Monthly Archives: September 2011

As electronic systems become more complex, troubleshooting manufacturing and field failures becomes more difficult.  Hard failures (i.e. permanent changes in a device that lead to reproducible failures) are straightforward to isolate and determine a root cause. The more problematic area is “no trouble found” or NTF – meaning the original failure is difficult (if not impossible) to reproduce.

There are several sources of NTF:

  • Noise – Signal reflections, cross-talk, ground bounce
  • Timing marginality – Variations in rise times and delay times over temperature and voltage
  • Weak cells (memory) – leakage in DRAM, read-write instability in SRAM
  • Soft errors – upsets in memory and logic due to radiation from alpha particles within the IC package or neutrons generated from cosmic rays

Each of these sources of errors can be difficult to reproduce.  This means that a failure initially observed at the system level will not be reproduced that the IC level.  The key to effective failure analysis is to recognize the possible sources and run the appropriate stress tests to reproduce and isolate the failure:

  • Noise and timing marginality are generally much worse at the system level than within the IC.  However, memory chips can be tested with special patterns that highlight these marginality issues.
  • Weak cells – failures will appear at random address locations from chip to chip, but refresh or voltage margin stress testing can be used to highlight the weak cell
  • Soft errors are difficult to test at the individual chip or system level.  Acceleration techniques are required:  high altitude testing or neutron beams for cosmic ray soft errors and sources containing high concentrations of radioactive materials for alpha particle soft errors.

Knowledge of the characteristics of each of these causes of NTF can then be used to help isolate what the most likely source.  Further testing at the system or component level with the correct source of stresses are necessary to confirm or refute the cause.

Zener diodes are among the most misunderstood and misused parts in electronic systems.  How can this be?  You might ask.  The diode only has two connections, run some current through it, and it simply clamps to a specific voltage.  Couldn’t be easier.  End of story.

Maybe.

This is the tale of a 50 mHz. low noise crystal controlled oscillator intended for a commercial spacecraft application.  The electrical specifications of the oscillator were tight; the size, weight and power budgets were even tighter.

Extreme stability requirements necessitated the use of a heater.  To save on power, weight and size, the manufacturer opted to heat only the crystal.  The main oscillator circuit was contained in a small hybrid package.  The hybrid, the heater controlled crystal and some tuning capacitors were mounted on a small printed wiring board.  The oscillator was powered by a simple shunt regulator using a 9.1 volt Zener diode mounted inside the hybrid, with a dropping resistor mounted on the board, feeding from a 12 volt regulated power supply.

Very simple, run some current through the diode and it clamps at the specified voltage.  Couldn’t be easier.

Another characteristic of the oscillator was that it had to turn on at -20⁰C and reach stability within fifteen minutes.  Reaching stability in the allowed time was no problem as the mass of the crystal and the heater assembly was small, and the operating temperature was reached in less than two minutes.

The operating frequency, on the other hand, was an issue.  In order for the oscillator to operate at 50 mHz., the crystal had to operate in overtone mode.  Overtone crystals are notorious for starting at some frequency other than the expected one.  With careful tuning, the oscillator could be made to start at the correct overtone.  Some crystals have spurious modes very near the expected frequency, and can start at one of those frequencies.  Once started, the crystals do not shift to their expected frequencies unless power is removed and the oscillator restarted.  There is no guarantee that the oscillator will come up at the expected frequency on restart.

As a result of this characteristic, each oscillator had to be tuned to match each crystal.  Since operation with a frequency error of even a few ppm would be catastrophic in the application, the spacecraft manufacturer required that each oscillator be tested for turn on and stabilization at -20⁰C.

The oscillators performed flawlessly on turn on and stabilization time at the manufacturer’s facility, so they were shipped.   The user retested the turn on and stabilization characteristics prior to installation in the next higher assembly.  Again, the oscillators performed flawlessly.

Until.

To ensure that there were no discontinuities in the temperature performance, the user brought the temperature up in 5⁰ increments while monitoring the oscillator output.  At approximately -5⁰C there was a sudden large burst of noise superimposed on the oscillator output.  This noise was evident until the oscillator temperature reached 0⁰C, whereupon the output noise dropped back into specification.  When the temperature was decreased, the noise reappeared.  This was very repeatable.

So, what happened?

The designer, pressed for power, decided to take advantage of the fact that the oscillator was to be powered from a regulated supply.  He knew that the Zener diode test current was specified to be 20 mA.  He also knew that the reason for biasing the diode at the test current was to minimize the Zener impedance, so that current variation would have minimal effect on the diode voltage.  He reasoned that with a regulated power supply, he didn’t have to concern himself with current variation due to changes in the input voltage, and further, the load was constant, so that he didn’t have to worry about current changes from that source.

The oscillator circuit drew 15 mA without the Zener diode.  It seemed a shame to waste more current in the diode than the circuit required for operation, and since the source voltage and load current were both constant, the designer squeezed out a power saving by reducing the entire current to 20 mA., 15 for the oscillator and 5 for the diode.

So far, so good.  Everything worked.  The oscillator turned on correctly at cold temperature.  Ship!!!

What did the designer miss?

He neglected to find out how much current the oscillator required at -20⁰C.  The oscillator current rose as the temperature went down, until the point was reached that all the current was flowing in the oscillator, and none in the diode.

Zener diodes have a peculiar, but not too well known, characteristic.  They are inherently noisy, but in the vicinity of the turn-on knee, they become extremely noisy.  The so called “noise diode” is nothing but a certain type of Zener reference diode that has been tested to have a specified noise spectrum under specified bias conditions.  Some series of Zener diodes include noise information on the datasheets, but only at the test current.  The diode used in this application had no noise specification.  Even if it had one, operation at the turn-on knee would not have been controlled.

The oscillator was quiet between -20⁰C and -5⁰C because the Zener diode was turned completely off due to lack of bias current.  At about -5⁰C the diode was just crossing the threshold of operation, generating significant noise.  By the time that 0⁰C was reached, the diode was on and operating normally.  This was confirmed by operating the oscillator as room temperature and reducing the supply voltage until the diode dropped out.  The same noise characteristic was noted at the threshold point.  Several oscillators were temperature tested, with the same results.  The only variation being the temperature at which the noise burst occurred.  Several oscillators were tested at reduced voltage at room temperature.  They were found to work well, even down to five volts.

Lessons learned.

  • Temperature testing  involves sweeping the entire range, not just testing at the extremes.
  • Understand the temperature performance of each part of your product.
  • The care and feeding of the Zener diode is paramount for a successful application.  A well fed Zener is a happy Zener!