Scope day

Give them a choice. Furniture, or paychecks.

If you get a deal on an 11k series, I'd go for it. They haven't been the most reliable box in the world. Spare are cheaper than repairs.

I have an 11402A. Nice. Make that nice and heavy. Yeah, I know, your BW is 50x mine. I got it at the Livermore swap meet at least 5 years ago for $225. Decent scope, but portable it isn't.

Phil, make sure you run the SPC test a number of times (I would recommend at least 10 times) and make sure it passes!

Thats interesting. Maybe someone hacked a scope together from several scopes. I'd give it a very thourough test with different signals at different frequencies before using it for anything serious.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

I believe his "second shift manager" is the BOSS. ;-)

I've been married to her for almost 30 years now, so the finances are a little bit intertwined. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

It shows 125+-2 ps rise times on all four channels, with nice flat pulse tops. I expect it fell off a shelf or something and banged up the front panel and maybe the CRT. The display is beautiful--much much brighter than my TDS 744A. So probably it was cobbled together with the good bits from some less valuable scopes, as you say.

It passed SPC fine, though I only ran it twice. (I'll check another 10 times, as JW suggested, though I'm not likely to do anything about it if it fails marginally.)

Any pointers on how to improve screen brightness on a TDS7xx? Are there intensity adjustments inside, or do I have to jack up the heater voltage? (Yes, I can find the service manual someplace, just wondering if anybody round here has done it.)

When the equipment budget (and the political climate) recover, I have to start buying more lasers instead of scopes--at my PPOE I had about 30 of them, and here I only have half a dozen, all He-Ne, plus a few boxes of unmounted diode lasers.

Next week I'm going to be doing some careful measurements on photodiodes, to see how their speed depends on where the beam hits the sensitive area, and whether jacking up the bias will help. Those SD-24 TDR heads are amazingly useful--it's easy to get sub-200 ps edges out of an ordinary diode laser with an SD-24 driving a 2 GHz Mini-Circuits amp.

My son and summer intern Simon is busy building Arduino-based data acq hardware so we can take C-V measurements on photodiodes using a Boonton

72A capacitance meter. He got his first gizmo working last week, and seems to be having a great time. (Of course his summer job last year was weedwacking and doing dishes at the Girl Scouts headquarters, so it should be an improvement in any event.)

A good summer so far--long may it continue!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

It's still good advice. :)

There are 4 or 5 pots facing the left side behind the floppy drive. The two closest to the front increase the raster and brightness. For the one closest to the front, you'll have to angle your tweaker a bit...

Keep in mind that this could shorten the life of the CRT a bit. The last time I bought some (about 6 months ago), the CRT was still available from Clinton displays for $165 if you feel adventurous. It's not an easy job as you'll have to run the HV assembly on the outside of the scope to tweak the new CRT; this requires making some extender cables. Getting the shutter off the face is probably the toughest part.

Thanks.

Clinton unfortunately closed that division, and they don't even have any glass to make tubes out of. They offered to re-gut my tube for $2500, because that would require setting up production again.

The fall-back position is a little VGA LCD display. Somebody makes a kit for $1700 or something, but I'd just use a small monitor and a cable wrapped around from the back. (I'm not _that_ posh.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

If that ever becomes an issue talk to Justin:

--
Regards, Joerg

http://www.analogconsultants.com/

I do my own. I've posted some more details on Dave's forum:

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

t

the

t e

Curiosity question: Is the sensitivity and/or output impedance of the receiving diode a function of the bias? the load? the received signal?

Now with that question posed, you can drive your intern nuts all summer.

I remember trying to improve the receive characteristics of a microwave diode including trying to speed up the leveling loop for a Swept Microwave Power Source. Everyone said, just bias the diode more, so I did a long hand analysis modeling the microwave diode 12-18GHz, and found that the solution was a modified Bessel Function of the first order. Surprisingly, bias did NOT improve output characteristics [as everyone expected], only lowered output impedance.

I guess what I'm trying to say is be careful the observed data does not get too affected by the way the data is observed. But, you know that, I'm preaching to the choir here.

Can you share results?

A few years ago, at Semco, we canned a dumpster load of those Boontons..

Jamie

The SD24 TDR stepper is a superb pulse generator. We use it as the source for jitter measurements, wideband amp testing, stuff like that.

Here's a TDR/TDT (reflection and thru-transmission) of a 7.5 inch test trace on PC board that goes from SMA to SMA on top and bottom layers, through five segments and four vias. On the top it crosses over a big slit in the layer 2 ground plane, and on layer 6 it crosses over two pour boundaries on layer 5.

Hope this link works...

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

Thanks. The behaviour of photodiodes is complicated, because there are all sorts of tradeoffs, and the headline specs of large area and high responsivity in the red and NIR tend to slow them down.

Schottky PDs biased above reach-through (when the potential well between the two Schottky barriers goes away) are about the fastest, but due to the interdigitated metal electrodes, they're inefficient and spatially nonuniform. Junction PDs are slower, and allow a much longer absorption depth (the full thickness of the wafer, if you bias them enough). The amount of reverse bias required to deplete a diode depends on the doping density, since every atomic layer of additional depletion adds a small sheet charge, which contributes a uniform electric field, so that the depletion depth (and hence the junction capacitance) is a sensitive probe of doping density vs depth.

Thick junctions have slower transit time, so thinner dice are better in general. However, carrier transport in the depleted region is by drift, which is comparatively fast, whereas in the undepleted zone it's by diffusion, which is slow. In fact it's quadratically slow with distance, so that larger PDs slow down quadratically with diameter. With a fixed bias resistance, the RC time constant goes quadratically with diameter too, of course, but diffusion is usually dominant in the better silicon PIN diodes.

Efficiency is a function of bias as well, mostly because when the chip isn't fully depleted, any photocarriers generated below the depletion zone will recombine before they can be collected. Interestingly there's an electrostatic sensitivity in plastic-packaged diodes due to static charge on the outside of the package. It turns out that surface recombination at the Si-SiO2 interface causes a few percent of responsivity loss, so a static charge that produces an E field big enough to deplete the epi a bit at the surface will often give you a few points of sensitivity increase.

Photodetection is fun.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

That was dumb. I love them.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Yeah. Like that.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

We have three, and we love them. We had four, until an intern decided to connect a Spellman HV supply to the bias inputs on the back.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators