DIY sampling "scope"

Sure. Email me.

Thats the easy part. I've been playing around with a similar idea. The biggest problem is triggering on signals with frequencies in the GHz range.

If it's just TDR, it doesn't need a trigger. TDR is probably the best, concentrated market to sell to. The old Tek 11801-series scopes are failing, and the Polar Instruments stuff is slow and ancient. There's room for a new player.

The fast stuff isn't all that hard at, say, 50 ps or so rise time. Some thought will have to be put into connectors, fixturing, interfacing to PCB test coupons.

What will be a bunch of work would be a good PC GUI application that PCB houses or receiving inspection folks could use to acquire and archive PCB impedance tests.

and

Aren't there a lots of TDR kits out there already? If I enter 'usb tdr' into Google I get lots of companies selling USB TDR kits.

Software is an important selling point these days.

I wasn't thinking about building a GHz scope, that would be way too ambitious. What I'd like would be a small part of what such a machine can do, for a very small fraction of the price. I'd just like to send a pulse into stuff and look at what comes out, at hobby prices (ie, couple hundred bucks not thousands). That would be an extremely useful tool. With separate output and input, I could also send the stimulus to logic circuits or mosfet drivers, and see what comes out, probe the supplies with a wide bandwidth, etc.

BTW it's not a commercial project, I live in a mostly communist country anyway, so profits ... lol.

On a sunny day (Fri, 14 Dec 2012 20:47:46 +0100) it happened PFC wrote in :

NNTP-Posting-Host: 88.168.96.119 ip_to_country -i 88.168.96.119 ip=88.168.96.119 (1487429751) "FR" "FRANCE" ??????????????

FR is more communist than CN if you look at the percentage of GDP that is government controlled.

It would make sense to make a GHz scope. There are so many TDR boxes out there already that it would be hard to come up with an ultra low cost solution that adds something to the current landscape.

Sounds like good fun. Do keep us informed!

and

player.

The ones that I've seen are klunky and/or expensive.

A USB differential TDR should be do-able for around $140 in parts. So it could be sold for under $1K.

On a sunny day (Fri, 14 Dec 2012 16:35:58 -0500) it happened Spehro Pefhany wrote in :

Yes, after posting that I realized that had their red revolution too, like Russia. Give it some time and revolution is here again.

That's way faster than I'd need, but if you can pull this off on FR-4 without megabucks, then I'm drooling.

What about using SATA, USB3, or other cheap, mass-produced, highspeed cables for signal ?

I'll try to make a quick mockup.

How many bits are realistic in the ADC ? 8 ? or is it possible to hope for more ?

For a TDR, there is :

- a clock, used to generate a pulse

- and a controlled delay, used to trigger the sampler

The delay could be a fast ramp + comparator, the ECL delay chip whose ref I don't remember, or one of the new, interesting fast clock buffer chips with embedded PLL and adjustable delay.

Using 2 oscillators doesn't seem too good, since it relies on long-term jitter being low, a delay relies only on period-to-period jitter, which is easier to get low.

For a scope,

- a trigger (ie, highspeed low jitter comparator)

- and a controlled delay

An equivalent-time scope can only examine perfectly periodic signals, with a very short period, and for triggering, it needs an edge on the signal that is always at the same spot... that's a pretty large restriction... observing more kinds of signals means more trigger complication.

OK, feature creep kills projects, so a possibility of plugging in a trigger/delay daughterboard (replacing the default clock/delay) would be nice.

FR4 is fine at those speeds if you keep the traces short.

Make the USB cable long and get the TDR pod right up against the stuff to test.

uP software just says GO.

Yup, linear ramp and a DAC and a comparator.

Most sampling scopes need an external trigger. TDR doesn't.

I think TDR it could be done with a modest effort and practically no risk, and it could be sold to PCB houses and anybody doing fast stuff. You could sell a lot of them for $1000 with basic software, and charge a bit for software options.

It is hard to make a clean TDR, something with beautiful non-ringy Gaussian response. But you don't have to. All you need to do is make something fast and ugly, and apply a deconvolution algorithm to clean it up.

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

I'm looking at the manual but don't see anything of the sort. Cite?

Tim

Page 372.

That's the full 7th edition, cost $2. There was also a "compact" edition.

That's restrictive. All you really need is jitter statistics such that you get enough samples falling in the view window to produce a trace in the time you're willing to wait.

It's important to realize the difference between a hobby project and a business plan. A typical example is the spectrum analyzer. Yes, there are lots of hobby projects that take a TV tuner and create some blips on the screen. But 30 Hz. resolution bandwidth and -124 dBm noise floor and 21GHz. range is 40 year old technology.

It's easy to think that you can engineer/produce/support a TDR system for less than an existing company with established marketing/service/manufacturing infrastructure.

It's easy to envision simpler ways to do stuff...unitl you try to implement it. The devil is in the details.

And, yes, innovation happens...but you won't likely get the secret sauce in a newsgroup...at least not from anyone who know how the patent system works.

Communism is dead. France is moderately socialist, but elitist and more inclined than most to believe in the advantages of central planning, but they aren't silly enough to let the highly educated bureaucrats in the central planning office execute any of the plans. That's left to regular commercial companies operating in the free market. Quite a few of them have significant state share-holdings, but French productivity is respectable and they have been known to do good stuff, though their "experts" do tend to be a little impractical.

The French bourgeoisie hate income taxes, and the French collect most tax as valued-added tax, which is not progressive and in fact collects a rather lower proportion of income from the rich than from the rest of the population. Their income inequality - Gini index of 32.7% - is marginally worse than Canada's at 32.6%

but it's way lower than China (42.5%), the US (40.8%) or Russia(40.1%), where the ruling oligarchies reserve most the goodies for themselves.

Motorola/ON-Semiconductor MC100EP195

It does 10nsec, so you have to use it with a 100MHz oscillator, counting clock edges for longer delays. Temperature stability isn't great (see figure 4 in the data sheet) and when I was planning on using it's predecessor I figured on automatic recalibration every few minutes - you use it to set up a pulse-width modulated waveform and measure the DC level of the filtered output with a decent A/D converter - 16-bits IIRR.

It might be easier to measure it's temperature with a thermistor and use a resistor to keep it's temperature stable.

DDS chip?

Protecting the fast comparator, and giving it a balanced signal - external trigger versus external ground - can keep busy for a while.

PDF scan posted to a.b.s.e.

Where you'd get the TDs at sensible prices, I don't know.