SED on ElectroOptical Innovations

A 150-Picosecond Time Domain Reflectometer for Under $2

Wed, 05 Feb 2025 12:08:10 +0000

One of the most enjoyable parts of electronics design is getting excellent performance with rock-bottom parts cost. The right circuit can produce exceptionally good speed, noise, and accuracy specs from very low-cost parts. A case in point was a project from December 2016: a time-domain reflectometer (TDR) for a liquid level sensing application in industry.

TDRs work by sending short pulses down a transmission line where they bounce off anything that disturbs their propagation. By measuring the time delay, you can tell how far down the line the disturbance is. It’s a bit like a one-dimensional radar, except that with TDR you can learn a lot more from the reflection than just its location. TDRs can find damaged optical fibres, waterlogged sections of coax cable, and many other things of that sort. This application used an air-dielectric coaxial probe built from two metal tubes sticking downwards into a tank, so that the first part of the probe had air as dielectric and the second part had liquid. The dielectric constant of a liquid is at least 2, whereas air’s is 1.0, so there’s a nice healthy impedance mismatch at the surface to reflect the pulse. This approach is very rugged and resistant to fouling (you can get all sorts of nameless crud in process water and diesel tanks, for instance).

Signal to Noise Ratio and You, Part 1

Sun, 24 Jan 2021 11:16:17 +0000

In building an ultrasensitive instrument, we’re always fighting to improve our signal-to-noise ratio (SNR). The SNR is the ratio of signal power to noise power in the measurement bandwidth, and is limited by noise in the instrument itself and the noise of any background signals, such as the shot noise of the background light or the slight hiss of a microphone.

If the signal is weak, it will have proportionally more noise, so that the apparatus has to be designed to get rid of as much noise as possible. There are a number of ways to do this. The best is to get more signal or reduce the noise, for instance by increasing the laser power and using a laser noise canceller, but eventually we hit a practical limit. At that point, we’re left with several options, all of which boil down to filtering in one form or another.

Mirror of www.analog-innovations.com (Jim Thompson's site)

Tue, 14 Jul 2020 07:56:59 +0000

James Elbert (Jim) Thompson was a well-known chip designer who used to be a regular on sci.electronics.design. He last posted in July 2018. As he was very sick at the time, we presume that he has died, but no obituary has so far turned up. He was born on February 29th, 1940, and used to say that he was looking forward to his 21st birthday in 2024.

Jim had a consulting company, Analog Innovations LLC, and a website, http://www.analog-innovations.com. You can find it on web.archive.org, but those copies are incomplete. A complete archive (minus a bit of javascript) is here at

Low Frequency Noise In InGaAs Heterojunction FETs

Sun, 23 Feb 2020 09:01:14 +0000

InGaAs heterojunction FETs are magic parts—fast, strong, and extremely quiet. They’re also called pseudomorphic high electron-mobility transistors (pHEMTs), because they use a 2D quantum well to to force the conduction electrons to move in a plane without much scattering. My fave Avago ATF38143 pHEMT was discontinued, but luckily Mini-Circuits stepped into the breach with their very nice SAV-551+ and its siblings, which are similar enough that the ATF SPICE model can be hacked up to work with them. (RF companies like Mini-Circuits never seem to supply SPICE models for some reason.) In one post on the ‘purpose of precision’ thread on sci.electronics.design, I noted that the Avago ATF38143 model I had posted awhile back predicted way, way too much low frequency noise. The real pHEMTs tend to have a pretty accurately 1/f PSD with corner frequencies between 10 and 50 MHz and flatband noise of around 0.3 nV/√Hz, about 10 dB quieter than the best JFETs, as well as being 20 times faster.

High-Value Ceramic Capacitors: They Stink, and You Can't Get Them Anyway

Thu, 26 Jul 2018 14:10:44 +0000

There are widespread shortages of electronics parts at the moment, especially passives. Quoted factory lead times are 40 weeks or thereabouts, and since the industry is capacity-limited, it isn’t clear that the situation is going to get better any time soon, so everybody’s starting to panic. Given all this churn I’ve been spending an unconscionable amount of time lately finding suitable replacements for out-of-stock parts.

High value ceramic caps are the worst–their capacitance drops by at least 60% and at worst 95% at rated voltage, so finding an adequate substitute involves a lot more than the package, value and voltage rating. Most of their data sheets are useless, which is frustrating. However, all is not lost: most makers have websites where you can look at the C(V) curves.

Temperature Measurement is Hard

Fri, 09 Mar 2018 09:52:10 +0000

Measuring temperature is surprisingly subtle. There are lots of sensors out there; Digikey sells thermistor sensors interchangeable to +- 0.1 C from several vendors for about $3 in onesies. IC sensors tout good accuracy and linearity, and come in both analogue and digital versions for way under a buck. So what’s the issue?

The issue is: temperature sensors measure the temperature of the sensor, whereas what we want is the temperature of something else: air, fluid, or some solid object we’re trying to control. So the problem is to get the sensor temperature to track the temperature we actually care about. IC sensors are especially bad, because they have stout leads made of copper (400 W/m/K thermal conductivity) and small packages made of plastic (0.1 W/m/K). Thus they basically measure the temperature of their leads, and are horrible at measuring air temperature, for instance.

