Hidden treasures revealed

When testing flat-panel displays, a large defense contractor needed to measure the panel's turn-on current. The test rack contained a single-channel DMM used to measure the power supply's voltages, but the system lacked shunts on the power-supply output, a digitizer card, and a second DMM input. Rather than add these components, engineers at system integrator VI Technology (Austin, TX; www.vi-tech.com ) found that the power supply from American Reliance (Arcadia, CA; www.amrel.com ) could measure its own output current. Through IEEE 488 programming, the engineers set the power supply to measure the display's startup current without adding equipment to the system.

Sometimes the extra input signal, output signal, or measurement function you need sits unused in the test rack you just built. Rarely do data sheets point out these hidden treasures (although manuals usually cover them), so take a close look at your DMM, oscilloscope, spectrum analyzer, power supply, logic analyzer, or other instrument. It probably contains inputs, outputs, or functions that can provide a needed signal or switch function, so you just may avoid buying another instrument (Ref. 1).

The list of hidden features in test equipment goes on forever. I'll describe a few of these features, including some buried treasure: embedded games. (See "Fun and games," p.44.)

When you need a low-power voltage source to drive an indicator or a relay, you can often find it on an instrument or in your PC. Oscilloscopes that use active probes must supply power to the probes. You can take advantage of an unused scope channel to produce a DC voltage source, usually ±12 V, and use it to power a CMOS device, relay, or indicator.

Figure 1. A connector on the rear panel of a Keithley 2000 Series DMM provides a signal (pin 1) that indicates a measurement is complete. Pin 2 provides an input for an external trigger. Courtesy of Keithley Instruments.

If you use an Agilent 34401A DMM, you can get 5 VDC from its RS-232 connector. Pins 1 and 9, not used for communications, provide access to pulses that indicate a pass or fail condition. Set either to a level that your measurements won't reach and you'll create a low-current 5-VDC voltage source.

A computer's RS-232 port can provide a DC voltage, too. Use the transmit-data pin (pin 3) and signal ground (pin 5) of an unused DB-9 connector. When not in use, the pin remains in the high-voltage state, usually 12 V, and can supply up to 50 mA, which can drive a relay, an LED, or several low-power CMOS devices (Ref. 3).

Test equipment is full of hidden AC signals that you can use for timing, synchronization, or calibration. You're probably aware of the 1-kHz square wave that oscilloscopes use for probe compensation. But did you know that scopes produce other signals? Mike Lauterbach, director of product management at LeCroy (Chestnut Ridge, NY; www.lecroy.com ) points out that LeCroy scopes can produce a square wave with variable frequency and amplitude. Lauterbach says that you can vary the frequency of the signal from 1 kHz to 2 MHz. You can use the signal as an external sample clock or as a trigger for another instrument.

Many spectrum analyzers and other RF test equipment provide a 10-MHz sine wave that you can use to synchronize test instruments. For example, the SR3452 CDMA channel emulator from Spirent Communications (Eatontown, NJ; www.spirentcom.com) provides a 1.23- MHz clock signal as well as multiples of 2, 4, 8, and 16 times that frequency, up to 19.68 MHz. The emulator also provides a 0.5-Hz square wave, which you can use to simulate an operator pushing a button as part of a functional test.

Rick Theiss, applications engineer at Boonton (Parsippany, NJ; www.boonton.com), points out that most RF power meters have a fixed 0-dBm calibrator output, typically at 30 or 50 MHz. He also notes that on Boonton 4530 Series peak power meters, you can vary the amplitude of the 50-MHz calibration output in 0.1-dB increments from –60 to +20 dBm. Boonton's 4500A or 4400A peak power meters and analyzers provide a 1-GHz variable-level calibrator output that can generate RF pulses with timing similar to signals used in radar applications.

Test equipment often requires sequential events. Trigger inputs and outputs let you start an acquisition based on a measurement result. Trigger outputs, often TTL-level pulses, usually indicate a pass or fail condition. Brad Byrum, product manager at Yokogawa (Newnan, GA; www.us.yokogawa.com), says that Yokogawa scopes can produce a pulse when a user-defined condition occurs; you can define the state of the pulse so that it goes high or low.

System and bench DMMs may also produce trigger signals. As I've mentioned, the Agilent 34401A has two signals on its RS-232 connector that you can use as a trigger or as a voltage source. Pin 1 and pin 9 can operate as pass-fail indicators. Each can produce a low-going, TTL-level pulse of 2-ms duration when the DMM measures a signal that passes or fails a test. You can use the pin 1 and pin 9 signals to alert a test operator of a failed measurement. If you just need a rising or falling edge to initiate an event, you can activate either or both of these outputs through a programming sequence.

A Keithley (Cleveland, OH; www.keithley.com) 2000-Series DMM can generate a pulse, too. The meter's rear panel contains an 8-pin connector (Figure 1), from which you can get a TTL-level pulse at pin 1. The low-going TTL-level pulse indicates that the meter has completed a measurement and remains low for 10 ms.

Like scopes, meters, and logic analyzers, power supplies also have trigger inputs. The Fluke (Everett, WA; www.fluke.com) PM 2800 has trigger lines that reset the supply to its previous voltage so you don't need to reissue a command to toggle the supply between two voltages.

Agilent system power supplies offer a fault output signal and an inhibit input. The fault indicator output signal is an open-collector signal you can use to indicate when the power supply detects a user-defined fault such as excessive current. An inhibit input signal lets you safely shut down the power supply's DC power output in response to an event from an operator button or from another instrument. For example, you can use the "measurement complete" signal from a Keithley 2000 DMM to remove power from the Agilent supply following a measurement.

Sometimes, you need a signal to initiate an event, emulate a pushbutton, or place a DUT into a known state before starting a test. Digital I/O ports address those needs. Several instruments supply digital I/O ports. For example, Agilent's system power products give you a three-channel digital I/O port that contains three signal pins and a digital ground. You can configure the port as a three-channel digital output or as a single-channel digital input.

When using a logic analyzer, you often need to set the DUT into a known state to run a test. With an Agilent 16700 Series logic analyzer, you can use the instrument target port to set, clear, toggle, or pulse any of the port's eight lines. If you don't have an instrument with a digital I/O port and need to control a circuit, look at the back of your PC. The parallel port has eight I/O lines that can control equipment or read data. To use the parallel port, you need pin diagrams and drivers, which you can find on the Internet. Start your search at "Parallel Port Central" from Lakeview Research (www.lvr.com/parport.htm ).

Did you ever build a DMM into your test rack, intending to measure just one signal, only to find that you needed to route another signal to the meter? If you use an Agilent 34401A or Keithley 2000 DMM, you won't need to add a multiplexer to route a second input to the meter. Instead, you can use the meter's front and rear input terminals to get two signals into your meter.

Both meters operate under manual control from the front panel. The Keithley meter also lets you select the front or rear terminals under program control. The Agilent meter won't let you make the selection automatically, but it will let you poll the meter to see which terminals are active.

What if you need to digitize a signal but your test system lacks a digitizer card, oscilloscope, or DMM? You're not out of luck, for your PC's sound card can serve as the needed digitizer (Ref. 4).

Sometimes, you find a digitizer where you don't expect it—in a signal generator. "If you use an Agilent 8664A, 8665A, or 8665B synthesized signal generator, you have a DC voltmeter, AC voltmeter, and AC power meter available," notes Chris Grachanen, manager of the metrology lab at Hewlett-Packard (Houston, TX).

Tektronix TDS5000/6000/7000 oscilloscopes hide several undocumented math functions in the math editor. You can enter the functions through a keyboard.

Min()

Max()

INSIDE()

OUTSIDE()

Filter()

These functions compare normal waveforms against an envelope waveform. For example, to compare a measured waveform to a limit, you can load an envelope waveform (alternating min-max pairs) into a Ref function and then enter the equation Math1=(CHx OUTSIDE REFy). Another function can count the number of violations, for example, "SUM(CH1 OUTSIDE REF1). Entering the Filter() function results in a 350 MHz bandwidth limit filter.

The list of hidden treasure in test equipment far exceeds the ones mentioned here. Send us the gems you've uncovered, and we'll add them to the online version of this article. E-mail: m.rowe@tmworld.com.