# Micro-strip gap capacitance for 0.062 inch thick FR – 4 PCB

The following chart shows the variation of the calculated microstrip gap capacitance for a gap ranging from 0.1mm to 1.0mm on a 0.062 inch ( 1.57 mm) thick FR4 PCB.

# Micro-strip gap capacitance data

It is interesting to note the lack of sufficient measurement data on the capacitance of a micro-strip gap. Based on conventional models, a micro-strip gap yields a series capacitance and two shunt capacitors for each side of the gap, thus forming a pi – network. However, measured data for these values of capacitance is not freely available. A recent study on the micro-strip gap model is constrained by this non-availability of data. IEEE has some data published in 1972 but this is restricted to a fixed dielectric constant and width/height ratio. Anyone with information on publicly available measured data for these capacitors please leave a comment.

# Multilevel FSK

Two level FSK signalling is well understood and used freely in the communications world. In this case one bit is sent per frequency level ( or symbol ). For example two frequencies can be chosen to be transmitted or received where frequency f1 represents a “0” bit and a frequency f2 represents a “1” bit. In this case each symbol ( frequency) contains one bit of information. However there is no reason to use only two level signalling. Four frequencies may be used. In this case each frequency ( symbol) can represent two bits. For example “00” can be represented by a frequency f1, “01” by frequency f2, “10” by frequency f3 and “11” by frequency f4.. Each symbol in this system represents two bits increasing the information sent. In this manner multilevel FSK can be used to advantage.

# Impedance of free space

It is interesting to note that free space has a definite impedance.
It is difficult to calculate exactly but an approximate number is
376.73031 Ohms.  The reason to know this number is that it figures in the calculations for characteristic impedances of microstrip ( and other transmission lines). .In addition for people working in the EM field free space impedance plays a very important part.