This paper was presented as part of IEEE Autotestcon 2002.
View the PowerPoint Presentation used at Autotestcon
EXTENDING YOUR TEST WITHOUT EXTENDING YOUR TEST SYSTEM
(c)2002 IEEE
Jay Nemeth-Johannes, Complete Test, (970) 290-9797, jay.johannes@completetest.com
Often a test system developer discovers that there is a need for just a bit more capability than the original specifications called for. This paper looks at ways of pulling out that little bit extra, using the spare, but less obvious capabilities that are built into all systems.
Keywords: Test Systems, extensibility, sources, switching, measurement, cost saving
Imagine this all too common scenario. The test system design has been submitted, reviewed and accepted. Implementation is well under way when it becomes painfully obvious that something isn’t being addressed. A necessary signal is missing, or an additional switch is required. This is often a cause for panic. The entire test plan needs to be sent back to design. The customer needs to approve changes to the equipment list. The schedule slips. Potentially, the entire project fails over a missing piece of functionality in the test system.
Another scenario involves writing the specification for a cost sensitive client. It is a competitive bid situation, and saving $1500 by not specifying a function generator will make the difference between winning and losing the contract.
Making use of the spare functionality which is built into every test system, but which is generally overlooked by the system designer can usually save the situation. This paper will help find those spare bits of capability that are just hanging around.
A needed signal can often be found in auxiliary outputs or can be derived from signals that are already present. Why spend money for a 1 VPP sine wave when one may be hanging around, doing absolutely nothing?
Just because it says Cal, doesn’t mean it isn’t a perfectly good source. Many instruments provide calibration signals, which have the advantage of being very high quality, although generally not adjustable in shape, frequency or amplitude. Look for periodic type calibration signals on most oscilloscopes, power meters, and counters. RF and microwave instruments generally have a cal output appropriate for the frequency range of the instrument. Some scopes and specialized voltmeters will provide stabilized DC voltage at the calibration outputs.
Don’t forget the trigger outputs on signal generators. The trigger output will generally provide a pulse output with fixed amplitude at the same frequency as the main signal.
RF and Microwave instruments contain a wealth of auxiliary outputs. Spectrum and network analyzers usually have a fixed frequency mixer output. There often is a tunable Local Oscillator output. Other common outputs include fixed frequency reference oscillators and a ramp that is locked to the sweep. These are often present in RF sources as well. Swept signal sources often have internal modulation sources with an auxiliary output. These can provide low frequency AM or FM signals. Most swept sources also contain marker outputs that will output short pulses coupled to a particular point in the sweep. This is nice for synching a measurement or for injecting a signal at a particular time stamp.
Many test engineers don’t think of the instrument controller as a test instrument. This can allow the developer to overlook a wealth of signals and measurement tools available for free.
All PCs currently ship with some sort of audio capability. Although it is easy to think of this as simply a way to make the computer alert the operator, it is in fact a reasonable arbitrary waveform generator. Many even have stereo outputs. The simplest way to drive these is to programmatically call the media player with a wav file and specify continuous repeat play. Similarly, the line input is a 1-channel audio frequency DAQ.
COM outputs can be programmed to provide flexible pattern generator capabilities. Many new PCs contain NTSC output on the video, which can provide a video synch signal. Finally, the I/O backplane is a source of 5Vand +/- 12 V power. It’s there for the taking as long as you are careful about power supply ratings.
Look at Probe power on instruments for DC power. This power is often switchable under program control. Many instruments have auxiliary power available to power accessories. The test system could certainly be considered an accessory to the instrument. Spare channels on digital output devices can be used to provide power as well.
Sometimes signals can be sourced from the most unlikely place. For example, the lowly multimeter would never be thought of as a source, but it does have a perfectly good current source in the ohmmeter section. This source has been successfully used to drive low power CMOS devices. Also, you might consider using a signal on the device under test as a signal source for a different section being tested. Naturally, you would want to order your test sequence to test this signal before using it. A digital output channel can be used as a square wave function generator by loading test vectors and initiating a sequence.
Most of the sources mentioned in this section have fairly modest power output. The test engineer needs to carefully consider the loads placed on the instrument by the test system, and remain within specs.
Measurement capabilities are generally the heart of a test system. As such designers tend to over specify capabilities and duplicate functionality.
Sometimes, especially in more expensive instruments, there is additional functionality hidden inside. For example, some microwave sources contain a fully functional voltmeter. The instrument is accessed from the diagnostic menu, but is fully programmable. Sometimes the terminals are external, and sometimes the cable must be routed into the case. Look carefully to see if there is such hidden functionality.
Similarly, instruments often contain functions that are documented, but are not promoted on the spec sheet. Voltmeters often contain a counter function. Counters sometimes contain a voltmeter function. Examine your test system carefully to determine its full capability before purchasing additional capability.
If you are looking at power requirements, make use of the measurement capabilities built into better power supplies. You can monitor current while switching portions of the device under test on and off and be able to derive information about output circuits without needing to measure them directly.
