really fast buffers

And at the other end of the scale some can produce a gain of far over 1,000 and be somewhat stable. That isn't the once round loop gain of course, it's V_out over V_in.

NT

There might be some mild hand-waving involved. The customer's machine has, in some boxes, microsecond delays that drift around, and maybe a nanosecond or two of of jitter, and all sorts electrical and optical propagation delays, and they want 1 ps jitter and 100 ps accuracy. That's meaningless, but I need to keep them happy.

I can use an 11801 sampling probe and poke around, and make a list of time fudge factors, to make up for cable and pcb and part delays. That can go into a cal table for the test set. Once that's done, I need stability. CMOS gates typically drift positive a couple ps per degree C. That BUF602 will help at least to get the fudge factors measured before everything drifts too far. Buffering and fanning out the fast signals allows better PCB geometry.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

We use lots of surface-mount beads, no problems.

A bead can kill RF oscillation but not contribute low-frequency noise like a resistor would.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I don't understand that. At low frequencies, as the base voltage goes up, the base current goes up. The base looks ohmic.

Can you Spice such a relaxation oscillator?

At high frequencies, the base can look like a negative resistance; some oscillators work that way.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Am 09.10.2018 um 06:00 schrieb John Larkin:

That does not work at low frequencies. It takes takes transistor- internal parasitic capacitance to have an effect. The effect gets more pronounced for high emitter resistance and high emitter-GND capacitance. In the pages from Rhea that I have referenced, there is a transformation that converts such a loaded follower to the usual feedback loop by re-assigning GND to the emitter itself and redrawing the circuit.

If you measure into base-gnd at RF, you see some pF in series with, say -50 Ohms. That is used in virtually all VHF/UHF VCOs from Z-Comm and others. I have once tried to pull down a Z-Comm VCXO by adding more varicaps since I had a few of them and needed lower frequencies. I didn't get very far because of the few pF that are in series to the base at RF. Disappointing, but finally understood.

In the case of my baseband amplifier with the 2 IF3602 pairs, there are nF capacitances, and the amplifier goes negative impedance below 100 KHz. Ferrite chokes have quite a good Q at these frequencies, and one can tune the oscillation frequency with them :-(

At higher frequencies where they work like resistors (as intended) they also generate voltage noise. You can't have your Kate, and Edith, too.

regards, Gerhard

I found that a unity gain buffer built from an AD8066 opamp also seems to have a negative resistance at its input. If you decide that you want to measure e.g. the DC offset, and short the input to ground with a 40cm jumper lead, it will oscillate at about 40MHz, and with shorter wires it will oscillate up to about 116MHz. Putting a 560R resistor in series with the input connector on the board, and a 3.3pF capacitor from the non-inverting input pin of the opamp to ground fixed it, but made it otherwise worse. I recall that there is nothing much in the datasheet to warn about this sort of problem.

I use discrete amplifiers with feedback to set the input impedance to 50 Ohms, because that's quieter than using an actual terminating resistor. These also have a negative input impedance in a certain frequency range, which causes instability with reactive sources. Fortunately --well no, by design-- this happens outside of my signal bandwidth, so I can muffle that with some filtering at the input.

Of course, like Gerhard says, the muffler produces thermal noise, but since that's outside my signal bandwidth, it doesn't matter.

Jeroen Belleman

In certain cases, one can achieve what LOOKS like negative time delay. Input pulse has a (relatively) slow rise time, and feeds a fast op-amp set to sense low threshold of the rise...output pops up beyond logic threshold BEFORE input.

What would happen if a computer were built of negative time delay gates? Th e output would appear before the input occurred. Since the input would some times be undecided when the output occurred, the output would determine the input. So take a standard computer, swap labels on inputs & outputs, and t here you have it, a faster than light future predicting computer. In fact t he slower it is, the faster it is :)

NT

y.

The output would appear before the input occurred. Since the input would so metimes be undecided when the output occurred, the output would determine t he input. So take a standard computer, swap labels on inputs & outputs, and there you have it, a faster than light future predicting computer. In fact the slower it is, the faster it is :)

Isaac Asimov invented it in 1947, and published his first paper on it in 19

48.

--
Bill Sloman, Sydney

snipped-for-privacy@ieee.org wrote in news: snipped-for-privacy@googlegroups.com:

Take a pair of scissors that span between the Earth and the Moon. It has straight cutting edges open at the moon and just beginning to shear at the Earth end.

As the scissors close, the shear point travels up the pair faster than light as the angle closes the Moon ends together. Of course, the depends on how fast the blades close. But the point of shear moves really fast.

Shear press for sheet metal works on this principle. The entire sheet gets cut in a single blade drop action, but the shear point is mobile allowing the event to occur with less shock and less force. For thick steel, the shear angle is not needed, but a cherry hot ingot may be.

