AN168

AN168 is one kind of TV/Satellite synthesisers-basic design guidelines.There are some noise problems of it:(1)I2C BUS RADIATION PROBLEMS:The main problem when designing PCBs using any I2C device is that data and clock are always being transmitted and fed to the transceivers. This can lead to problems with radiation unless suitable precautions are taken.Coupling from the SCL and SDA lines can often occur where these lines are long tracks leading to the synthesiser.The SCL and SDA lines pose particular problems as clock anddata are always present on the I2C databus, regardless of whether the synthesiser is being addressed or not. These can couple into the synthesisier through any of the pins; it is therefore important to ensure that all pins are decoupled where possible. Unused ports should be taken to ground.Small decoupling capacitors may be placed directly on the pin to cut radiation into ports.

I2C BUS LINE FILTERING:I2C bus specifications permit a maximum of 400pF on the SDA andSCL lines. This figure refers to the maximum total capacitance present on the bus so therefore includes other devices. Most applications use a combination of 100pF decoupling capacitors on each line together with a series resistor of up to 100k , depending on the clock rate.

VARACTOR LINE FILTERING:Special care should be taken with the varactor line. A low pass filter may be placed in the varactor line to prevent ripple being fed along the line and mistuning the oscillator.