View source for 7A11

{{Plugin Sidebar | manufacturer=Tektronix |
type=7A11 |
summary=250 MHz FET-probe amplifier |
image=tek-7a11-front.jpg | 
caption=7A11 front view |
introduced=1969 |
discontinued=1984 |
series=7000-series scopes|
manuals=
* [[Media:070-0984-00.pdf|Tektronix 7A11 Manual]] (PDF) / [[Media:070-0984-00 (2).pdf|Alternate]] (OCR, PDF)
}}
The '''Tektronix 7A11''', [[introduced in 1969]], is a vertical plug-in for [[7000-series scopes]].
It includes a permanently attached FET probe.  The bandwidth of a 7A11 in a [[7904]] is 250 MHz.

{{BeginSpecs}}
{{Spec | Bandwidth | 250 MHz (in [[7904]]) }}
{{Spec | Deflection | 5 mV/Div to 20 V/Div in 1—2—5 sequence }}
{{Spec | Input resistance | 1 MΩ }}
{{Spec | Input capacitance | 
* 5.8 pF @ 5 mV/Div to 50 mV/Div 
* 3.4 pF @ 100 mV/Div to 1 V/Div
* 2.0 pF @ 2 V/Div to 20 V/Div
* Add 1.2 pF for BNC input adapter
* Add 1.2 pF for AC coupling plug-on capacitor
}}
{{Spec | Features | 
* Variable DC Offset (with output connector)
* 20 MHz Bandwith Limit switch
* Invert switch
}}
{{EndSpecs}}

==Internals==

Regarding the 7A11, its designer [[John Addis]] says:
<blockquote>
The 7A11 input capacitance is 5.8 pF from 5 mV/Div to 50 mV/Div, goes down to 3.4 pF from
100 mV/Div to 1 V/Div, and down again to 2.0 pF from 2 V/Div to 20 V/Div.   Because the AC
coupling is attained with a plug-on capacitor (that adds 1.2 pF to the input C), a DC offset 
is supplied to reduce the need for AC coupling.
</blockquote>
<blockquote>
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.
</blockquote>
<blockquote>
The 7A11 was admittedly clumsy to use because of the probe size.  It had some trouble —
DC drift, RF pickup at 5 mV, 100 mV and 2 V/Div. It was not very popular partially 
because it was expensive, $850 for single channel vs. dual channel [[7A12]] (105 MHz) at $700.  
But there was never another 1 MΩ 7000-series plugin as fast as the 7A11 ... and it was 
one of the originals! 
</blockquote>
<blockquote>
The [[7A16]] (single channel plugin) was also 150 MHz, but that was a year later.  
Then the [[7904]] came out in late 1971.  That made the 7A11 a 250 MHz plugin (for $950).  
The [[7A16]] was then 225 MHz (for $625).  The 7A16 disappeared rapidly and became the [[7A16A]], 
still 225 MHz in 1973. 
</blockquote>
<blockquote>
In 1972, the [[485]] came out with a 250 MHz, 1 MΩ input. 
The 485 used a faster IC process 
("[[SH2]]", 3.5 GHz) but the 1 MΩ to 50 Ω converter used only discrete devices. 
(See Electronics June, 1972.)
</blockquote>
<blockquote>
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 50 mV/div with a (9.5 pF, 3 foot, × 10) 
probe, five years after the 7A11 introduction.  [[Tom Rousseau]] designed the 7A26 which used the 
faster IC process and a [[155-0078-00|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.
</blockquote>
<blockquote>
Remember also that the 7A11 was introduced at the same show as the HP183A/1830A/1840A, 
faster (250 MHz), 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.
</blockquote>
<blockquote>
The star-crossed [[7A12]], which was supposed to be the dual trace flagship of the original plugins, 
was only 105 MHz, not the hoped-for 150 MHz.  That was partially due to the fact that it used the 
existing Tek IC process (about 1 GHz).  The HP IC process was about 3 GHz.  The 7A11 used discrete 
NPN and PNP transistors with 4 GHz f<sub>t</sub>.
</blockquote>
<blockquote>
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 ×10 attenuation.  I wanted the 7A11 to be able to handle the 
full useful range of sensitivites that other plugins attained when adding a ×10 probe ... that 
meant going to 20 V/Div.  But you could certainly get to 5 mV/Div, which you could not attain with 
a 5 mV/Div plugin and a ×10 passive probe.  
</blockquote>
<blockquote>
But using that scheme, if the 7A11 went to 20 V/Div, it would have to stack three ×10 attenuators 
inside the probe,  making it a behemoth, and even if you did, that would still leave the ×2 and ×5 
attenuations up to gain switching in the amplifier.  
</blockquote>
<blockquote>
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 parasitics.
</blockquote>
<blockquote>
To get a 1-2-5 sequence from 5 mV/Div to 20 V/Div, it had the worst of all possible worlds.  The 7A11 
needed two ×20 attenuators in the probe and that meant that it needed not just ×1, ×2 and ×5 gains in 
the amplifier, but ×1, ×2, ×2.5, ×4, ×5 and ×10!  This had not been done before, but the 7A11 does it 
without any change in transient response!
</blockquote>
<blockquote>
The secret is three entirely passive, [[miniature relays | relay-switched]], O-pad attenuators:  ×2, ×2, and ×2.5 in a 
balanced transmission line environment.  Stack them up (as you can do with matched pads) and you get 
all the combinations you need: ×1, ×2, ×2.5, ×4, ×5, and ×10.  From 5 mV/Div to 20 V/Div, twelve 
different V/div settings, more than any other high speed plug in!  
</blockquote>
<blockquote>
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.
</blockquote>
<blockquote>
[[Ron Peltola]] (of [[Peltola connector]] fame) designed the probe.  I helped on its amplifier.
</blockquote>

==Links==
* [http://www.barrytech.com/tektronix/tek7000/tek7a11.html Tek 7A11 page @ barrytech.com]
* [http://www.amplifier.cd/Test_Equipment/Tektronix/Tektronix_7000_series_amplifier/amplifier_7A11.htm Tek 7A11 page @ amplifier.cd] including repair report (relay replacement!)

==Prices==
{| class="wikitable"
|-
! Year
! 1971
! 1980
! 1984
|-
! Catalog price
|align=right|   $850
|align=right| $1,725
|align=right| $2,700
|-
! 2019 value
|align=right| $5,340
|align=right| $5,320
|align=right| $6,600
|-
|}

==Pictures==
<gallery>
Tek-7a11-front.jpg
7A11_2.JPG| right side, probe unplugged
7A11_3.JPG| probe accessoires: tip cover, AC coupling capacitor, [[GR-874]] adapter ([[017-0088-00]])
7a11-right.jpg| right side detail, cover removed
7a11-left.jpg| left side detail, cover removed
7a11-input.jpg| input jack with probe cable 
7a11-probe-interior.jpg| interior of probe
7a11-probe-interior-back.jpg| interior of probe, back side with range relays
Tek 7a11 on.JPG
</gallery>

[[Category:7000 series vertical plugins]]
[[Category:Active oscilloscope probes]]