A compact time domain reflectometer probe based on high speed logic gates
By using a compact time-domain reflectometer probe based on high-speed logic gates, the problems of insufficiently steep pulse edges and impedance mismatch in existing TDR instruments for measuring small structures are solved, achieving high-resolution, low-cost, and accurate measurements, suitable for the detection of small structures in the laboratory.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CHINA RAILWAY ENGINEERING EQUIPMENT GROUP CO LTD
- Filing Date
- 2026-04-01
- Publication Date
- 2026-07-10
AI Technical Summary
Existing TDR instruments suffer from problems such as insufficiently steep pulse edges, output impedance mismatch, and lack of convenient interfaces in laboratory or amateur electronics production environments, making it difficult to accurately measure small-sized structures.
The compact time-domain reflectometer probe, based on high-speed logic gates, includes a power supply filtering module, an ultrafast edge pulse generation module, an impedance matching drive module, and a signal separation and test port module. It utilizes 74LVC series high-speed CMOS logic gate chips and a carefully designed resistor network to generate extremely fast pulses and achieve 50-ohm impedance matching, all integrated on a small PCB.
It achieves high-resolution measurements, accurately detects impedance discontinuities in small structures, is low-cost and compact, and is plug-and-play via a standard interface, simplifying design and improving measurement accuracy.
Smart Images
Figure CN122362017A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of electronic measurement technology, and specifically relates to a portable time domain reflectometer probe that can be used with a general-purpose oscilloscope to detect transmission line impedance discontinuities and fault locations. Background Technology
[0002] A time domain reflectometer (TDR) is a commonly used instrument for characterizing and locating impedance discontinuities (such as open circuits, short circuits, and connector failures) in transmission lines (such as coaxial cables and PCB traces). Traditional TDRs are usually large, stand-alone benchtop instruments, which are expensive and mainly used in professional applications.
[0003] In laboratory or amateur electronics production environments, there is often a need for rapid testing of short cables, homemade connectors, or transmission line structures on PCBs. While some simple TDRs based on high-speed logic chips exist, they often suffer from problems such as insufficiently steep pulse edges (leading to low resolution), output impedance mismatch (causing signal reflection interference in measurements), or lack of convenient interfaces with general-purpose oscilloscopes, making it difficult to accurately measure small structures. Summary of the Invention
[0004] The technical problem to be solved by the present invention is to provide a time domain reflectometer probe that is compact, low in cost, has extremely fast pulse edges, and has good output impedance matching.
[0005] The present invention adopts the following technical solution:
[0006] An improved compact time-domain reflectometer probe based on high-speed logic gates includes a power supply filtering module, an ultrafast edge pulse generation module, an impedance matching drive module, and a signal separation and test port module. The power supply filtering module supplies power to the ultrafast edge pulse generation module and the impedance matching drive module. The ultrafast edge pulse generation module is a relaxation oscillator that outputs a square wave signal to the impedance matching drive module. The impedance matching drive module performs impedance matching on the square wave signal and then inputs it to the signal separation and test port module. The signal separation and test port module includes a triangular resistor distribution network consisting of three resistors. One endpoint of the triangular resistor distribution network is electrically connected to the output terminal of the impedance matching drive module, one endpoint is a test port, and one endpoint is an oscilloscope signal output port.
[0007] Furthermore, the power filtering module includes an input power interface, a 10uF tantalum capacitor and two 1uF ceramic capacitors, and an output +5V voltage.
[0008] Furthermore, the relaxation oscillator is a Schmitt trigger inverter. The input terminal of the Schmitt trigger inverter is grounded through a capacitor, and the output terminal is grounded through a resistor. The input terminal is also electrically connected to the output terminal through the aforementioned resistor.
[0009] Furthermore, the relaxation oscillator outputs a 1MHz square wave signal.
[0010] Furthermore, the impedance matching drive module includes an inverter whose input is electrically connected to the output of a Schmitt trigger inverter, and whose output is grounded through two series resistors R3 and R4.
[0011] Furthermore, R3=82Ω and R4=130Ω. The parallel equivalent resistance of R3 and R4, combined with the output impedance of the inverter chip, can approximately match a 50-ohm transmission line, limiting the drive current to below 32mA.
[0012] Furthermore, the resistance of each of the three resistors in the triangular resistor distribution network is 49.9Ω.
[0013] Furthermore, one endpoint of the triangular resistor distribution network is electrically connected to the connection point of resistors R3 and R4 via a DC blocking capacitor.
[0014] Furthermore, the test port is an end-emitting SMA connector.
[0015] Furthermore, the oscilloscope signal output port is a PCB-mounted BNC female connector.
