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In a previous article, we described an ultralow cost time-domain reflectometer (TDR) with 150-ps edges, which is used in fuel gauges for heavy equipment, basically a radar dipstick. For that job, moderate performance was OK, and somebody else was doing most of the system. Our bit was all analog and pretty minimalist—a single-diode sampler, silicon-germanium BJT edge generators, two-stage BJT amplifier, and JFET track/hold. Its rock-bottom BOM cost ($1.30 @ 100 pcs) made it possible for the whole gauge to sell for under $40. That performance is far from the limit for low-cost samplers, as we'll see.
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.
Internal Developments
In the last year or two we've been doing a lot of work aimed at replacing photomultiplier tubes (PMTs) in instruments, using avalanche photodiodes (APDs) and silicon photomultipliers (SiPMs). These devices are arrays of single-photon detectors, so they're also known as multi-pixel photon counters (MPPCs). Our main application areas include biomedical instruments such as flow cytometers and microplate readers, which have to measure low light levels very precisely but don't need the ultralow dark current of PMTs. (Follow-on articles will talk about our SiPM work in airborne lidar and SEM cathodoluminescence, as well as on improving the performance of actual PMTs.)
PMTs have been around since the 1930s, and remain the undisputed champs for the very lowest light levels. We love PMTs, but we have to admit that they're delicate and not that easy to use—they tend to be bulky, they need high voltage, and they need regular replacement. Most of all, PMTs are very expensive.
In Part 1, we discussed ways to get better measurements by improving the signal to noise ratio (SNR), and saw that although it was often a win to measure more slowly and use lowpass filters, going too far actually makes things worse, because of the way noise concentrates at low frequency. Here we introduce a more sophisticated approach that generally works better: the lock-in amplifier.