Decap Photo of Terabeam APD Photoreceiver

Sat, 24 Feb 2018 20:53:26 +0000

Decap picture of a Terabeam CD3109 APD/TIA module, taken with a lens glued to a cell
phone camera

Thermal Runaway Found Useful

Sat, 24 Feb 2018 20:51:39 +0000

This odd circuit is an on-chip temperature balancer that uses thermal runaway to force N transistor arrays to all run at the same temperature. BJT dissipation goes up at low temperature, with very high gain. Here’s its step response.

Sine Wave Generation with TANH Wave Shaper

Sat, 24 Feb 2018 20:46:04 +0000

Sine wave generation is a perennial problem.

Direct-digital synthesis (DDS) uses a bunch of counters, lookup tables, and DACs,
but that’s a relatively heavyweight solution that doesn’t fit all problems.
BJT differential pairs naturally have a hyperbolic tangent (tanh) characteristic, which can be used to round off a triangle wave into a very passable sine. I’m not old enough to have invented this technique, but here are a couple of illustrations of how it works: TANH Sine Wave Shaper (PDF) and TANH Sine Wave Shaper (Mathcad).

Care and Feeding of Tantalum Capacitors

Sat, 24 Feb 2018 20:40:06 +0000

Solid tantalum capacitors have a lot of advantages: very low inductance in surface mount packages, ESR low but not so low that your LDO regulators start oscillating; and good capacitance per unit volume. Unfortunately they’re also prone to burn up when mistreated, which makes many engineers wary of them. This war story, entitled What a Cap-astrophe! talks about how to treat them properly, and how a bit of TLC after soldering can restore their full performance.

High Dynamic Range FET Bridge Mixers

Sat, 24 Feb 2018 20:34:12 +0000

For RF folks, one of the perennial quests is for better frequency mixers: lower distortion,
lower power, better spurious performance. FETs can help. Nowadays CMOS muxes are the devices of choice for HF mixers, but to get the best performance, you still have to know how they work. Ed Oxner was a long-time Siliconix apps guy, and his paper on High dynamic range FET RF mixers is still right up there with the best.
(From a Siliconix databook, 1985.) The FET mux approach is often credited to Dan Tayloe, but since they work just the same, the “Tayloe mixer” should really be called the “Oxner mixer”.

Noise Peaks From Linear Voltage Regulators

Sat, 24 Feb 2018 15:10:29 +0000

Erroll Dietz is a remarkable fellow. He started out at National Semiconductor as Bob Pease’s
technician, and rose to become Chief Technology Officer.

Feedback amplifiers generally have an output impedance that rises linearly with frequency—in other words, it’s
inductive. As Dietz’s short paper from Electronic Design shows, this effective inductance can
resonate with the output bypass capacitor to cause really nasty noise peaks in the 1-100 kHz region. If you have an inexplicable noise peak in that range, a small resistor (a few tenths of an ohm to an ohm or two) in series with the regulator output can be just the ticket. You can also put it in series with the cap, but if you do that, make sure it’s a pulse-withstanding type, or it’s liable to blow up by an output short-circuit, or even the inrush transient, e.g. if somebody runs your gizmo off batteries.

Random resources from my Usenet posts on sci.electronics.design

Sat, 24 Feb 2018 15:03:20 +0000

Sometimes it’s useful to add supporting documents, schematics, scope photos, simulations or other things to Usenet posts. By popular request I’ve added links to some of these, in no particular order.

A trick for turning the usual proportional/integral (PI) loop filter of a phaselocked loop (PLL) into a sweep generator for lock acquisition.

A big slide deck with ideas on Getting Photodetection Right (from a talk given at BBN in Cambridge MA)

How To Read SED: Archive Sites and Newsfeeds

Sat, 24 Feb 2018 14:34:57 +0000

One of the nice things about sci.electronics.design is that it’s widely redistributed by archive sites, some of which you can also use for posting, which is good since Google Groups no longer links to Usenet.

Some examples:
electronics-related

Narkive

Electron Depot

It’s really better to use a proper newsreader such as Thunderbird or Forte Agent. You can get a free Usenet account from Eternal September, and cheap ones from Supernews or astraweb.

SED has been around for a long time. Archives going back to 1993 are available from Archive.org, and have a lot, a lot of good stuff. (Plus the usual allotment of Usenet flame wars and so on.)

Cascode Enhancement pHEMT Photodiode Preamp

Thu, 07 Sep 2017 12:30:22 +0000

This is pretty small, because it has to be–those are microwave transistors, and will oscillate at the slightest provocation. The axial resistors and TO-92 parts are all for biasing—the actual amplifier is the part between the output coupling cap (the small orange thing in the middle) and the photodiode, which is the white square with the black middle at the right.

The 0.1-inch pitch holes round the outside and 25-mil pad pitch set the scale.

Transistor Tester for laser noise canceller

Thu, 07 Sep 2017 12:27:50 +0000

As discussed in medium-gory detail in this paper, laser noise cancellers can let you do shot-noise limited measurements at baseband with lasers that are as much as 70 dB noisier than that.

They’re limited by two main effects: beta nonlinearity (1/hFE-1/hfe) and log nonconformance (d ln(IC)/dVBE - kT/e).

This tester measures both of these quantities directly. It’s a one-off, of course, so it’s done with discrete logic and instrumentation amp parts. It has certain points of interest, for instance the use of a unity gain instrumentation amplifier as a precision +1/+2 gain amplifier. One loose end: U1 is a LT1043 switched-capacitor building block—a glorified MUX that has very low charge injection and very good common-mode rejection.