A quick plug for a little gem of a book that all fans of early radio should know about: “Super-Regenerative Receivers” by J. R. Whitehead (Cambridge University Press, 1950). It’s part of the Modern Radio Techniques series, where a bunch of the technical movers and shakers document the advances that were made during the war, e.g. centimeter radar. This one is about the theory and practice of superregenerative radios. I learned a lot from it and had a lot of fun.
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.)
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.
We were considering a typical baseband signal, one that goes from near DC to some much higher frequency. Audio is a typical example, with a bandwidth usually quoted as 20 Hz to 20 kHz. To escape the low frequency noise, we need to move our signal up in frequency, out of baseband. In lock-in detection we make the signal periodic in time at some carrier frequency fc chosen to be several times higher than the required bandwidth. This is generally pretty easy to do, as we’ll see, and doing so ensures that none of the signal we care about remains near DC. Our noise rejection filter now needs to be a narrow bandpass centered at fc, so as to reject both low-and high-frequency noise. We’ll also need some means of measuring the amplitude and phase of the AC signal. That’s more complicated, of course, but with this setup we can narrow the bandwidth as much as we like and still get the full SNR improvement. A lock-in amplifier is a device for making such narrow-band AC measurements conveniently. It’s basically a radio that measures the phase and amplitude of its input, so that we recover a lowpass-filtered version of the baseband modulation signal that we care about, with no 1/f noise pollution to worry about. At this point we need to geek out a little bit and talk about modulation, which is what we mean by moving the signal away from baseband.
A thermoelectric cooler is a solid-state device made from two alumina ceramic plates with an array of metallized pillars in between. The pillars are also ceramic–they’re made of alternating p-type and n-type bismuth telluride (Bi2Te) semiconductors, alloyed with antimony telluride (p-type) or bismuth selenide (n-type), and connected in series electrically. The Peltier effect makes them electric-powered solid state heat pumps. (Thermocouples work the other way round, via the Seebeck effect, but the physics is the same.)
From the cutting room floor at Building Electro-Optical Systems, Third Edition:
Photographic Film Okay, okay. Photographic film isn’t a detector of the sort we’ve been discussing. Film is so out of fashion, so inconvenient. It needs messy chemicals. Getting it to be highly sensitive requires all sorts of 1960s alchemy such as pre-flashing and hypersensitizing in a forming gas or hot hydrogen atmosphere. Why do we care about it at all, in these days of 4k x 4k CMOS imagers?
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.
