How to reduce switching noise in test systems

　　Designing switching systems for testing electronic and electromechanical devices presents as many challenges as designing the products themselves. With the advent of high-speed logic in devices and interfacing with more sensitive analog circuits, reducing noise in test switching systems is more important than ever.

　　The noise reduction technical criteria described in this article are for signal frequencies below 300MHz, voltages below 250V, currents below 5A, and voltage-by-Hertz products below 107.

　　Any modern test system uses many signal and power lines to simulate and measure the DUT (device under test), and has various switches to make automatic connections. A general test system architecture is shown in Figure 1. The control bus is shown on the left. The analog, digital, and power buses are shown as vertical pairs behind the different subsystems.

　　The switch is the center of the entire system, interconnecting many test points to measurement instruments and routing signals and power to the DUT. Almost all analog and digital signals and power pass through the switching system.

　　If not carefully designed, the switching system can be a source of measurement errors. Sometimes the errors are inexplicable. The reason is simple, many interconnects are usually close to each other, which provides ample opportunities for noise coupling. Every noise problem has a noise source that couples to the receiver in some form, which in turn is sensitive to noise.

　　There are 3 steps to solve the noise problem

　　The source of the noise must be identified;

　　The receiving point must be identified;

　　The coupling method must be determined.

　　Internal noise sources in a switching system can be caused by the following reasons: the circuit driving the switch, thermal instability on the switch, coupled noise from other conductors in the system, and noise generated outside the system.

　　Noise from adjacent channels coupled into the measurement channel is a threat to signal integrity. The most important causes of coupled noise are conductivity coupling, common impedance coupling, and electric and magnetic fields.

　　In addition, some systems are sensitive to noise from electrical action, thermal coupling noise, electrolytic action, thermoelectric effects, and conductor movement. Switching system circuits are also sensitive to electromagnetic radiation from radio, television, and other wireless broadcasts.

　　Thermal instability can be minimized by mechanical design, ensuring that all contact points in the relay receive the same temperature gradient in the lead wire, or by using latching relays to minimize thermal instability. Whenever possible, latching relays are used. The latching relay winding is energized only for a moment, usually 15 to 20 ms, to allow the relay contacts to transfer and latch. This reduces the source of heat generation, which is the main source of heat generation for non-latching armature relays.

　　Proper shielding and grounding techniques can effectively solve many noise coupling problems in hardwired systems. However, the problem becomes more serious when the signal must be switched to an oscilloscope, counter, or other measurement instrument.

　　In many cases, the source of noise is adjacent channel crosstalk in the system. In the simplified equivalent circuit (Figure 2), most of the parasitic capacitance in the switch system is across the open contact and the adjacent conductive path. As with any capacitance, noise coupling is a function of area and distance. Therefore, a simple way to reduce coupling is to separate the switch and the wire from each other.

　　However, there is a desire to increase switch density to provide more capabilities in a smaller package. Today’s systems under test tend to be more complex and have more points and lines. Therefore, test engineers face the difficulty of increasing component density while increasing the distance between channels at the same time.

　　In some cases, a solution to one noise problem may be less effective than a solution to a different noise problem. The noise sources, coupling methods, and noise receivers must be well understood in order to make appropriate tradeoffs between these factors.

　　As a rule of thumb, a physical separation of 40 times the conductor diameter will attenuate the noise by about 8dB. Greater separation between conductors has little effect.

　　The tree switch separates the switch columns from each other, which is quite effective in reducing the stray off switch capacitance in a large system, which is caused by the unused parallel relays in the connection system. As shown in Figure 2, the tree switch is placed between the H, L, G lines on the left and the 3 columns of the 16 channels on the left. The introduction of relays in series with the input relays can reduce this stray capacitance.

　　

　　For a 16-channel multiplexer, this series switch configuration effectively reduces the stray capacitance of the measurement circuit. This results in low crosstalk and fast measurement settling time.

　　T-Switch

　　The T-switch isolates all unused channels from the measurement bus with a low capacitance path to ground. This isolation is achieved on a single conductor, inserting two additional contacts in the signal path. The result is good signal isolation between channels at high frequencies.

　　The T-switch principle is illustrated in Figure 3. The upper source VN is shown disconnected from the load resistor because switches A and B are newly open and switch C is closed. Therefore, the T portion of the switch is effectively connected to ground. However, the lower source VS with corresponding contacts in another position is connected to the load R2.

　　

　　Another way to reduce switch capacitance and coupled noise is to make the switch and switch contact gap large or make the contact area very small. For example, the Agilent 876A coaxial SPDT switch utilizes a very long make in its switching action. This maximizes the break contact gap. This switch is packaged in a precision metal case to ensure signal integrity greater than 18GHz.

　　Optimized grounding

　　Equal attention should be paid to the design of the rest of the system. Grounding and shielding can solve most noise problems. Improper grounding can be a major noise source. An effective grounding system must minimize the noise voltage generated by currents flowing through a common ground impedance from two or more circuits and avoid the creation of ground loops that are sensitive to magnetic fields and ground potential differences.

　　Although there are many possible reasons for grounding, the two most common reasons are to provide safety and to provide an equipotential reference for signal voltages. A safety ground is provided so that the impedance between the instrument chassis and the high voltage line of the power supply line is broken down through a low impedance path to ground. Such a ground is always at zero voltage potential. A signal ground may or may not be at zero voltage but can be considered an equipotential circuit reference point for a circuit or system, or a low impedance path for current to return to its source.

　　First, it is determined to be a standard description of an ideal ground plane. Second, it emphasizes the fact that IR drops can occur in the ground plane and couple noise into the signal conductors.

　　A properly designed system will have a signal path and a defined return path, as both are essential to a working system. However, the return path is often overlooked.

　　A poorly designed return path can change dependencies. Changing the return path can cause discontinuity problems and generate unwanted noise.

　　In most systems, separate ground return paths are required for different components of the system. Low-level signal grounds should be separated from hardware grounds and noisy grounds (such as relay and motor grounds). In sensitive systems, separate signal grounds for low-level and digital grounds prevent higher-level, noisier digital signals from coupling to low-level signal lines.

　　If AC power is distributed throughout the system, the power ground should be connected to the chassis or hardware ground. A single ground reference point should be used for low-level operation. In addition, any differences in ground levels will appear as noise in the signal path.

　　As shown in Figure 4, if the low end of the instrument is grounded (Z2=0), the ECM is directly connected across Rb, which is in series with the input signal. However, the floating low end of the instrument (see Figure 4) increases Z2 to a larger value and forms a voltage divider, which can reduce the measurement path noise by approximately Rb/Z2 times.