Fast-assed signal switch

It's a sup'd up 4066?? D from BC

It's more conventional to keep the diodes off most of the time and couple SRD-generated impulses into them to turn them on very briefly. The resulting signal is usually only a few per cent of the input (the ratio is called "sampling efficiency") so the resulting charge glitch is gained up, sample/holded (is that a verb?) and fed back to re-center the diode bias. This gets around the problem that the tau formed by the diode resistance and the load capacitance can be really slow.

I'm guessing that any imbalance in your SRD drive would blast through into the output and swamp the signal.

I have a bunch of HP and Tek sampler manuals; I'll post some schematics when I have a chance. There are probably manuals on the web somewhere... HP1810, HP187, Tek 1S1, 1S2, and the S1/S2/S3 sampling heads. Most of the old manuals had a good technical discussion.

There's also Tek's wonderful paperback "Sampling Oscilloscope Circuits", 1970, original price $1. They show up on ebay.

John

John

Ive made this and it works fairly well,

I since modified it to be a SRD type design but was with a monolithic device including the coupling capacitor and schotky diodes.

you can get very fast comparators wich are easier than using discrete transistors.

as stated in other post, the way SRD are used here is to use the turn off spike to turn on the schotky diodes for a breif time. theres no point in having any other mechanism to turn them on in a sample circuit with using a SRD.

Colin =^.^=

Heh, an analog type D flip-flop. With diodes.

device

Yeah, but that's cheating. ;-)

Why? Isn't the instant when the schottkies are turned off the instant when the signal is disconnected? Therefore, it shouldn't matter how long the thing is switched on, so long as enough of the input voltage gets through the diodes (perhaps into a rather small initial holding cap, as the 1pF in the above circuit). Turn-off should be with as high a dV/dt as possible, to maximize the input dV/dt that can be handled without slew errors (in an SE switch, the slew is skewed; in a balanced switch like this, both up and down slew rate).

The "hump" of an SRD turn-off, coupled appropriately, will turn on, and off, the schottkies fast enough, but not necessarily in enough time to get anywhere up the RC time constant of the input, diodes (whatever effective resistance they have) and holding capacitor. "Sampling efficiency" doesn't make sense to me: after coupling in a bit of charge, another bump of the same level will go more in that direction, despite the input being constant. Therefore, it is approximately an integrator circuit, and therefore needs a differentiator to have any use as an oscilloscope attachment.

Tim

-- Deep Fryer: A very philosophical monk. Website @

Get the Tek book. It explains all of this stuff.

John

Way to dodge the question ...NOT ;-)

Unless you're offering, of course.

Tim

-- Deep Fryer: A very philosophical monk. Website @

Oh all right.

Imagine a 25 ohm source (50 in, terminated), a series switch, and some small hold capacitance. Say that the switch has another 10 ohms of series resistance, and the hold cap is 3 pF. If the switch is closed, the hold cap follows the input with about a 100 ps time constant. So your off-switch will be speed limited by that tau.

If we blip the switch closed only for, say, 25 ps, the cap samples the signal for 25 ps but it only charges up to about 25% of the difference between the pre-sample cap voltage and the new voltage. That's a 25% sampling efficiency, but we're 4x as fast. All it takes to fix the efficiency problem is a cheap opamp, to give low-frequency gain. Most sampling scopes only run a few percent efficient.

And fast samplers usually use only 2 diodes, to reduce the switch resistance.

This was all worked out in about 1961, when HP discovered the SRD (known then as the Boff Diode) and invented the dual-diode feedback sampler, first used in the HP185 scope.

The only two serious advances since then have been the shockline sampling pulse generator and the 6-diode traveling-wave sampler. The fastest electrical samplers are now upwards of 250 GHz, using shock lines and dual diodes. PSPL sells a commercial 100 GHz sampling head, but not the whole oscilloscope.

John

thers no D type there that I see, although I added an ecl one in mine becuase i dont see how that triggers cleanly.

