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I have encountered an unusual situation in which I require an XOR gate that will function reliably when presented with a square wave input with a frequency between 2 and 3 GHz. I know that desktop CPUs have logic gates that can function at these speeds, but I don't know of any IC that will do this. Should I try to build the gate out of transistors?

Also, at these speeds, do I need to worry about using ground planes, mitered bends, and microstrip?

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  • \$\begingroup\$ That certainly is a fast response time needed... I'm guessing a 74 series just won't even come close to cutting the mustard... ;) \$\endgroup\$ – Majenko Jul 14 '11 at 21:06
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    \$\begingroup\$ @okw, very important element to this. Most people will spec based on the frequency of their clock. You actually need to spec on the knee frequency. This is 1/rise time of your signal. This means that if you have a 3GHz clock your knee frequency is what will determine bandwidth requirements. I have some hope though, you know what a mitred bend is, most do not. \$\endgroup\$ – Kortuk Jul 14 '11 at 22:46
  • \$\begingroup\$ Thanks for all the detailed answers - I will probably have to rethink the design and lower the frequency requirement to something lower like 900MHz. \$\endgroup\$ – okw Jul 14 '11 at 23:45
  • \$\begingroup\$ This question reminds me of how the "A20 gate" was an actual external AND gate before the 486. \$\endgroup\$ – Yuhong Bao Dec 25 '13 at 3:25
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The fastest logic family long has been and still is ECL. While often overlooked in recent times, developments such as PECL and LVPECL (essentially positive supplty ECL and differential PECL) have kept the family at the forefront of logic switching. The previous limitations of multiple supplies and negative voltages have been eliminated, but with backwards compatability available in many cases.

The MC10EP08 / MC100EP08 devices would meet your requirement http://www.onsemi.com/pub_link/Collateral/MC10EP08-D.PDF

Not quite as good but also almost meeting your spec http://www.onsemi.com/pub_link/Collateral/MC10EL07-D.PDF

Available from Digikey (in stock) http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MC100EP08DTGOS-ND

In PECL mode these will operate from Vcc = 3.3V to 5V and Vee = 0V.

Maximum frequency is rated as > 3 GHz typical with propogation delays of 250 picosecond (!) typical and 300 picosecond max at 25C with cycle to cycle jitter of < 1 ps.

Digikey list a range of ECL gates.

While 3 GHz operation is probably best left to existing gates such as these ones, it is relatively easy to implement extremely high speed gates yourself using discrete parts with ECL type topology. Looking at the equivalent circuits of older ECL gates gives a good start (modern datasheets typically just give overall functional diagrams with no clues as to how the results are achieved). Gates are essentially very familiar long tailed pair type arrangements. Performance per effort and cost is liable to be vastly better than for most other approaches.

An excellent TI tutorial on "Interfacing Between LVPECL, VML, CML, and LVDS Levels" with discussions on impedance matching, transmission lines, reflections, biasing ... , and includes diagrams of how functionality is achieved.

http://focus.ti.com/lit/an/slla120/slla120.pdf

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I suggest a change of approach. You don't say why you need such an XOR, but I will propose that if you're asking questions about mitered corners and ground planes then you don't really have what it takes to do this kind of circuit. Don't take offense at that, as I suspect that 99.99% of the people on this site couldn't do that-- including me, and I have done GHz circuits before! So, rather than trying to do a 3 GHz XOR, I suggest that you find a different way of accomplishing what you want in a way that doesn't need such fast speeds.

Just to make myself clear, here's why I suggest changing your approach... Let's say that you could do a 3 GHz XOR, then here's some of the issues & solutions that you'd face:

  1. You wouldn't do this out of individual transistors, too slow. TTL type parts are also way too slow. Instead you would have to think about some high speed logic parts. Back in the day you might use ECL or PECL parts (a different family, like TTL but not). I have no idea what you would use now, or even if the ECL/PECL parts are still around. Of course custom chips will do it too, at a huge cost.

