ELEC3106 Electronics. Lab 3: PCB EMI measurements. Objective. Components. Set-up

Transcription

1 ELEC3106 Electronics Lab 3: PCB EMI measurements Objective The objective of this laboratory session is to give the students a good understanding of critical PCB level Electromagnetic Interference phenomena and effects. Components For this laboratory exercise, you will need 1 ELEC3106 EMI test board 7 jumper connectors lab-book Set-up In this lab, you will be using a pre-manufactured printed circuit board. This has been constructed in such a way that many EMI phenomena can be observed. The full schematic is available on the course web site; for the purpose of the laboratory exercise, it is sufficient to use the simplified TL/lab3/March 9, 2016 J1 VCC analogue VCC P5 P3 P10 P1 P2 board loop noisy digital VCC digital VCC P7 logic en jumper legend: three way left connection J2 GND analogue GND P11 P8 P15 digital GND noisy digital GND poor load return path board loop P16 P18 two way connected two way open J3 VEE Figure 1: Simplified schematic for EMI test board (initial set-up jumper connections). ELEC3106/lab3 p. 1/5 School of Electrical Engineering and Telecommunication

2 schematic shown in Figure 1. The board can be configured in a number of different ways by means of jumpers (little pins protruding from the PCB surface that can be shorted by means of little jumper connectors; see Figure 3). The board consists of a high gain amplifier and two logic blocks. Power supplies are (normally) distributed to each of these three blocks from a star-connection where power enter the board. A digital clock is feed into the first digital block and is distributed to the second block. The second digital block does not have good power supply decoupling; further, it can be configured to drive a heavy load (simulated by three 22 nf capacitors) at the clock frequency. Noise from this second digital block have many ways to find its way into the sensitive analogue circuit. Elec3106 EMI Test Board 2008 P12 power supply +5V 0V 5V J1 J2 J3 P6 P5 P2 P1 P4 P3 P7 P13 P17 P18 P21 P14 P16 P8 P11 P9 P10 P15 P19 P20 pulse gnd pulse generator gnd ch2 ch1 gnd CRO Figure 2: Initial set-up of EMI PCB. top view: side view: (a) (b) (c) (d) Figure 3: Jumper legend. Left-connected three-way (a), open circuit three-way jumper (b), closed circuit two-way jumper (c), open circuit two-way jumper (d). The positions of the jumpers, measuring points and supply/generator connections are shown in Figure 2. Three-way jumpers are used for measuring points; for these jumpers, the left-most pin is connected to the reference ground (except for P7) which is indicated by a black pin on those jumpers in the figure. The middle pin on these jumpers are all floating, while the right pin carries the signal to be measured. When measuring a signal, it is best to use a ground connection close to the signal however, you may find that the crocodile leads too easily short if you use ELEC3106/lab3 p. 2/5

3 the ground connection on the same jumper as your signal. Please be careful. The function of each connector/jumper on the board is described in the table below. supply/generator connections symbol function J1 +5V power supply J2 0V power supply (reference ground) J3 5V power supply P6 high-gain amplifier input P15 digital clock input, TTL level configuration jumpers symbol function when connected P1 amplifier power supply (left analogue, right noisy digital) P3 amplifier power supply decoupling capacitor P4 amplifier gain reduction P5 amplifier input board loop connection P8 amplifier ground board loop connection P11 amplifier ground (left analogue, right noisy digital) P14 noisy digital power supply decoupling P16 buffer toggling, heavy load P17 buffer toggling, moderate load P18 ground return for buffer load measurement jumpers symbol measuring point P2 amplifier power supply P7 amplifier output P9 amplifier input P10 amplifier ground P12 noisy digital power supply P13 quieter digital power supply P19 flip-flop A output P20 flip-flop B output P21 flip-flip data input left pin is reference; middle pin is floating; right pin is measuring point Before you start The PCBs are easy to break, both physically and electrically. Please handle them carefully; check the board orientation before you start making connections; check connections, instructions and voltage levels before applying power or signals to the PCB. Remember, other students need to use the boards as well, after your turn! Also, please do not loose the jumper connectors. Measurements Carry out the measurements in this section during your laboratory session in pairs of two students. Make sure that you show your result to the laboratory demonstrator, and that you both record everything neatly in your lab-book. Connect a +5V/0 V/ 5V bench power supply to the board, as shown in Figure 2, and connect your pulse generator to the P15 connector pair, using a ELEC3106/lab3 p. 3/5

4 1 khz square wave taken from the TTL level output. Connect your CRO channel one to the amplifier power supply and channel two to the amplifier output as shown on the figure. AC couple both CRO channels. Power-supply separation First, configure the board using the jumper settings as follows (also indicated in Figure 2): L O O O O L O C O C Using this setup, the EMI generated by the digital buffer couples to the amplifier power supply and output. Sketch the amplifier power supply (P2) and output (P7) traces on the CRO, noting period of signals, peak values and pulse lengths. Comment on your observations. Now add the power supply decoupling capacitor to the op-amp power supply by connecting jumper P3; what happens to the traces? Remove jumper P16; what happens now? Why? You will probably observe some high-frequency ringing (at about 20 MHz); this is caused by inadequate grounding of the measuring equipment. If you move the ground lead to the CRO channel one to the P20 jumper you will get more ringing, as the ground connection is now even further from your signal; if you move the ground lead to P2, the ringing will reduce. Better measurements are possible using ground planes and probes. Now investigate the effect of not separating the analogue and digital power supplies: move jumper P1 to the right and replace jumper P16 such that you jumper configuration looks like this: R C O O O L O C O C Sketch the CRO traces of the op-amp output and power supply as before, and comment on your observations. Why does the low-frequency square-wave power supply noise not couple to the op-amp output? Return current path The big load capacitor driven by the digital buffer is provided with a local return current paths through jumper P18; if this is removed, the current to charge the load has to travel around the board when discharging. To investigate this effect, move jumper P1 back to the left position, and move jumper P11 to the right position (this cause the op-amp ground to be taken at the noisy digital point) such that your jumper configuration looks like this: L C O O O R O C O C Further, move CRO channel one to jumper P10 to measure the op-amp ground on that channel. Sketch both CRO channels as before. Now remove jumper connector P18, and sketch the CRO channels; comment on your observations. ELEC3106/lab3 p. 4/5

5 Reactive noise coupling Last, we want to explore the effects of capacitive and inductive coupling to the amplifier input. Move jumper P11 to the left again, and replace jumper P18, such that your jumper configuration looks like this: L C O O O L O C O C We cannot observe the amplifier input without influencing the measurements, so remove the leads to CRO channel one, and observe only the amplifier output (P7). The amplifier input is pulled to ground with a large resistor and thus is prone to noise pick-up. Observe and sketch the amplifier output on the CRO. Now, connect jumper P5; this connects the amplifier input to a trace that runs close to the digital power and ground lines all around the board, capacitively coupling the amplifier input to the digital supply lines. Sketch the amplifier output trace as above and comment on your observations. Now, connect both jumpers P5 and P8; this completes a loop around the board, connecting this to analogue ground in one end and the amplifier input in the other. Current can now flow in this loop which have a good magnetic coupling to the digital power and ground tracks. Sketch the amplifier output trace as above and comment on your observations. Report A short report (5 pages max) on the laboratory exercise must be prepared and uploaded as a.pdf-file on the course Moodle site by Friday the week after the lab finishes. The report should include answers to all questions asked above, and brief comments on the results. Submit one report per two-student group and make sure that both students names and student ID appear clearly on the report. ELEC3106/lab3 p. 5/5