What can you see, test and measure with a traditional time domain reflectometer (TDR)? The answer is “more than you might expect”:
- measure impedance variations in connectors/filters/antennas/PCBs
- locate short/open circuits and damage in cables
- locate intermittent faults in cables and connectors
- locate connectors in cables
and can resolve discontinuities around 2cm apart. That resolution is at least 10 times better than can be obtained with the typical homebrew logic pulse + oscilloscope combination.
I recently bought a couple of cheap 1970s Tektronix 1502s in the hope that I could make a single working frankenmachine. My initial assessment was depressing: one had a cracked and broken case (so I assumed the CRT was also broken), the other’s electrolytic caps had spewed acid across the PSU and had a faulty 2kV PSU, and both had defective NiCd batteries – and it won’t even start without a working battery. But eventually I managed to get both working: I recapped the PSUs, rewelded the case with methylene chloride, used my “new” 12kV scope probe and 40kV meter to repair the HV PSU, created a “NiCd emulator”, and the CRT wasn’t damaged after all. Later reading of a TekScope magazine indicates it isn’t surprising the CRT survived: it is mechanically completely isolated from the chassis to protect against up to 26 12″ drops.
So I am now the proud possessor of two nice little portable waterproof instruments, literally designed for field use – one of the service manuals indicates they were used with Patriot missile defence systems.
The fundamentals of TDR measurements are well defined elsewhere and will not be repeated here. As usual, the scope trace shows a graph of voltage versus time, but in a TDR:
- the X axis is calibrated in metres, not seconds. The 1502 provides three different scale factors for different dielectric constants.
- the Y axis is calibrated in the reflection coefficient, ρ, not volts.
The relationship between reflection coefficient ρ, incident and reflected voltages and impedances are the well known . Since there is no processor in the TDR, Tektronix provides nomograms and tables for the operator to convert between reflection coefficient and impedance.
The “hello world” of TDR testing is the mismatch between 50Ω and 75Ω cables, with open circuit termination:
Strictly speaking in that graph the “0Ω” isn’t actually 0Ω, it is merely that the step hasn’t reached the sampling diodes, which leads us to consider the TDR’s architecture…
The architecture is that a tunnel diode generates a 200mV step with ~50ps risetime, and launches it into a 50Ω “sampling stripline” and, via a special BNC connector, into a 50Ω cable. The step and any reflections from the unit under test (UUT) are measured with a conventional diode-bridge sampling scope. In the picture below the tunnel diode (TD) is at the right, the stripline extends to the BNC connector on the left, and the sampling diodes (SD) are ~7.5cm away from the BNC connector. The scale can be inferred from the DIL IC with 0.1″ pitch pins.
Any voltage in the UUT could damage the tunnel and sampling diodes. While it is reasonable to define that the operator must not connect the TDR to a “live” UUT, it isn’t reasonable to presume there’s no static electric charge in the UUT out in the field. To discharge any such voltage, Tektronix have a very unusual BNC connector containing a shorting bar. As the cable is being attached the bar shorts the static to the shield, and when the cable is fully inserted the shorting bar is disconnected.
When a cable isn’t connected, the graph shows the voltage step passing the sampling diodes followed by the voltage returning to zero when it meets the short circuit in the BNC connector. It is thus a pulse where the duration corresponds to the distance between the sampling diodes and the BNC connector:
Note the sharp rising edge, which corresponds to about 50ps.
The next example is two 50Ω cables with an SMA connector. The expanded X and Y scales indicate that the connector is “visible” as a 2.5cm ~55Ω discontinuity corresponding to a return loss of ~26dB and a VSWR of 1.1.
The fourth example is two SMA T-connectors in the middle of a 50Ω cable. Each connector is clearly visible as a separate “capacitative dip” in the reflected voltage, in between wiggles due to the connector/cable connection. Discontinuities 3.2cm apart are easily resolved, and it looks like it could resolve discontinuities 2cm apart. (Tektronix claims 0.6″/1.5cm)
The last example is two traces from a subtly broken cable. The damage was very visible when the faulty cable was touched or moved, since the trace moved up and down.
I suspect this indicates a poor connection in either the core or the shield.