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Frequency limitations for beginners

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mg:
Hi there,

Today, I had opportunity to visit a high frequency digital dev lab where they develop customized ASICs and put them on HPC PCI cards.
As the name suggests, guys working there play with high frequencies, thus they have proper toys, like (>12GHz) o-scopes for 150k EUR, RF scanners, crazy soldering machines etc.
I never seen lab like that  so I opened my mouth wide ;)

This gave me thinking about a frequency which I shouldn't cross or after crossing I will get hit by problems at different level (dimension), hard to solve without sophisticated tools.
I assume that such problems exists and for instance I can't simply solder IC which will work at 1GHz?

I would like consider this question under different frequency categories:
(a) bellow 100 MHz
(b) bellow 300 MHz
(c) bellow 500 MH
(d) bellow 1GHz

I guess (a) fits into the beginners range, but what for instance if we start touching things like DDR3, Cortex, Spartan6 which operate up to 1GHz?

jahonen:
It is not the frequency what matters in digital electronics, it is the edge rate (rise/fall time) which is the main factor determining how difficult things are. Of course, timing margins get narrower when clock rate rises but those highest frequency things are usually serialized differential links so problem reduces to maintaining proper signal integrity.

Signal integrity stems basically from good control of impedance all the way from transmitter to receiver chip, which is mainly dependent on the materials and construction geometry. Of course, at highest frequencies, normal FR-4 becomes too lossy, but then we are talking something like >1 Gb/s serial links.

If you take the time to study basic properties of transmission lines and know how to create one within a PCB, then it certainly possible to make usable signal paths up to 1 GHz or so, without any special equipment. Of course, this almost always requires multilayer PCB (unless you are dealing with RF stuff, where there are only few signals, so wide traces are possible. Furthermore, RF signals are usually narrowband and good matching is required only on that narrow band, not from DC to n GHz or so as is the case with most digital signals).

Regards,
Janne

Bored@Work:

--- Quote from: jahonen on May 10, 2011, 08:11:53 pm ---It is not the frequency what matters in digital electronics, it is the edge rate (rise/fall time) which is the main factor determining how difficult things are.
--- End quote ---

Which boils down to a frequency issue again, as Mr. Fourier figured out in an other context in the 18th century. Sharp edges = lots of high frequency components. Failure to properly transmit those high frequency components = kiss your signal goodbye.

mg:

--- Quote from: jahonen on May 10, 2011, 08:11:53 pm ---Signal integrity stems basically from good control of impedance all the way from transmitter to receiver chip, which is mainly dependent on the materials and construction geometry. Of course, at highest frequencies, normal FR-4 becomes too lossy, but then we are talking something like >1 Gb/s serial links.

--- End quote ---

Is it only about SI?
I guess, it has to be more complicated than that?

What happens when after designing a PCB and assembling yourself components the thing does not work,  and you don't have 500MHz oscilloscope?
Where do you start - can you make it running?

sacherjj:
Coming from the Amateur Radio side of things, there are some techniques that help with hacking on higher frequency circuits.  Building techniques such as Ugly Bug or Manhattan where circuits are build up from a solid copper board that is at ground.  Solder points are created with Meg Ohm resistors to ground.   With DIP chips on their back and pins our to the side, you get some advantage of the small capacitance from the pin to ground.  Freebie filter caps, as it were.

Even then, most HAMs don't play up much higher than 2 meter, or 144-148 MHz.  (Of course, some are hacking microwave transmitters...)

I'm perfectly happy down here at 20-30 MHz max, where my Rigol can play and the micros are cheap. :)

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