TDR Impedance Test
More and more PCBs require impedance test to ensure proper signal function. Impedance issues can cause performance and reliability problems that are easily detected through TDR test. CTS has the expertise to help with your impedance test needs.
Controlled Impedance Explained
Signal integrity in PCBs is becoming more important as clock speeds and data rates move faster and faster. At high speeds, PCB traces act as transmission lines and the electrical energy can reflect back and forth. Controlled impedance traces are designed to minimize electrical reflections and ensure an error free signal transmission throughout the PCB.
PCB designers typically attempt to match the characteristic impedance of the signal traces to that of the actual load the signal trace will receive when the PCB functions in a working system. This will maximize the signal transfer.
Uncontrolled Impedance
When there is a mismatch between the characteristic or inherent impedance and the working impedance, there is signal loss and reflection. This will cause suboptimal or poor signal quality (e.g. low gain, noise and random errors).
Here is simple example: When a television antenna and coax cable have impedance mismatch, the television picture quality is poor and may have multiple images due to reflection.
Time Domain Reflectometry (TDR) Test
The best way to ensure correct PCB impedance is to test using TDR methods.
TDR test applies a very fast electrical step signal to the PCB or coupon through a controlled impedance cable and probe. Whenever there is a change in impedance value, part of the signal power is reflected back to the TDR tester.
The data is recorded and saved by the TDR tester as well as graphed. The graph depicts the impedance values over the distance of the PCB or coupon under test. Data such as average, standard deviation, minimum and maximum values are also displayed.
Capabilities
- 0-150 Ohms
- Single and Differential readings
- Coupons and PCBs
- 1% tolerance at 50 ohms
- Bandwidth up to 7 gigahertz
- Comprehensive graphs and reports
- Risetime 70 picoseconds