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 7A11 input capacitance is 5.8pF from 5mV to 50mV/div, goes down to 3.4pF from 100mV/div to 1V/div, and down again to 2.0pF from 2V/div to 20V/div. Because the AC coupling is attained with a plug-on capacitor (that adds 1.2pF to the input C), a dc offset is supplied to reduce the need for AC coupling.

A nice thing about the 7A11 was that the probe cable length was adjustable in six steps to just over 7 feet in length. You just uncoil however much cable length you want.

The 7A11 was admittedly clumsy to use because of the probe size. It had some trouble dc drift with rf pickup at 5mV, 100mV and 2V/div. It was not very popular partially because it was expensive, $850 for single channel vs dual channel 7A12 (105MHz) at $700. But there was never another 1 megohm 7000 series plugin as fast as the 7A11...and it was one of the originals!

The 7A16 (single channel plugin) was also 150MHz, but that was a year later. Then the 7904 came out in late 1971. That made the 7A11 a 250MHz plugin (for $950). The 7A16 was then 225MHz (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 485 used a faster IC process ("SH2", 3.5GHz) but the 1 megohm to 50 ohm converter used only discrete devices. (See Electronics June, 1972)

By 1974, the 7A11 cost $950 and the new, popular 7A26 (dual trace) was $1,050, not counting probes. But that would still only get you to 50mV/div with a (9.5pF, 3 foot, X10) probe, five years after the 7A11 introduction. Tom Rousseau designed the 7A26 which used the faster IC process and a vertical IC I designed for the 485. The 7A26 sold so well that Tektronix presented Tom with an entirely gold plated 7A26! Obviously, he still has it.

Remember also that the 7A11 was introduced at the same show as the HP183A/1830A/1840A, faster (250MHz), smaller, lighter, and less expensive with better triggering than 7000 series had. 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, not the hoped-for 150MHz. That was partially due to the fact that it used the existing Tek IC process (about 1GHz). The HP IC process was 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 few with 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 you could certainly get to 5mV/div, which you could not attain with a 5mV/div plugin and a X10 passive probe.

But using that scheme, if the 7A11 went to 20V/div, it would have to stack three X10 attenuators inside the probe, making it a behemoth, and even if you did, that would still leave the X2 and X5 attenuations up to gain switching in the amplifier.

Problem with gain switching is the bandwidth and transient response tend 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 due to longer leads and more parasites.

To get a 1-2-5 sequence from 5mV to 20V/div, it had the worst of all possible worlds. The 7A11 needed two X20 attenuators in the probe and that meant that it 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 three entirely passive, relay-switched, O pad attenuators: 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. From 5mV to 20V/div, twelve different V/div settings, more than any other high speed plug in!

I got a patent on the variable attenuator, which was just a JFET shunting the O pads. The patent had to do with making the gain vs control rotation linear, which does not simply happen with linear gate-source voltage control. The JFET causes a small change in transient response, but not a bad one.

Ron Peltola (of Peltola connector fame) designed the probe. I helped on its amplifier.

Manuals

• Tektronix 7A11 Manual (PDF)
• Tektronix 7A11 Manual (OCR, PDF)

Pictures

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  Front

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  front above

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  right side, with removing probe

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  probe accessoires

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  right side detail, cover removed

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  left side detail, cover removed

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  input jack with probe cable

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  interior of probe

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  interior of probe, back side with range relays