I cheated even more and replaced those transistors in that diagram with the emiter folowers from the ecl trigger flip flop

well theres just no point in having an srd and turning it on as well. you can get a very fast fall time of the switching waveform without srd. the voltage over wich the diodes turn from on to off isnt much and you dont need a fantastically high dv/dt to acheive 250ps switching time.

extending the on time has no such effect whatsover becuase you are only interested in the last 250ps or so anyway. so if the hump is ~250ps thats about right, whatever state the waveform is in before then is of no interest to you. in fact it can cuase problems.

its not realy an integrator, but the two types of circuit behave differently. with an srd the voltage on the capacitor remains from one sample to the next, so adjacent samples that are similar voltage increase the voltage on the capacitor. therefore at slow sweep rates the capacitor charges up closer to what the input is.

therefore with an srd and capacitor the sweep rate affects the high frequency roll off.

with the on for longer aproach its harder to recover the sampling loss as the previous sample voltage is destroyed, however the frequency response is flatter but ofc lower voltage.

Colin =^.^=

Here's a dual-channel sampler I did for fun. Only one channel is implemented. It worked pretty well, 70 ps risetime, 5 GHz bandwidth. It drives a dual-diode half-bridge with complementary impulses, starting with a step generated by the srd in the center, then differentiated by the short, fat, shorted transmission lines.

John

It picks off a little of the input signal for the blowby compensation. "Blowby" is the small amount of non-sampled signal that gets through the diode capacitance. I figured the same signal could be used for an internal trigger, which would be handy for high rep-rate signals. The blowby amp is to the lower left. I'd use an opamp nowadays.

Yup. That pair goes off the the charge amp, slow sample-hold, and bias feedback stuff, not shown.

The topside flood seems to work. And there's lots of capacitance between the topside and bottom copper pours. There are 8 vias sort of generally surrounding the sampler circuit.

The only thing that matters here is the peak of the spike that forward biases the sampling diodes. Otherwise, it can ring and bounce and twang all it wants, and it doesn't matter.

John

What is the piece of white coax doing?

Where is the sampled output? In my HP8411A, the output comes from the DC bias resistors to the Schottky diodes, which would be equivalent to the to fatter wires that go down through the hole closest to the sampling diodes. Is that what you do on your sampler?

BTW I can't help thinking that it might go better if you put in a few more ground "vias" at the ends of the fat microstrip lines above and below the SRD. Maybe it wouldn't actually work better but I guess the current pulse into the top transmission line eventually has to be returned to the bottom transmission line through the ground plane under the SRD.

Chris

Thanks - makes sense.

Ok.

I was just thinking that for maximum bandwidth, the reflection from the short had better be nice and crisp to avoid smearing out the shape of the sampling pulse. If it worked well, then that's all that matters.

Chris

Has optical sampling been done? Two photo diodes switched by a short laser pulse.

So, the SRD plinks the short bit of trace (between its caps and the rest of the ground plane, acting as an inductor), which causes a wave to bounce down both ends, and I suppose reflecting off the right side, so the total pulse length is approximately the width of the dual dipole structure? Or is the pulse the height of the dipole base (from the SRD to where it joins the ground plane), so the pulse is that short, and the right end is merely terminated? (I can't get a good look at the right side.) But if it were terminated, why any right wing at all?...

The schottky diodes are in the SOT-23 I take it, so that when the pulse hits it, it gets forward biased, grabbing some of the input and dumping it into

1. nearby capacitances and 2. the twisted pair line exiting through the hole?

So the other end of the output twisted pair contains coupling and bias which, combined with the 2.7k's, keeps the schottkies biased off?

Where does the DC (from the rectified pulse) go? Seems to me it might build up, though obviously DC /per se/ bleeds off just fine, being 0Hz. I guess I should say, charge built up from one cycle.

Tim

-- Deep Fryer: A very philosophical monk. Website @

One sampler is a thin film of radiation-damaged GaAs, which forms a photoconductor with a picosecond-range lifetime. It's been used with a fs laser to make an optically-gated electrical sampler. One problem is the cost, and another that the fs lasers sort of fire when they feel like, and can't be triggered with appropriately low jitter.

