Compact Time Domain Reflectometer
Introduction
TDRs don’t exclusively belong in $10K benchtop boxes. Ours fits in your hand, handles field abuse, and still gets sub-100 ps resolution. We’ve been developing this one for a few precision agriculture and infrastructure monitoring applications. TDR has a lot going for it as a measurement scheme, particularly in industrial, agriculture, and field applications. It’s insanely robust compared to other common methods and can be embedded directly into the sensing element if you design it right.
For ag or other field deployments, TDR has the enviable characteristic of being able to use 304 stainless spikes as the sensing element. You don’t need to have spend too many days setting up system in Fort Mac or West Texas to know why this is a feature We use an aluminum potting enclosure and encase the entire board in epoxy, leaving just the waveguides and the connector exposed. The potted board becomes a replaceable assembly for servicing, though, in practice the potting keeps service calls to a minimum. While it’s built for dirt and diesel, the same module is surprisingly capable in the lab.
See Phil’s Post on electrooptical.net for some of the gritty analog details.
Characteristics Overview
- Serial interface with adapters to RS-485, SDI-12, and USB-C available
- 1" × 3" × ¼" board size and < 100 grams
- 60 ps 10–90% rise time at the receiver
- 1 ps timing control
- 5 V power @ >50 mA typical, microamps in low power mode.
- Software and hardware triggers
- Serial and analog readout: allows time-stretching to view pulses on a standard scope
- High linearity receiver
- Designed for high vibration environments and extended industrial temperatures (just don’t expect the exact same performance at 125 °C as at 20 °C)
OEM Version
For OEM applications, we connect to the device using 3.3 V UART and +5 V over a 2 mm JST header. SDI-12 or RS-485 Modbus are common interface choices, but it will integrate just as easily in an instrument talking protobuffers over a 2 MBaud UART.
The OEM version is shipped as a PCB with a 5-pin JST connector. All units are factory calibrated.
Lab Version
The lab version adds a status LED, output control button, and uses USB-C for power and control. The interface is based on the legendary SD-24, which we’re great admirers of.
The module speaks SCPI over USB-serial, making it compatible with VISA (NI, python or other), used with LabView, or scripted directly over the serial port. It’s ideal for testing cables, connectors, trace impedances, characterizing PCB pours, and developing TDR-based measurements.
One of our favorite features is using self-triggered mode with analog output. The output is time-stretched 1,000,000:1 by default—so a picosecond becomes a microsecond on the scope.
Applications
TDR is used when you either know the length of a waveguide and want to measure its dielectric (e.g., for water or fuel sensing), or know the dielectric and want to determine physical length (e.g., for cable diagnostics).
Soil Moisture & Salinity
This is a well-established measurement, but the TDR01 brings higher performance than what’s currently available at an incredibly low BOM cost.
Flood & Leak Detection
TDR isn’t just a binary “wet or dry” sensor, it gives spatial information. A single cable or waveguide can detect both presence and position of a leak. Perfect for infrastructure edge sensing.
Level Sensing
Our first TDR was a low power, compact, extremely low cost level sensing device intended for industrial diesel tank monitoring. See our blog post here.
Lab Use
Even though the TDR01 is built for field work, it’s very much at home on our bench. It’s analog output time stretching mode means you can use your scope instead of a laptop.
- Cable and connector testing
- PCB trace impedance profiling
- Evaluating pours and controlled impedance regions (We add U.FLs on sensitive pours for VNA and TDR probing).
- Development of TDR-based measurement systems and proof-of-concepts
TDR can be one of the most insightful and reliable measurements you can make especially when it’s compact, programmable, and rugged. Extra points for when it’s cheap enough to integrate at the edge and replace when there’s trouble.
Want to try one out? Drop us a line or pick one up from Hobbs ElectroOptics.