology offers here. Then you might have to familiarize yourself with class-D amplifier technology, PWM schemes, synthesizing of envelopes etc. It also helps to look at how very modern AM transmitters in the "big league" work. The days of the classic big old plate modulator are numbered ;-)

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Reply to
hananl

Do you mean a transmitter in the US AM broadcast band? 550 to 1620 KHz...

Methinks you and your advisor should consider a few points:

(1) An AM transmitter is usually implemented using rather simple analog techniques. One transistor as an oscillator, either LC or crystal controlled. Another transistor as a series modulator. Total parts count: under a dozen parts. You could do this on an IC I guess, but it's nowhere near VLSI. And to meet FCC regulations you'll need a LC tuned circuit of non-negligible size, so the space-saving aspect of VLSI won't be of much help.

(2) You could do it digitally, but even then you just have a clock, a phase accumulator register, an adder and a multiplying D/A converter. Hmm, not all that much digital there either.

Quite a few challenges there--- a transmitter requires a few milliwatts of DC power, hard to do on a dust speck. Plus a transmitter requires an antenna, preferably of a quarter-wavelength or more-- that's a lot larger than dust-size in the AM band.

An AM band transmitter is not a very good match to digital, VLSI, or smart dust. Time for a rethink.

Reply to
Ancient_Hacker

Joerg ... tell me it isn't SO!!!! I've got a whole stockroom full of 6146s that will never fulfill their destiny.

{;-)

Jim

The days of the

Reply to
RST Engineering (jw)

Hello Jim,

T'is so, I am afraid:

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Those should still hold some value in the ham radio community. Now just imagine what these tubes could do if operated in pulsed mode. Although nowadays one would probably use big FETs instead :-(

At least you have the tubes. Out here the steel (!) tube in my old Rohde&Schwarz SMF has croaked. I like that generator because it has a real dial with coarse and vernier instead of having to press some buttons until the fingers cramp up.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Receiver? That won't help you much building a transmitter.

I assume you meant milliwatts. Anyhow, the classical way to do this would be a full custom chip design. For new transmitter architectures that move towards the digital domain check publications such as this one:

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As Ancient Hacker has posted keep the antenna in mind. It needs to have certain dimensions or it won't radiate.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

On Thu, 26 Oct 2006 11:22:27 -0700, RST Engineering (jw) top-posted: [top-posting repaired]

Just gold-plate their bases, and sell them to audiophools for $900.00 apiece. ;-)

Cheers! Rich

Reply to
Rich Grise, Plainclothes Hippi

Hello Rich,

And don't forget fancy plate connectors to go with them. Maybe like a small moped cylinder with gold plated fins, with an official endorsement paper from the RST Institute, Audio Research Department :-)

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Well no.

The audio signal is mixed with the carrier, and the output of that mixer is the original carrier, unmodified, and two sidebands on either side of that carrier. Taken as a whole, the amplitude varies, but the carrier itself stays constant.

This is likely a useful hint, because it's far more common to see a balanced mixer in ICs than something called an "AM modulator".

Michael

Reply to
Michael Black

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Is it a good red-blooded American tube with a number that goes

6-letter-number-letter, or is it one of those weirdo euro-things?

You do know that there are tubes available NOS, as from

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don't you?

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Technically correct, but misleading as hell. The OP's explanation is also technically correct, and less misleading to the transmitter designer, although of less use to the receiver designer.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Actually, most of us know what "AM" means, and we also know that "AM" doesn't mean "North American AM broadcast" unless you say so.

You can make the RF portion of an AM transmitter with a single transistor if you have sufficient audio power available. But you can't modulate 100%, and you'll have significant FM riding on your AM. Consequently your signal will be splattered all over the band and it won't be as efficient as it could be.

I can't give you an upper limit, because (a) you still haven't stated all your requirements, (b) you haven't said if you're going to do this on a process that lends itself to analog and (c) I'm not a chip designer.

I suggest that you do more web searching, or be more specific with your questions, or both. You may find the ARRL Handbook to be informative for background study -- you may even luck out and find an AM transmitter block diagram.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Hello Tim,

It's even weirder. A Euro thing built from steel instead of glass. EBF11, six of them, vintage 1938 :-(

Not that one. The EBF11 is a dual diode plus pentode with an odd form factor. It is only 1.5" high but also about 1.5" wide. Looks unbreakable, like it was designed to survive world wars two through five but unfortunately one of them is now experiencing episodes of unconsciousness.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Interesting, a digital modulation approach takes 72 stages, presumably of equivalent output power? Doesn't that yield a pretty poor dynamic range?

Or is it a 72-bit D/A, which sounds totally crazy.

And they must be in perfect phase and at perfect power levels across the band.

