During PCB layout, designers typically have to indicate the impedance of traces to ensure a proper balance of low-loss characteristics and power-handling capabilities. Before layout, the designer determines the trace width in some field solver software based on the desired impedance and other stackup factors like the surrounding substrates‘ dielectric constant(s) and the distance between the trace and the nearest plane. While these models are extremely accurate, testing must still confirm the impedance of these traces post-manufacturing using time domain reflectometry PCB analysis.
| Analyzing Time Domain Reflectometry PCB Readings | |||
|---|---|---|---|
| Matched | Mismatched (real) | Mismatched (imaginary) | |
| Loadless | N/A |
|
N/A |
| Load |
|
|
|
Time Domain Reflectometry PCB Analysis Simplifies Signal Integrity
Time domain reflectometry is a relatively straightforward process conceptually: signals propagating through a medium encounter various impedance discontinuities. By comparing the reflected signal against those of reflections from standard impedance circuits, the reflectometer can accurately measure the device under test’s (DUT) impedance. Reflections occur at each impedance discontinuity, so the final waveform results from each discontinuity encountered over the transmission path. These impedance discontinuities occur regularly: solder joints, components, vias, connectors, and anywhere else the transmission medium changes can have a portion of the transmitted signal reflect toward the source. To the reflectometer, these discontinuities act as lumped elements that change the signal characteristics:
- Short-circuit termination – Appears as a step function where the pulsed signal jumps to a voltage equivalent to the source impedance times the current (i.e., Ohm’s Law).
- Open-circuit termination – Appears as consecutive step functions where the pulsed signal first reflects at the load, then a second time at the end of the transmission path.
- Matched load termination – There are no reflections (i.e., a 1:1 voltage divider); the pulsed waveform looks identical before and after encountering the load.
- Mismatched load termination – The pulsed signal shows an increase in voltage (when the load impedance is greater than the source impedance) or a decrease (vice versa).
- Capacitive load termination – The pulsed signal sags before increasing according to its time constant (analogous to an open circuit – after the capacitor reaches steady state DC, it acts as an open).
- Inductive load termination – The pulsed signal spikes before decreasing according to its time constant (analogous to a short circuit – after the inductor reaches steady state DC, it acts as a short).
- Shunt capacitance termination – The pulsed signal experiences a small sag before quickly recovering (analogous to a matched load).
- Series inductance termination – The pulsed signal experiences a small spike before quickly recovering (analogous to a matched load).
With this analysis, designers can alter their prototype’s circuitry to account for the parasitic contributions from fabrication and assembly. As a reminder, impedance is a complex-valued measurement that combines real resistance and imaginary reactance (a positive reactance indicates inductance, while a negative reactance is for capacitance). Matching the impedance is as easy as providing the complex conjugate – the resistances are equal (to ensure 1:1 voltage division), and the signed inductive and capacitive reactances zero out. Therefore, engineers can account for the inherent parasitics in the subsequent design revision for improved system stability.
Limitations of Time Domain Reflectometry
Like all tests and measurements, time domain reflectometry has to account for accuracy and precision. Some of these requirements fall on the device (and its regular servicing), while others result from the testing environment or design. Consider all of the following factors when performing measurements:
- Noise – Random signal variations become more disruptive at low-level measurements due to their relatively outsized effect. Modern reflectometers often average multiple pulsed signal reflections to control for noise contributions. Averaging comes at the cost of increased processing time, which can become an issue when testing numerous DUTs.
- Probe reflections – Measurements should always account for the length of the cable, as large inductive reflections can form before settling with time. Whenever possible, probe tips and ground leads should remain as short and direct as possible.
- Cable losses – During transmission, conductor loss dominates over dielectric loss. As frequency increases, the skin effect reduces the area electrons can flow to the conductor’s surface, significantly increasing the impedance. Therefore, impedance will appear much higher during these measurements than the actual value of the DUT. Additionally, the rise time of the signal pulse degrades during the cable transmission and can detract from the accuracy of impedance measurements considerably higher or lower than 50 ohms.
Your Contract Manufacturers’ Guarantee of Signal Integrity
Time-domain reflectometry PCB analysis is an excellent tool for determining fabrication and assembly quality. When used early in a product development lifecycle, it allows designers and engineers to accurately assess signal integrity and incorporate any circuit changes necessary for smooth signal response. Bridging the gap between design and device can be challenging when dealing with the nonidealities of parasitics, but VSE is here to help. Our engineers are committed to building electronics for our customers, including a thorough understanding of signal integrity for optimal reliability. We’ve been realizing life-changing and life-saving devices for over forty years with our valued manufacturing partners.