Agilent makes an optical signal sampler, the 86119A, that uses some nonlinear optical effect. It's very expensive but hits 500 GHz.

HP and Tek could certainly make 250 GHz electrical samplers, but seem to have called a truce at about 70G, probably because signals don't pass through cables and connectors very well up there.

John

Yes, the latter. The very short stubby traces are the clipper lines. The longer traces to the ends transport the 50-ohm glitches to the diode pairs.

Imagine that the SRD is a fast-rise current source. The stubby lines are 25 ohms. The current enters the 3-way junction and the leading edge goes in all directions. When it hits the end of the 25 ohm stubby, it reflects back as zero volts, that hits the junction, and zero volts chases the initial step down the 50 ohm lines. It's all terminated at the diodes.

(I can't get a good look at the right side.) But if it were

That's for the 2nd channel, which I never built.

Yes.

Yup. The diodes are biased off about 2 volts.

A small shot of charge gets to the twisted pair, the amount depending on the difference between the instantaneous signal level and the previous diode bias. A charge amplifier senses the step, and it's amplified and goes into a slow (1 us roughly) s/h and then a gated integrator. The resulting voltage is fed back to approximately re-center the diode bias for the next shot. It's a feedback sampler, invented by HP in about 1961 or so. The diodes are sort of a sampled error detector.

John

Alright, excellent. Then, to apply this to things I have on hand, I might... hmm, I probably have some signal schottkies. I have some RF circuits, one board here has four glass body diodes arranged in a ring modulator (inbetween two ferrite transformers- could it be any more obvious?). I can make out "HP2", and maybe "305" on the next line. HP2305, any ideas?

Anyway, for pulse generator, I don't recall having any diodes with reasonable snap, so I might try an avalanche generator. 2SA1206 or something, I discovered, works at bench supply voltages, while I have some PH2369's that plink from +100V, as Jim uses in AN47. So I could set up one of these on, say, 6" of shorted coax, no, twisted pair -- so that when this thing fires, it runs a good voltage for a moment, then reflects back and shorts out. In the mean time, some schottkies or whatever are biased by said pulse and pick up some input from across the input cable, passing it to something, which I suppose has to be a charge amp, to restore the frequency response.

Ironically, looking at the finer details of an avalanche generator is one of the goals of my wanting to look at hell ass fast signals. Fortunately I have more than two 2369's...

Tim

-- Deep Fryer: A very philosophical monk. Website @

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you could let the laser fire at random, then you measure the time it fires from you trigger point then plot the sample at the right place.

Colin =^.^=

It's true that lasers in general, and especially solid state lasers like Ti:Sapphire, aren't triggerable any time you happen to want. Their jitter, though, is lower than any other known system whatever--below 1 part in 10**20, iirc. You can make them wider than an cctave, so that you can harmonically lock one end of the frequency comb to the second harmonic of the other end, and make something like an old-time ham marker frequency generator, producing peaks at 100 MHz intervals across the whole visible and near-infrared spectrum, whose frequency is known to an accuracy of 10**-18 or better. The key to the accuracy is that you aren't multiplying up the 100 MHz by 4,000,000 times--you're doing the mixing at 1x, which gets you a cool 126 dB improvement, and the frequency accuracy of each comb peak (in Hz) is the same as the rep rate's.

This got Jan Hall and Ted Haensch the Nobel Prize in 2005--more for the optics than the electronics, because they figured out how to do the spectral broadening without messing up the time coherence. The nice thing is that once they figured it out, it's really easy to do.

There are also streak cameras, which can take data continuously, not stroboscopically like a sampling scope, and can display sensibly at 1 ps/division. Getting the electrical signal onto the optical one is usually the tough part, but I have an electrooptical modulator in my lab that is flat to 1 dB out to 30 GHz, and the technology could go further. Full scale is about +20 dBm.

If you get good enough coverage this way, it could work. Alternatively, with a fs sampler, you could use a variable optical delay, but that would probably have to be mechanically scanned, which would make the update rate slowish.

Cheers,

Phil Hobbs