7A11
The 7A11, introduced in 1969, is a vertical plug-in for 7000-series scopes. In includes a permanently attached FET probe. The bandwidth of a 7A11 in a 7904 is 250MHz.
Regarding the 7A11, its designer John Addis says:
The capacitance at the probe tip of the 7A11 is 6pF max, and goes down to 3.4pF at 100mV/div and down again to 2.0pF at 2V/div.
The 7A11 was expensive: $850 for single channel vs dual channel 7A12 (105MHz) at $700. There was never another 1 megohm 7000 series plugin as fast as the 7A11...and it was one of the originals! The 7A16 was 150MHz, but was a year later. Then the 7904 came out in late 1971. The 7A11 was 250MHz (for $950). The 7A16 was then 225MHz (but for $625). The 7A16 disappeared rapidly and became the 7A16A, still 225MHz in 1973. In 1972, the 485 came out with a 250MHz 1 megohm input. The scope used a faster IC process ("SH2", 3.5GHz) but the 1 megohm to 50 ohm converter used only discrete devices. By 1974, the 7A11 was up to $950 and the popular 7A26 (dual trace) was $1,050, not counting a probe, and that would still only get you to 50mV/div with a X10 probe, five years after the 7A11 introduction. The 7A26 used the faster IC process and a vertical IC I designed for the 485. Remember also that the 7A11 was introduced at the same show as the HP183A/1830A/1840A faster (250MHz), smaller, lighter, less expensive with better triggering than 7000 series had. HP cleaned our clock until the 7904 came out. Their secrets were: Al DeVilbiss and a faster IC process. We had neither. The star-crossed 7A12, which was supposed to be the dual trace flagship of the original plugins was only 105MHz. That was partially due to the fact that it did use the existing Tek IC process (about 1GHz), but it was a slow process compared with HP's (about 3GHz). The 7A11 used discrete NPN and PNP transistors with 4GHz Ft.
There is one noteworthy point I would like to make about the 7A11. In those days, the different V/div settings were generally attained using fixed high impedance attenuators, usually stacked one after another and none more than X10 attenuation. I wanted the 7A11 to be able to handle the full useful range of sensitivites that other plugins attained when adding a X10 probe...that meant going to 20V/div. But why not go to 5mV/div, which you could not attain with the X10 probe? If you did, you could not stack three X10 attenuators inside the probe without making it a behemoth, and even if you did, that would leave the X2 and X5 attenuations up to gain switching in the amplifier. Problem with that is the bandwidth tends to change when you change amplifier gain. The greater the gain change, the greater the bandwidth/transient response change in the amplifier. The more gain settings you had, the worse it got with longer leads and more parasites. But if you did 5mV/div to 20V/div, you had the worst of all possible worlds, Turns out you needed two X20 attenuators in the probe and that meant that you needed not just X1, X2 and X5 gains in the amplifier, but X1, X2, X2.5, X4, X5 and X10! This had not been done before, but the 7A11 does it without any change in transient response! The secret is an entirely passive, relay-switched, balanced O pad attenuator: X1 (straight through), X2, X2, and X2.5 in a balanced transmission line environment. Stack them up (as you can do with matched pads) and you get all the combinations you need, X1, X2, X2.5, X4, X5, and X10. Twelve different V/div settings, more than any other high speed plug in! I got a patent on the variable attenuator, which was a JFET shunting the O pads. The patent had to do with making the gain vs control rotation linear. The JFET causes a small change in transient response, but not a bad one. Ron Peltola (of Peltola connector fame) designed the probe.
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Front
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side, with removing probe
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probe accessoires
