February 5, 2025
— updated June 20, 2025
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).
Read more…May 1, 2023
— updated May 2, 2023
We’ve published our first full-scale application note: AN-1: Photoreceiver Testing System
. Based on our LP870 Nanosecond Light Source and LA22 Lab Amplifier, the system gives an easy and economical way to test photoreceivers as fast as 5 ns. In the time domain, it measures rise and fall times with excellent accuracy. In the frequency domain, it measures the noise floor (down to below 1 nV/√Hz), both dark and light, and gives a neat hack for measuring the transimpedance vs. frequency to very high accuracy.
Read more…February 24, 2021
— updated June 20, 2025
Specifications
| |
|---|
| 3dB Bandwidth | From DC–200 kHz to DC–300 MHz |
| Rise Time | 4ns (highest bandwidth configuration) |
| Gain Control Method | Gain settable by serial or analog voltage. Analog voltage control profile mimics behaviour of similar PMT modules. |
| Detector Type | Hammamatsu S13361/S13362 series or On Semi MicroFC series SiPm |
| Coupling | DC |
| Output Impedance | 50 Ω |
| Dynamic Range | All configurations support analog and photon counting |
| Power Requirements | +5V 100mA, -5V 10mA |
| Signal Output | SMA |
| Applications | Flow cytometry, Microplate readers, TOF Lidar |
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.)
Read more…January 25, 2021
— updated August 5, 2022
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.)
Read more…January 24, 2021
— updated January 20, 2022
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.
Read more…