A Traditional TDR Cable Tester With <2cm Resolution

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.

Tektronix 1502 TDR Cable Tester

Tektronix 1502 TDR Cable Tester

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 \rho = \frac{V_{reflected}}{V_{incident}} = \frac{Z_L-Z_0}{Z_L+Z_0}. 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:

50Ω - 75Ω - open circuit

50Ω – 75Ω – open circuit

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.

Tektronix 1502 tunnel diode and stripline sampler

Tektronix 1502 tunnel diode and stripline sampler

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:

7cm Between Sampling Diodes and BNC

7cm Between Sampling Diodes and BNC

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.

Intermittent Cable Fault

Intermittent Cable Fault

I suspect this indicates a poor connection in either the core or the shield.

Advertisements

3 thoughts on “A Traditional TDR Cable Tester With <2cm Resolution

  1. Pingback: Measuring Input Protection Components’ Effects With a TDR | Entertaining Hacks

    • Good question. Since I have two Tek 1502s, I was able to try two ways.

      The 1502’s charging circuit contains a constant current trickle charger (~140mA), an overvoltage protector which kicks in at about 15V and crowbars the PSU, and an undervoltage protector which protects cells from reverse charging also by crowbarring the PSU. Both protectors are fast acting.

      NiCds can be replaced by NiMH batteries, but care is required because the allowable NiMH trickle charge is significantly lower than NiCd. That constraint can be satisfied using smaller cheaper cells and lowering the current by changing a resistor; I have used 2500mAh AA cells and reduced the current to ~100mA. Alternatively it ought to be possible to use 3000mAh tagged sub-C NiMH cells. With this solution it is again possible to use the 1502 away from a mains power supply. However even when turned off, an unmodified 1502 draws 2mA from the battery; over several weeks the cells will be discharged, and then some will start to be reverse charged. I do not know the effect of that on NiCd nor NiMH cells.

      In the absence of a battery, it is possible to satisfy the steady-state trickle current and voltage requirements by simply replacing the battery with a 12V/0.140A => 900 ohm resistor dissipating 1.7W. However that is insufficient for two reasons: the trickle charging current is a pulse which would trigger the overvoltage protection, and when turning on such a supply the transient inrush current would trigger the undervoltage protection. The solution to that is twofold. In parallel with the resistor, I raided my junkbox to use somewhat dubious capacitors (nominally 8800uF) that were conveniently cell-sized. That was almost sufficient, but sometimes the undervoltage protector still tripped. To prevent that, I slugged the undervoltage protector by adding a capacitor across the voltage sensing resistor, so that the undervoltage would need to last ~1s before the PSU is crowbarred. If you have a 1502, I can dig out the details of the resistors and capacitor.

      I’m undecided whether it would be better to sell a 1502 with the “emulator” or to fit “proper” NiMH cells. I may compromise by selling the emulator and giving advice on replacement cells.

      Like

      Reply

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s