[0016] The beneficial effects of this invention are:
[0017] The time-domain reflectometer probe disclosed in this invention has the following significant advantages compared with existing simple TDR solutions or expensive professional equipment:
[0018] (1) High resolution: Due to the use of 74LVC series high-speed CMOS logic gate chips and optimized circuit layout, the pulse rise time is less than 530 picoseconds, which can distinguish impedance discontinuities at closer distances, making it very suitable for measuring small structures on the laboratory scale.
[0019] (2) Excellent impedance matching: By carefully calculating the combination of discrete resistors (R3, R4) and the output impedance of the logic gate chip, a source impedance of close to 50 ohms is achieved, which minimizes the reflection generated from the signal source itself and improves the measurement accuracy.
[0020] (3) Compact structure and low cost: The entire circuit consists of a small number of general-purpose surface mount components and chips, integrated on a small PCB. No complex power supply or control is required, the manufacturing cost is extremely low, and it is easy to use as a spare accessory for oscilloscopes.
[0021] (4) Plug and play, easy to use: Connect to the oscilloscope via the standard BNC interface and the device under test via the SMA interface. The reflected waveform can be observed directly on the oscilloscope without additional configuration, and the distance to the fault point or the transmission line speed factor can be directly calculated via the cursor function.
[0022] (5) Ingenious signal separation design: The triangular resistor distribution network is used as the core coupling and separation component, which realizes the signal separation function required by the single-port TDR, eliminates the expensive directional coupler, and simplifies the design.
[0023] The time domain reflectometer probe disclosed in this invention can be directly connected to a common laboratory oscilloscope to form a simple but high-resolution TDR test system, which makes it convenient for users to quickly detect impedance abnormalities in transmission lines and locate fault points. It is especially suitable for measuring small-sized cables and structures. Attached Figure Description
[0024] Figure 1 This is a circuit diagram of the time-domain reflectometer probe disclosed in this invention;
[0025] Figure 2 This is a schematic diagram of the PCB layout of the time domain reflectometer probe disclosed in this invention;
[0026] Figure 3 It is the fast step pulse displayed on the oscilloscope;
[0027] Figure 4 It is the subsequent reflected waveform displayed on the oscilloscope. Detailed Implementation
[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.
[0029] Example 1 discloses a compact time-domain reflectometer probe based on high-speed logic gates, all integrated on a small PCB. It includes a power supply filtering module, an ultrafast edge pulse generation module based on high-speed CMOS logic gate circuits, a precision 50-ohm impedance matching drive module, and a signal separation and test port module. The power supply filtering module supplies power to the ultrafast edge pulse generation module and the impedance matching drive module. The ultrafast edge pulse generation module is a relaxation oscillator that outputs a square wave signal to the impedance matching drive module. The impedance matching drive module performs impedance matching on the square wave signal and then inputs it to the signal separation and test port module. The signal separation and test port module includes a 50-ohm triangular resistor distribution network consisting of three resistors for separating incident and reflected signals. Of the three endpoints of this triangular resistor distribution network, one endpoint is electrically connected to the output of the impedance matching drive module, one endpoint is the test port, and one endpoint is the oscilloscope signal output port.
[0030] In this embodiment, as Figure 1 As shown, the power filtering module includes an input power interface, a 10uF tantalum capacitor C2 and two 1uF ceramic capacitors C3 and C4, and outputs a +5V voltage to provide a stable, low-noise power supply for subsequent logic chips.
[0031] The ultrafast edge pulse generation module is the core of generating high-speed test signals. It consists of a 74LVC1G14 Schmitt trigger inverter U2 forming a relaxation oscillator. The input terminal of the Schmitt trigger inverter is grounded through a 1nF capacitor C1, and the output terminal is grounded through a 1.2kΩ resistor R6. The input terminal is also electrically connected to the output terminal through a resistor R6, thus forming an oscillation circuit to generate a square wave signal with a frequency of about 1MHz.
[0032] The impedance matching driver module is used to buffer and drive pulse signals and achieve precise 50-ohm source impedance matching. It includes a 74LVC1G04 inverter U3 as the output driver. The input of this inverter is electrically connected to the output of a Schmitt trigger inverter. The output is grounded through two series resistors R3 and R4, where R3 = 82Ω and R4 = 130Ω.
[0033] The resistance values of R3 and R4 are specially designed. Their parallel equivalent resistance, combined with the output impedance of the inverter chip, achieves approximate matching of the 50-ohm transmission line, while limiting the drive current to below the chip's maximum rated current of 32mA.
[0034] The signal separation and test port module is key to achieving simultaneous excitation and sampling at a single port. Its core is a delta resistor distribution network, where three resistors, R1, R2, and R5, are connected in a delta configuration, each with a resistance of 49.9Ω.
