Controlled Impedance

Controlled impedance/Controlled dielectric PCB is a specially accurate PCB Manufacturing techniques used for the Circuit Board Design. Which means by controlling the entire board traces with finite trce dimensions(width,Height,Dielectric Strength,Copper quality(1oz,2 oz) etc).So while during the manufacturing time,Controlled impedance can be achieved by measuring each trace impedance through out the running length by using TDR(Time Domain Reflectometry). Controlled impedance Board keeps all the copper trace routes with in a Accurate tolerance level. If you specify controlled impedance/controlled dielectric, they will test your board to ensure that the traces are at the specified impedance. In your fabrication notes on your PCB printout, specify the nets and their targeted impedance (with tolerance for the USB, e.g., 90 ohms +/-10 ohms or 50 ohms +/-5 ohms).When board traces carry high frequency signals(SDRAM,DDR,DDR2&3,PCI etc), care must be taken to design traces matching the impedance of the driver and receiver devices. The longer the trace or the higher the frequencies, the higher is the need to control the trace impedance.If the High Frequency trace impedance tolerance is higher that might cause serious signal integrity issue.