The CircuitMaster is a completely new type of circuit test instrument, designed with two aims in mind: -

• To simplify the safe probing of fine pitch PCBs and PCA's

• To enhance the available measurement capability

On fine pitch ICs and tiny components, probing IC pins and component packages is becoming more and more difficult, and there is an ever-present risk of damage by shorting pins together. Furthermore, with traditional instruments you have to probe “blind” because you normally have to adjust instrument controls and ranges while probing. The CircuitMaster 4000M is equipped with automatic range switching to ensure that you never need to take your eyes off the pin during probing.

Measurement data can be stored using the WaveStack so you can review readings later after removing the probe.

On unfamiliar boards, you often have to use different instruments on the same pin, compounding the probing problem. The CircuitMaster includes DC and AC voltage analysis, and also a stimulus function allowing it to not just measure but also actively inject signals onto the board under test.

Although it resembles a traditional oscilloscope, and can be used as such if needed, it is capable of much more. The ability to inject signals onto the board allows new types of test to be carried out, including FirmFlex, SignalTrack and VI tests. In addition to looking at 2 analog or digital signals, you can also look independently at 4 digital signals on the same waveform using the LogicView function.

With traditional test equipment, the choice of operating mode and settings is often left up to the user, who may be relatively inexperienced. Even an experienced user may take time to set up an instrument for particular task. The CircuitMaster 4000 PCB Diagnostics unit contains several preset test strategies to automate the setup for common tasks, simplifying operation. We will described the various strategies briefly here – detailed descriptions are given later

•  Standard test strategy

The simplest test strategy is Standard. In this mode, the instrument is used purely in passive mode, and is then similar to using a traditional digital oscilloscope and multimeter together. A typical application for this would be to look at signals or voltages at points in a working circuit and make measurements or do comparisons accordingly.

•  FirmFlex test

The FirmFlex test uses the built-in stimulus generator in DC mode to analyze  the impedance at the node under test . The waveform and/or voltage at the node is also displayed. This allows the unit to differentiate between a 0V signal caused by an open circuit and a 0V signal caused by a short circuit – two extremely different circuit conditions which do not show up on conventional test equipment.

•  VI test

The VI test uses the stimulus in AC mode and plots a graph of current into a node against the applied voltage. Comparison of the shape of this graph or image with a known good component or PCB leads to rapid identification of faulty components on boards. This test would normally be used on boards without power to find severe faults which prevent the board from being powered, such as short circuits or overloads due to faulty components. This works like the Huntron equipment.

•  SignalTrack test

Based on a traditional circuit diagnosis technique, the SignalTrack test used channel 1 to produce a programmable waveform that can be connected to the input of a circuit under test. Channel 2 can then be used to track the signal through the circuit, the waveform being at all times synchronized to the stimulus signal.

•  LogicView test

The LogicView test adds 4 digital channels to the 2 normal analog channels on the display, making 6 channels in all. Since many signals which you wish to examine will be digital, you can use this strategy when looking at digital circuits. For example, the digital inputs to a DAC can be examined using logic view, and the analog outputs using the normal channels, all on the same time scale so you can get the maximum amount of information.

•  MultiWay test

Most of the available tests can be executed in MultiWay mode using the supplied cable and IC test clip, or you can make your own interface to suit the board under test. This allows waveforms to be captured for each pin of an IC under the same conditions, greatly speeding up circuit analysis and minimizing probing.

The new test functions in the CircuitMaster allow tests and measurements that would be difficult with traditional equipment.

Some simple examples: -

 Example 1

A pin on a board should be connected to 0V through a 10k resistor. On 1 board the resistor is missing, on another a 1k resistor has been fitted by mistake, on another it is shorted to 0V. If we use a multimeter to measure the voltage at the pin with the power on, it reads 0V in all cases including the correct one. An oscilloscope would show also the same reading.

However, the CircuitMaster would use the FirmFlex test to dynamically measure the source impedance at the node, quickly identifying the differences between the fault boards in this example.

 Example 2

A precision ADC voltage measuring circuit is checked with a multimeter and found to have the correct input and reference voltages. However, the measured results are erratic. On investigation with the CircuitMaster, it is found that the reference input to the ADC is oscillating, causing the errors. In a precision circuit, a traditional oscilloscope is not accurate enough, but a multimeter cannot show such oscillations as they are averaged to give a reading which looks about right. The CircuitMaster combines DC accuracy and waveform acquisition into a single measurement to quickly identify the problem.

 Test method

VI testing is a technique which is excellent for fault finding on PCBs, and is ideal when diagrams and documentation are minimal. When using VI testing, no power is applied to the device under test. Therefore this technique is an extremely effective test for so called dead boards which cannot safely be powered up. A current-limited, ac signal (usually a sine waveform) is applied to the device under test and the characteristic impedance is displayed by plotting voltage against current on an X-Y graph (the X axis representing voltage and the Y axis representing current).

