really fast buffers

torsdag den 4. oktober 2018 kl. 00.03.49 UTC+2 skrev John Larkin:

clock buffers ?

TI's BUF602 looks pretty good for this. It's designed to be a gain-of-one buffer. Weird pinout, not like an opamp.

GBW is 1 GHZ, pretty good, but the slew rate is 8 v/ns.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

8 v/ns is too slow? how much voltage do you need? (sorry I mostly live in 'audio' land, ~10 v/us.)

George H.

My intuition thinks to skip the op amp and CMOS gates and go directly for the source follower, maybe using like a phemt and fast bipolar or phemt and mosfet in a compound arrangement, will get there.

No, the 1 GHz is mediocre. The 8v/ns is great.

I'll be following about a 4 volt logic swing from the DUT. This is a real analog buffer, so if I route the buffered signal to a scope I'll see pretty much what's there, not a digitally cleaned-up or attenuated version. I'm guessing it won't add much jitter.

How do you tolerate waiting around?

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I'm with you on the follower, perhaps a 42GHz BFP650, $0.285CAD @QTY100 at Arrow:

How would you use these in a compound arrangement? (LTspice ASC wellcome for the schematic. I wouldn't expect it to run.)

That would work, as a couple or few emitter followers to fan out my incoming signal. It has decent beta at, say, 50 mA or even 100 mA (driving something like 5 volts into 50 ohms).

The Vbe offset would be more predictable than the turn-on of a phemt. I'll be timing edges to picoseconds so I don't want unpredictable offsets.

You could servo out the Vbe offset with an opamp, but that would be a hassle to do lots of times on my board.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

All the low prop/zero prop clock buffers I know are PLL based and are no good for random signal buffering.

Their low drain impedances make pHEMTs very disappointing as followers, unless you bootstrap the drain. The late lamented ATF38148 could have a gain of 0.5 as a follower.

offsets.

You'll need a base resistor to keep it from oscillating. Don't let anybody hang a cable on it.

Cheers

Phil Hobbs

nless you bootstrap the drain. The late lamented ATF38148 could have a gain of 0.5 as a follower.

y hang a cable on it.

I hope that anyone playing with 42GHz GBW would automatically add a base re sistor to kill parasitic oscillations. Even a lowly 2N3904 can go into para sitics with a long base lead.

Often the real problem with parasitics is they can occur at a frequency abo ve your fastest scope, so you can't see them. Sometimes even touching part of a circuit can kill weak oscillations, making you believe the circuit is safe.

Don't believe it. Add a base resistor to any circuit connected to the outsi de world, or any place where fast transistors are connected together.

Often, a simple hand wave above a circuit can change the operating conditio ns, or a circuit may only go into oscillation at a certain point on the inp ut waveform. Both issues are hit or miss. Add a base resistor.

The value may range from 100 ohms for low frequency circuits, to perhaps 5 or 10 ohms for fast circuits. You can easily change the value as needed whe n you gain more experience with the circuit. But it is extremely difficult to add a base resistor to a smd transistor when you have forgotten to inclu de it in the layout. Add a base resistor.

would they list a minimum frequency then?

Looks like the best, fastest, lowest delay, lowest delay TC, and lowest jitter digital logic buffer will be an opamp!

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Well, I have a couple of 40-50 GHz ones, so that probably won't be an issue. ;)

I've posted a few times about my "manual wavemeter". From July 2015, concerning a pHEMT bootstrapped with a SiGe:C BJT:

"The first proto oscillated at around 12 GHz. I didn't have a microwave spectrum analyzer at the time, so I measured the frequency by an interesting manual wavemeter method: if I held my hand over the board at different heights, the oscillation amplitude varied periodically, with a period of a bit less than half an inch."

"The actual board was fine, after a bit of Dremelling and adding a bunch of parallelled bypass caps. With 40-50 GHz transistors and lots of gain in a small space, half a nanohenry here and there can ruin your whole day."

AKA a snivet.

That blanket rule doesn't cover all the bases, so to speak. ;) Base resistors trash the speed and noise performance, so a tight layout and close attention to minimum-inductance bypassing help a lot. Then I use a zero-ohm jumper as a placeholder for a possibly-needed resistor or bead.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Lowest power and lowest cost too? ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Or as an emitter follower with the base well bypassed to ground. The wire bonds inside are nice high-Q inductors.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I'm building a test set (for an astronomical heap of dollars) so that doesn't matter.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

How about SN65LVDS20? The 'enable' input can solve some of the switching issues. Differential gives you both polarities...

ECL is good, too, because the slew rate of your input pulse will be high when passing the threshold (and any ECL gate has a bit of voltage gain anyhow, so output slew rate will be healthy).

MC10H104 would be another candidate, and comes with a lovely opportunity to pay a Trump-era tariff, according to DigiKey.

A zero ohm short may not be the best idea. You have to pay

1. to install the part.

1. to remove it when you find a problem. This can damage the pads.

2. to install a base resistor. This can also damage the pads.

When you multiply the costs by the number of transistor involved, it can mean a substantial amount of money and time lost.

If you install the resistor in the first place, you can eliminate these costs.

After you gain some experience, you can pretty much tell how much resistance to use for each type of transistor and the kind of circuit used. I recommend using the highest value that won't degrade the performance of the circuit.

Also remember you may have a slow transistor in this production run. The next batch may be on the hot side.

Here's a problem you may not see often. This was in an audio amp. The top trace in blue is the input signal. The bottom trace in red is the parasitic oscillation.

Just because you are working in the audio range doesn't prevent a circuit from oscillating at VHF.

This doesn't exactly fit the "single-ended, easy termination" bill, but I've recently had some fun (and success) with the ADCLK950:

2 inputs, 10 outputs, propagation delay ~210 ps, 75 fs rms jitter.

How stable do you need the propagation delay to be?

? David