SioL

Reply to
SioL

SioL a écrit :

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equivalent output power?

Seems to be 72 identical PA, plus one linear one to interpolate between the discrete power levels. (top of p5, point 2)

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Thanks,
Fred.
Reply to
Fred Bartoli

Well, your explanation is mighty confusing, and partially incorrdct, IMHO but it's not your fault.

Originally AM modulation was done in a mighty crude but effective way-- by putting a regular old telephone's carbon microphone in series with the transmiting antenna!

That setup should give you a clue as to what "AM" really is, in the time-domain that is-- The carbon mike's resistance varies up and down as sound waves hit it-- resistance goes DOWN as a high pressure wave hits, then goes UP as the lower pressure hits, each happens once each audio cycle.

Later in "grid modulation" was figured out. here things get a bit fuzzy, but you could look at it as the audio moves the tube's bias point around and effects it's efficiency. You can also look at it as a "mixing" action is going on, which leads you to think a frequency-conversion is going on. Both viewpoints are correct, and of course both are misleading.

Later on "plate modulation" came in, where you put the audio signal in series with the plate DC supply. Again it's obvious what's happening in the time-domain: the plate voltage goes up and down with the audio, hitting twice the voltage on the positive peaks, and zero voltage on the negative peaks. Looked at PURELY in the time-domain, it looks like the carrier is going UP and down in amplitude. In the frequency domain of course it looks like sidebands are popping up, which they are.

Much later on, the theory of sidebands, "mixers", and "multipliers" was cleared up. mathematically, the old "mixers" were revealed to be kinda like a poor non-linear multiplier, which multipled the two input voltages (not terribly linealy, but good enough). And the math said when you multipled two sie waves you end up, in the frequency domain, with their sums and difference frequencies.

But people kept calling capacitors "condensers", and multipliers "mixers", so the confusion continues.

You can look at it as "multiplication" or "mixing"-- both are at least partially correct.

But if you look at the output waveform on a scope, you have this waveform where the RF amplitude gores UP and DOWN, all the way down to ZERO, so in some sense the carrier sure LOOKS like it's going down to zero. And if you think of the B+ voltage going to zero, it's hard to imagine how the carrier can still be going out when the plate voltage is zero.

So it's at least partially incorrect to say the carrier "stays the same" and "sidebands pop up".

Reply to
Ancient_Hacker

Joerg wrote in sci.electronics.design:

They're glass under steel. The tube itself is a more-or-less conventional glass tube, apart from the stubby shape. I sawed one apart as a youngster. Didn't quite make it, the hood *is* steel.

Anno

Reply to
anno4000

To get back to the poor original poster's question, and to point out some POSITIVE and realistic options, instead of all our poo-poohing:

If you want to make a transmitter, a really small one, one that can actually broadcast some distance more than a few millimeters, and still be detectable over the background noise:

(1) Think hard about the physics of the antenna situation-- a tiny antenna implies a HIGH frequency. For instance, if your "dust" is going to be on the order of an IC chip size, the frequency, in order to have a 1/4 wave antenna, is going to have to be in the tens of gigahertzs region.

(2) I suspect your vlsi process is not up to building gigahertz-region digital frequency synthesizers. It may be capable of low-gig rizetimes, but for a true synthesizer you'd need at least 20 times the output frequency to make effective synthesizer components, like adders and D/A's.

(3) Also think about the power situation-- even if your vlsi is low-power (which it won't be at GHz sppeds), a transmitter will need several milliwatts of ouput power to overcome background noise level, and that requires several times the input milliwatts.-- figure out how large a battery or solar cell has to be to generate a few milliwatts for even a few seconds. Prolly a whole lot larger than ic-size.

(4) Also think about the rules of your country's FCC. In the USA you can't just broadcast willy-nilly, there are specific bands and emission modes required (I think, unless there's some loophole). As far as I know, you have to stick to 100mw or so max power, and in the AM or FM bands, or around 13.56MHz, or twice that, or the 47 and 4xx MHz old wireless phone bands, or the microwave 2.6 GHz band, or a few other narrow spots. And I suspect you have to do AM in the AM band, FM in the FM band.

Just a suggestion, but the choices seem to narrow to:

VLSI digital synthesizer for the AM BC band, but with a long antenna (up to 3 meters in the USA).

VLSI analog synthesizer in the FM band, with a several inch antenna.

IC analog oscillator/modulator in the 4xx or 2.6 GHzMHz band, with an inch or so of antenna.

.. and for power source, the tinyiest of lithium hearing-aid batteries, so I hope your VLSI process can run on 1.5 or 3 volts :)

Hope this helps.

Reply to
Ancient_Hacker

equivalent output power?

discrete power levels. (top of p5, point 2)

Interesting.

SioL

Reply to
SioL

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