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PC port interface hardware

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GARTHWILSON

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Posted: Mon Mar 09, 2009 11:32 pm

Joined: Fri Aug 30, 2002 1:09 am
Posts: 8520
Location: Southern California

I tried setting up an experiment to measure it where the rubber meets the road, but my cheap RF signal generator does not put out a strong enough signal at the frequencies of interest for my 'scope to be able to measure. (I didn't want to take the time to build an RF amplifier either, something I used to do for a living. The lab where I worked had $100,000 of RF test equipment per engineer in the mid-1980's, something I don't have at home.) My next idea was to look up (since I couldn't remember) the inductance of a straight piece of wire in my books. This seemed to be an illusive piece of info, so I did a web search for it, and found a wire inductance calculator at http://www.consultrsr.com/resources/eis/induct5.htm . [Edit, 4/3/14: That domain seems to have expired. There's another online wire inductance calculator at http://www.eeweb.com/toolbox/wire-inductance/.] I measured the lead diameter of a .1µF ceramic disc capacitor and got .022", then plugged that and 2" of wire into the calculator (2" being approximately the length across the corners of a 40-pin DIP), and got 52nH. Changing the wire diameter to that of 30-gauge wire-wrap wire changed the inductance from 52 to 60nH, which is not much difference for this application. 52nH resonates with a .1µF capacitor at only 2.2MHz, and with a .01µF capacitor at 7MHz, assuming the capacitor were perfect. Above the resonance of course, the impedance of the capacitor with those long leads rises quickly with increasing frequency, whereas ideally it would keep dropping. At 100MHz (which is definitely a frequency of interest for a 14MHz square wave), this piece of wire has 32 ohms of reactance. Putting that in your power or ground connection is terrible on an IC with fast output slew rates and which may switch states on many output bits at once and each of those is driving several capacitive loads. Unfortunately there's inductance also between the pins and the die of the IC. The 6502 in a PLCC package dramatically reduces that not only by having a smaller package with shorter internal connections, but more of them, two Vdd pins and three Vss pins.

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As you can see, 90MHz is the only the fourth harmonic of 10MHz

It's the fourth one with any power on a true square wave that's perfectly symetrical, but 90MHz is still called the 9th harmonic of 10MHz. Ideally for this application, the theory would be treated in the time domain and not the frequency domain, but a very basic understanding of circuit properties at the harmonics of interest is nevertheless helpful.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

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kc5tja

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Posted: Tue Mar 10, 2009 4:20 am

Joined: Sat Jan 04, 2003 10:03 pm
Posts: 1706

Interesting; the amateur radio literature I've seen counts the number of harmonics, not the ordinal position of harmonics. Or, maybe I'm confusing it with music? Anyway, I think my point was taken.

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