The resulting characteristic represents the impedance of the node under test (NUT)For frequency dependent components such as capacitors and inductors, the impedance is frequency related. Therefore a variable frequency stimulus source is required for these types of components. It can also be seen that the current limiting resistor and the NUT form a potential divider. To achieve a reasonable trace the current sense resistor should be the same order of magnitude as the impedance of the NUT at the test frequency. Thus, in order to use this technique on a wide range of NUTs, a wide range of current limiting (or source) resistors are required. On the CircuitMaster the source impedance can be adjusted from 100R to 1M in decade steps.

It is not necessary to understand the technique to be able to use VI testing for fault diagnosis. Most applications use VI testing in a comparative manner where understanding the displayed characteristic is not important. Indeed, in real board fault diagnosis situations, many components will be connected to a particular node and the resulting analogue VI curve will be a complex composite of the individual components’ VI characteristics making it extremely difficult to completely understand it.

• Understanding The Display

The VI curve display plots the voltage across the device under test on the horizontal axis, and the current through the device under test on the vertical axis. Different devices in different configurations produce different signatures, depending on the current flow through the device as the applied voltage changes. A short circuit, for example, would be displayed as a vertical line, because the current flow for any applied voltage would be theoretically infinite, whereas an open circuit would display a horizontal line because the current is always zero irrespective of the applied voltage. A pure resistor would give a diagonal line whose slope is proportional to the resistance, because the current is proportional to the applied voltage. More complex curves are obtained with frequency dependent components such as capacitors and inductors, and also for non-linear devices such as diode and transistor junctions.

Even though the curves can sometimes be quite complex, it is not necessary to understand them in order to use the VI test. The comparison of the curves for a known good board and a suspect board can often identify faults with a minimum of knowledge. Bear in mind that in a typical circuit the displayed VI curve would normally be for a number of components in parallel. A better understanding of the operation of the VI test can be gained by using the system with known components out of circuit.

• Component VI Tests

The signatures of resistors are straight lines. The value of the resistor under test affects the slope of the line, the higher the value, the closer the line gets to the horizontal (open circuit). The source impedance of the VI test should be selected so the slope of the line, for a good resistor, is as close as possible to 45 degrees. A difference in the slope of the curve when comparing a good and suspect board would indicate a difference in the resistor values on the two boards.

Capacitors with relatively low values have flattened, horizontal, elliptical signatures and capacitors with relatively high values have flattened, vertical, elliptical signatures. The optimal signature is a nearly perfect circle which can be obtained by selecting the appropriate test frequency and source impedance. Typically, the higher the capacitance, the lower the test impedance and frequency. A leaky capacitor would give a sloping curve due to the effective resistance in parallel with the capacitor.

The signature of an inductor is elliptical or circular, sometimes showing hysteresis. Inductors with relatively high values have flattened, horizontal, elliptical signatures similar to those of capacitors. The optimal signal is a perfect circle.

Inductors may have ferrite, iron, brass or air cores, which may or may not be adjustable. Inductors with the same value may have very different signatures if they use different core materials or if the core is positioned differently. Inductors usually require a low source impedance and higher test frequencies to exhibit an elliptical signature. An open circuit inductor (a common fault with small PCB mounting devices) can easily be detected by the sharply contrasting VI curves when comparing two boards. The signature of a silicon diode can be identified easily. The vertical part of the curve shows the forward bias region, and the turn-on voltage and the forward voltage drop can be easily identified. The curved area of the trace shows the changeover from fully off to fully on as the applied voltage increases. The horizontal part of the curve is the reverse voltage region where the diode is non-conducting and is effectively an open-circuit. Faulty diodes can easily be identified by a deviation from this characteristic, for example a diode which exhibits significant reverse leakage would have a diagonal curve in the reverse region, similar to a resistor.

Zener diodes conduct in both directions. The forward current characteristic is similar to that for a diode (see above). The characteristic in the reverse direction is also similar to a diode until the breakdown or Zener voltage is reached, at which point the current increases rapidly and the diode voltage is clamped. The test voltage should be chosen to be higher than the Zener voltage for this curve to be obtained. A suspect Zener diode may not have a well-defined "knee" and the horizontal part of the curve in the reverse region may exhibit leakage effects in a similar way to a normal diode.

NPN and PNP bipolar transistors have a signature similar to that of the diode when tested between the base-collector and base-emitter junctions. If tested between the collector-emitter terminals the signature would appear to be open circuit. The pulse output (available on the MultiWay connector or on the P1/P2 4mm sockets) can be used to apply a bias voltage, via a suitable resistor, to the base of the transistor, so that the switching action can be observed. The pulse outputs can also be used to trigger devices such as triacs and thyristors and MOSFETS, so that again the switching action can be observed. Transistors with open circuit or leaky junctions can easily be identified by the marked differences between curves.