  2. Ground planes, absolutely. Controlled impedance PCB's, yup. Maybe 6 or 8 layer PCB's, depending on other requirements. At least 4 layers, for sure. Mitered bends, might as well. Microstrip/microplane traces, absolutely. And of course you'll have to play very close attention to PCB layout. Remember that 3 GHz is about 0.333 ns.

  3. Once you get it all built, let's say that it doesn't work. Then what? Get out the o-scope! Most hobbyiest o-scopes top out at around 100 MHz. In my office I have a 1 GHz, 4-channel scope that cost US$10K, but the 1 GHz probe costs an extra US$2K. You'll need at least 5 or 6 GHz and 3 scope probes. I haven't priced them in a while, but that will cost at least US$10K, and maybe up to US$30K.

So, to do it you'll have to use parts that are hard to find, do a complex layout on a multilayer PCB, and when it doesn't quite work right (odds are that it won't) you'll have to spend lots of money on an o-scope to help you figure it out. Then repeat the process again, because at 3 GHz you can't rework your PCB to fix the flaws. Ouch!

And lastly, here's a link to some On-Semi ECL XOR Gate: http://www.onsemi.com/PowerSolutions/product.do?id=MC100EL07 It looks like it might, just barely, be able to do 2 GHz. 3 GHz looks like a stretch, but not completely out of the question. They do have an eval board for that chip (wow, I've never seen an eval board for an XOR gate before). If you insist on going down this path, that eval board might be your best option (US$137 at Digikey). But you'll still need an o-scope.

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    \$\begingroup\$ Check out NBSG86A and NB7L86M for parts that On Semi claims can be used as XOR/XNOR at 8 and 12 GHz, respectively. They're actually 2:1 MUXes, which can be wired up as essentially any gate. They don't explain how to wire the XOR function to get the right termination though. \$\endgroup\$ – The Photon Nov 17 '12 at 7:11
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3Ghz? Dude, you have real troubles :-)

Making out of transistors is not an option - You won't go far over 100 Mhz even with fastest transistors. The main problem is trace lengths & EM interference & sloooooww transistors.

Even if you have separate chip with required speed - You will have to worry alot about transmitting signal with up to 10-15Ghz bandwidth (to have at least some visible fronts, you need to be able to transfer multiple of your target digital frequency). Also, at this speed, signal reflections will require impedance matching everywhere (=i.e. you need not only ground plane, but also specific PCB thickness & trace widths + termination)... World of hell.

The only reliable solution is to leave that XOR gate inside custom ASIC having rest of your device. Even at 0.25um you can have 3Ghz XOR easily.

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    \$\begingroup\$ This is exactly why most modern high speed microprocessors have a comparatively low speed interface to the motherboard and operate at many multiples of that speed internally. The distances inside the chip are miniscule compared to what is on the motherboard. While it is possible to have say a 2.66GHz front-side bus on a processor, it makes the motherboard pretty much impractical. \$\endgroup\$ – Majenko Jul 14 '11 at 21:57
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    \$\begingroup\$ A nanosecond is a light-foot in propagation terms at lightspeed. Longer on PCB. While not for the faint hearted, IC's such as those I listed are actually made to be used and can be. Even if implemented in an ASIC etc the signals have to be dealt with. The requirement may be "impractical" but IF he has it in practice then PECL will allow it to be met with due care and skill. Without due care and skill, at 3 GHz NO solution is going to work. \$\endgroup\$ – Russell McMahon Jul 14 '11 at 22:21
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Probably a bit much for you, but the HMC721LC3C from Hittite is good for 14 GHz. Digikey has 10 in stock as of this writing.

There's some design info that might be useful that you could glean from their evaluation PCB, much of which would be applicable to less demanding requirements.

It's really useful to have a fast sampling scope-- you can see the discontinuities introduced by PCB bends, connectors, visa etc. such a beast can be cobbled together from boat-anchor eBay findings for a few K (dollars) but it won't be very portable.

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