Sometimes it is possible to derive an accurate measurement from other measurements that may be easier to make. For example, most voltmeters cannot measure power directly, but they can measure voltage and current and thus easily derive power. A counter can be used as a voltmeter by adjusting trigger level, hysteresis and duration, and using successive approximation to determine the input threshold. Consider using an external timebase, say from a source to trigger a voltmeter and thus improve its timebase stability. A voltmeter can measure dutycycle if the waveform shape and peak voltage is known.
Is there a scope in your test system? A scope is a versatile piece of test equipment and can be used to make a wide variety of measurements in the time domain. Perhaps the scope can make measurements well enough eliminate the need for an extra piece of test equipment. The basic function of the scope is to capture Waveform data and display it in a voltage vs. time format.
Scopes are versatile in providing a variety of measurements including ACV, DCV, frequency/period, and all of the common pulse measurements. Built in math functions can provide even more specialized measurements. For example, you can measure instantaneous power by measuring voltage and current and using waveform multiply. Although scopes tend to be expensive, the total cost may be lower if you can replace several instruments.
Do you really need to make a parametric measurement? Perhaps you just need to make a comparison to a standard waveform or an industry standard template. With the scope, you can define an upper and lower boundary waveform and then test to see if the device under test is within the boundaries.
You can’t measure a signal if you can’t find it. If you are thinking in terms of DAQ, this can lead to multi-channel acquisition, a lot of data being collected and then analyzed in the computer. This is both slow and expensive in terms of channel count. If you have an instrument in the system that can handle complex triggering, you should consider using this hardware trigger capability. These instruments invariably have a simple trigger output circuit. You can use the complex triggering to qualify the event and then output a simple trigger to the instrument needed to make the measurement. Examples of instruments that provide this capability include digitally triggered oscilloscopes, logic analyzers, time interval analyzers and spectrum analyzers
Switching is one of those areas which some have compared to being pecked to death by a flock of ducks. The first few aren’t bad, but sooner or later it gets to you. Although relays are generally priced out at $10 to $50 per relay, the actual cost is based on the assumption that you are using a card, which contains a number of relays. If you exceed that number by a single relay, the actual cost of that last relay is the cost of the entire card, plus the fractional cost of the mainframe slot. This can mean a relay which costs more like $1000-$3000!
Minimizing your switching topology is an entire paper in itself, so this paper will stick to ways of finding some switching in unlikely places.
Sometimes the right instrument is in the test system, but in the entirely wrong place. For example, you need to take a voltage measurement in the audio section, but the voltmeter is wired into the switch matrix to measure power supply voltage. The first impulse is to add switches to route the signal to the voltmeter. However, switches can be costly. Is there a measurement instrument already in place that can do the job? For example, we have already shown how the oscilloscope that is being used to measure jitter, or the counter that is measuring frequency output can be pressed into service. The point of the operation is to get a voltage measurement of acceptable accuracy. If the local instrument is “good enough”, routing the better instrument gives no better answer and simply adds cost to the test system.
Many instruments have switching built in that can be utilized as part of the switch topology. A good example is a voltmeter that has front and rear terminals. This is effectively a 2 position MUX. Better still, the MUX will most likely switch high, low and guard, thus providing full isolation.
Sometimes you can avoid the use of a switch entirely. For example, if a voltmeter and a counter both have high impedance inputs, you may be able to wire both up to the same common MUX and avoid switching between them. Similarly, a continuity/shorts test can be accomplished with half as many switches if you use a common mode point and diode isolation.
Data Acquisition Systems generally consist of a voltmeter and a collection of switches. Rather than specify a voltmeter and a switchbox, consider the integrated solution. Often the overall cost is lower and performance is improved as well.
You might want to consider using the instrument as the primary operator interface. Although the obvious choice is to use the computer as the operator interface, there are several advantages to using the instrument instead. First, the measurement itself is available from the instrument. This is especially important on waveform instruments. Using the instrument for the interface allows the operator to concentrate on a single point of contact. Second, it is easier to restrict operator interaction through the instrument. If the computer is used, it is difficult to secure the system against inadvertent or malicious input. Third, in some union shops, allowing the operator to interact with the computer may require the worker to be reclassified from semi-skilled to computer operator.
Many instruments will allow message display and key press capture from the remote interface. Usually this level is about right for the limited interaction you want to provide to the operator.
Some instruments are now web enabled and even act as web servers. You can use these features to provide web access to the test system, relieving the need to bring up a web server on the test machine and also providing firewall security for the test computer.
Instruments often provide mass storage capabilities. This is generally used for waveform capture, but it generally can be used for storage of any test data. If the instrument being used as the operator interface also has mass storage, it can be used for archival result storage as well.
With all the options and flexibility available, test systems can be made more cost effective. Further, it is almost always possible to squeeze that little bit extra out of the system, thus saving time, budget and possibly the test engineer’s job.