Re-Working on critical electronics design
Keep all old designs and take a good snap shot to remember old and changes made thereafter. It make sense to keep proper records with some version identity and Date when changes were made and perhaps a document which states reasons for making changes and something about achievements and recommendations for reproduction or perhaps end of life of the design is a drastic change has been made.
Termination resistances, filter capacitor and some correction or modification jumper wires are often become part of the modified PCB. These all go away in the new design after they are fully incorporated in new circuit schematic. Make sure one does not short the ground shield as it may be as close to track as 3-10 MIL. Thick lines often carry large current and have low impedance for DC power and rarely have breaks. However for high frequency signals needing 50 ohms, 75 ohms or 91 ohms impedance matching, one needs to take care of the suitable line width. Shield if it is very close to the signal then it forms a long capacitor along the signal trace and may cause loss of signal or change of slopes. Keep this in mind if signal frequencies are above 100 MHz. Above design is for 10 MHz clock and with transition edges of rise time and fall time of 3ns.
Packing the instrument in a shielded metal case is essential to keep the RF interference at the lowest level. I use 3mm aluminum instrument cases for housing measurement and control instruments. Keep the power switch on one side and control switches with some separation for easy access without interfering with one another and all signal BNC or other connectors on one side fully marked for identification. Computer interface and power source cord on back panel is ideal Also high voltage should be on back panel as shown below.
Also show signals in oscilloscope for greater understanding. Adding signals with documentation helps one to find problems on later stage when equipment or circuit starts failing. DSO helps in analyzing rise time, fall time, frequency, pulse width, noise and analog signal current and voltage. Use better tools for greater accuracy and DSO may have limited precision and accuracy. I use both kind of tools. Some I make and some I simply buy from market.
Discuss your problems with me and I can help you develop instruments for medical, biological, nuclear, poser, safety, defense, avionics and environmental usages. I am at Gwalior, India for about 4-6 months and then i UK and USA for the rest of the time with University research students. You can fund my address here at www.sensorstechnology.com/ or just email me to [email protected]