[0035] One endpoint of the delta resistor distribution network is electrically connected to the junction of resistors R3 and R4 via a DC blocking capacitor. One endpoint is the test port J3, and the other is the oscilloscope signal output port J1. The test port is an end-emitting SMA connector used to connect the cable or device under test. The oscilloscope signal output port is a PCB-mount BNC female connector used for direct connection to the oscilloscope's input channel.
[0036] like Figure 1 As shown, the circuit connection of the time domain reflectometer probe is as follows:
[0037] The power supply VCC (+5V) is connected through J2 and filtered by C2 (10uF), C3 (1uF), and C4 (1uF) before supplying power to U2 and U3. U2 (74LVC1G14) and its external resistors R6 (1.2kΩ) and capacitor C1 (1nF) form a relaxation oscillator, generating a square wave of approximately 1MHz at the output of U2. This square wave is fed into the input of U3 (74LVC1G04). U3 buffers and drives the signal, outputting it from its output. The output signal passes through an impedance matching network consisting of R3 (82Ω) and R4 (130Ω). The connection point of R3 and R4 is connected to one end of a delta resistor distribution network consisting of R1, R2, and R5 (all 49.9Ω) through a DC blocking capacitor. The other two ends of the delta resistor distribution network are connected to the oscilloscope output port J1 (BNC connector) and the test port J3 (SMA connector), respectively. All resistors and capacitors are surface-mount components, and all ground terminals are connected to the PCB's ground plane. The PCB layout is as follows: Figure 2 As shown.
[0038] In use, connect J1 to a channel of the oscilloscope via a BNC cable, and connect the cable or device under test (DUT) to J3. Connect a 5V power supply to the input power interface; the oscilloscope will then display an indicator such as... Figure 3 The rapid step pulse shown and its subsequent pulses, as shown Figure 4 The reflected waveform is shown. By measuring the time difference Δt between the incident step and the reflected wave, and calculating the distance L = (v*Δt) / 2 of the impedance discontinuity point, the location of the impedance discontinuity point can be accurately calculated. In the above formula, v is the propagation speed of the signal in the transmission line under test (v = speed of light x cable speed factor).
Claims
1. A compact time-domain reflectometer probe based on high-speed logic gates, characterized in that: The system includes a power supply filtering module, an ultrafast edge pulse generation module, an impedance matching drive module, and a signal separation and test port module. The power supply filtering module supplies power to the ultrafast edge pulse generation module and the impedance matching drive module. The ultrafast edge pulse generation module is a relaxation oscillator that outputs a square wave signal to the impedance matching drive module. The impedance matching drive module performs impedance matching on the square wave signal and then inputs it to the signal separation and test port module. The signal separation and test port module includes a triangular resistor distribution network consisting of three resistors. One endpoint of the triangular resistor distribution network is electrically connected to the output terminal of the impedance matching drive module, one endpoint is a test port, and one endpoint is an oscilloscope signal output port.
2. The compact time-domain reflectometer probe based on high-speed logic gates according to claim 1, characterized in that: The power filtering module includes an input power interface, a 10uF tantalum capacitor and two 1uF ceramic capacitors, and outputs a +5V voltage.
3. The compact time-domain reflectometer probe based on high-speed logic gates according to claim 1, characterized in that: The relaxation oscillator is a Schmitt trigger inverter. The input terminal of the Schmitt trigger inverter is grounded through a capacitor, and the output terminal is grounded through a resistor. The input terminal is also electrically connected to the output terminal through the aforementioned resistor.
4. The compact time-domain reflectometer probe based on high-speed logic gates according to claim 1, characterized in that: The relaxation oscillator outputs a 1MHz square wave signal.
5. The compact time-domain reflectometer probe based on high-speed logic gates according to claim 3, characterized in that: The impedance matching drive module includes an inverter whose input is electrically connected to the output of a Schmitt trigger inverter, and whose output is grounded through two series resistors R3 and R4.
6. The compact time-domain reflectometer probe based on high-speed logic gates according to claim 5, characterized in that: R3=82Ω, R4=130Ω. The parallel equivalent resistance of R3 and R4, combined with the output impedance of the inverter chip, can approximately match a 50-ohm transmission line, limiting the drive current to below 32mA.
7. The compact time-domain reflectometer probe based on high-speed logic gates according to claim 1, characterized in that: In the triangular resistor distribution network, the resistance of each of the three resistors is 49.9Ω.
8. The compact time-domain reflectometer probe based on high-speed logic gates according to claim 5, characterized in that: One end of the triangular resistor distribution network is electrically connected to the connection point of resistors R3 and R4 via a DC blocking capacitor.
9. The compact time-domain reflectometer probe based on high-speed logic gates according to claim 1, characterized in that: The test port is an end-emitting SMA connector.
10. The compact time-domain reflectometer probe based on high-speed logic gates according to claim 1, characterized in that: The oscilloscope signal output port is a PCB-mounted BNC female connector.