Am 07.10.2018 um 16:19 schrieb Winfield Hill:

< >

and the pictures to the left & right.

The noise simulation tends to be optimistic.

There is now a negative version of LT3042 / 3045 : LT3094 :-)

negative voltage regulator, 2.2 nV/rt Hz

regards,

Gerhard

Version 4 SHEET 1 2048 1328 WIRE -240 -1136 -320 -1136 WIRE -240 -1120 -240 -1136 WIRE -48 -1104 -192 -1104 WIRE 176 -1104 -48 -1104 WIRE 224 -1104 176 -1104 WIRE 336 -1104 224 -1104 WIRE -192 -1072 -192 -1104 WIRE -240 -1056 -240 -1120 WIRE -368 -1024 -384 -1024 WIRE -320 -1024 -320 -1056 WIRE -320 -1024 -368 -1024 WIRE 176 -1024 176 -1104 WIRE -320 -1008 -320 -1024 WIRE -192 -944 -192 -992 WIRE -288 -928 -320 -928 WIRE -240 -928 -240 -1008 WIRE -240 -928 -288 -928 WIRE -192 -928 -192 -944 WIRE -48 -928 -48 -1104 WIRE -288 -912 -288 -928 WIRE 48 -880 16 -880 WIRE 96 -880 48 -880 WIRE 176 -880 176 -944 WIRE 48 -848 48 -880 WIRE -48 -752 -48 -832 WIRE 176 -656 176 -880 WIRE 256 -656 176 -656 WIRE 320 -656 256 -656 WIRE 176 -624 176 -656 WIRE 352 -576 240 -576 WIRE 352 -560 352 -576 WIRE 176 -496 176 -528 WIRE 176 -464 176 -496 WIRE -224 -416 -288 -416 WIRE -208 -416 -224 -416 WIRE -48 -416 -48 -672 WIRE -48 -416 -144 -416 WIRE 112 -416 -48 -416 WIRE -288 -352 -288 -416 WIRE -48 -352 -48 -416 WIRE 176 -272 176 -368 WIRE -288 -208 -288 -272 WIRE -288 -176 -288 -208 WIRE -288 -64 -288 -96 FLAG -192 -848 0 FLAG -288 -64 0 FLAG 224 -1104 vcc FLAG -288 -208 vgen FLAG -288 -912 0 FLAG -368 -1024 v3p FLAG -240 -1120 v3r FLAG -192 -944 v1p FLAG -48 -272 0 FLAG 48 -784 0 FLAG 256 -656 raus FLAG 352 -480 0 FLAG 176 -496 c1 FLAG 176 -272 0 FLAG -288 -576 0 FLAG -224 -416 rein SYMBOL voltage -192 -944 R0 WINDOW 0 -32 56 VBottom 2 WINDOW 3 32 56 VTop 2 WINDOW 123 0 0 Left 2 WINDOW 39 -52 151 VTop 2 SYMATTR InstName V1 SYMATTR Value 7 SYMATTR SpiceLine Rser=0.001 SYMBOL voltage -288 -192 R0 WINDOW 123 24 160 Left 2 WINDOW 39 24 132 Left 2 WINDOW 3 -155 185 Left 2 SYMATTR Value2 AC 1 SYMATTR SpiceLine Rser=0 SYMATTR Value SINE(0.0 0.001 2000000 0 0 0) SYMATTR InstName V2 SYMBOL cap -144 -432 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2

SYMATTR SpiceLine Rser=1m SYMBOL e -192 -1088 R0 SYMATTR InstName E1 SYMATTR Value 2 SYMBOL res -336 -1152 R0 SYMATTR InstName R1 SYMATTR Value 60 SYMBOL voltage -320 -1024 R0 WINDOW 0 -32 56 VBottom 2 WINDOW 3 32 56 VTop 2 WINDOW 123 0 0 Left 2 WINDOW 39 -57 138 VTop 2 SYMATTR InstName V3

SYMATTR SpiceLine Rser=0.1 SYMBOL res -64 -368 R0 SYMATTR InstName R2 SYMATTR Value 2k2 SYMBOL res -64 -768 R0 SYMATTR InstName R3 SYMATTR Value 2k2 SYMBOL res 192 -896 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 22k SYMBOL cap 32 -848 R0 SYMATTR InstName C3

SYMBOL npn 16 -928 M0 WINDOW 0 -8 -39 Left 2 WINDOW 3 -55 -11 Left 2 SYMATTR InstName Q3 SYMATTR Value ZTX851 SYMBOL res 160 -1040 R0 WINDOW 123 37 107 Left 2 SYMATTR Value2 m=3 SYMATTR InstName R5 SYMATTR Value 51 SYMBOL npn 112 -464 R0 WINDOW 0 91 30 Left 2 WINDOW 3 89 86 Left 2 WINDOW 123 91 58 Left 2 SYMATTR InstName Q4 SYMATTR Value ZTX851 SYMATTR Value2 m=16 SYMBOL npn 240 -624 M0 WINDOW 0 91 30 Left 2 WINDOW 3 89 86 Left 2 WINDOW 123 91 58 Left 2 SYMATTR InstName Q1 SYMATTR Value ZTX851 SYMATTR Value2 m=16 SYMBOL voltage 352 -576 R0 WINDOW 0 -32 56 VBottom 2 WINDOW 3 32 56 VTop 2 WINDOW 123 0 0 Left 2 WINDOW 39 -52 151 VTop 2 SYMATTR InstName V4 SYMATTR Value 2.9 SYMATTR SpiceLine Rser=0.001 SYMBOL res -304 -592 R0 SYMATTR InstName R6 SYMATTR Value 0.05 SYMBOL res -304 -368 R0 SYMATTR InstName R7 SYMATTR Value 1m TEXT 272 -840 Left 2 !;tran 0 2m 1.97m TEXT -104 -1128 Left 2 ;Vcc + E1 * 1n VRtHz noise TEXT 272 -864 Left 2 !.op TEXT 272 -808 Left 2 !.noise v(raus) V2 dec 1000 1 1e6 TEXT 272 -784 Left 2 !;ac dec 1000 10 10Meg TEXT 264 -728 Left 2 !.temp 20 40 60 TEXT -344 -208 VLeft 2 ;50/0.005 Ohm = - 60 dB attenuator TEXT 432 -256 VLeft 2 ;BZX84C2v7 + cap + Res to Vcc TEXT -128 -216 Left 2 ;Q1 is really one 2SC3420 because avail. but model missing. TEXT -104 -168 Left 2 !*ZETEX ZTX851 Spice model Last revision

21/1/93 (C) 1993 ZETEX PLC\n\n* 200 pV/rtHz lt AOE3\n\n*\n\n.MODEL ZTX851 NPN IS =1.0085E-12 NF =1.0001 BF =240 IKF=5.1 VAF=158\n\n+ ISE=2E-13 NE =1.38 NR =0.9988 BR =110 IKR=5.5 VAR=46 \n\n+ ISC=4.6515E-13 NC =1.334 RB =0.025 RE =0.018 RC =0.015 \n\n+ CJC=155E-12 MJC=0.4348 VJC=0.6477 CJE=1.05E-9 \n\n+ TF =0.79E-9 TR =24E-9

How about using a step-up transformer and some jfets?

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Am 10.10.2018 um 17:31 schrieb John Larkin:

A step up transformer would preclude measuring small AC signals riding on a DC; a DC block would have to be really large and would probably resonate with the transformer stray inductance.

Transformer + BJT would require a smaller step up ratio, which eases things, and BJTs usually have the better 1/f corner.

I have also tried FET amplifiers with a heroic numbers of FETs, and also tried multiple IF3602. That leads to a huge input capacitance and also questionable stability.

A BF862, RIP, or its new successors from ON semi are somewhere at 900 pV/rtHz and the ZTX851 at 200 pV/rtHz; that is quite a head start for the BJT, given that halving the voltage noise requires quadrupling the number of transistors.

I have also tried transformers in the chopper amplifier after the chopper switch; that resulted in very bad ringing. It first seemed attractive to step up the voltage after the switch so I could use an ADA4898 after the transformer. Snubbering away the glitches probably converts precious input signal energy to heat, not only the charge injection of the switches.

Noise-optimizing a chopper with Spice is close to impossible. With the (large-signal) switching going on, there is no such thing as an operating point where one could sum up the noise contributions. That would work probably with a harmonic balance simulator, but I don't want to spend a fortune on ADS.

BFP640 is also at 300 pV/rtHz, even less at cryo temp, but it has a 50 KHz 1/f corner, being a microwave transistor.

cheers,

Gerhard

Split the paths: a chopper amp for the low frequency part, a transformer and some jfets for the faster stuff. Someone used to make a permalloy shielded step-up transformer for just such applications. I think it worked down to 1 Hz or something like that.

Obviously, don't design it wrong.

Bipolars will make a lot of base current noise.

It ocurred to me that a distributed amplifier will be very low noise, but that's not practical at low frequencies.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

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

Is there any trick to getting this into LTSpice? Cutting and pasting as straight ASCII text yields various cryptic errors.

I cleaned it up to this point:

... but it still doesn't like the .MODEL statement, apparently.

-- john, KE5FX

save is as a text file. rename it to something.asc, open it with ltspice.

--
  Notsodium is mined on the banks of denial.

Weird. I just opened the .asc file in gedit on the Linux side of the virtual Win7 machine and copy-pasted it into the thunderbird message.

Let's see if they let me post attachments.

No. It does not work for .asc and not for .asc.txt Watch your mail.

regards, Gerhard

Did i not say "what LOOKS like negative time delay"?