PRL 50 Ω Coaxial Interconnect Modules
50 Ω Coax Interconnections for Gigadapter Motherboards
For GHz circuits, proper interconnection among circuit components is of the utmost importance. A practical guide line in the time domain is that when an interconnect distance, measured in transit time, is longer than tr/5, it must be treated as a transmission line, where tr is the 10%-90% rise time of the signal. In the frequency domain, the equivalent parameter is λ/20, where λ is the wavelength of the signal.
Using the MC10EL16, for example, the 10%-90% rise time is about 250 ps. Therefore, the critical distance, measured in transit time, is 50 ps. Assuming FR-4 is the PC board material, the propagation delay, a function of the dielectric material only, is approximately 150 ps per inch. This translates to a maximum physical distance of 0.3-in.
In the frequency domain, the wavelength of a 1 GHz signal is 30 cm, or 11.8-in. For the same FR-4 material, the maximum distance is approximately 0.59-in.
Using this guide line, one has several options when interconnecting Gigadapters on the motherboard. When the critical distance involved is < 0.3-in., a wire jumper, a zero Ω chip jumper or a 1.2 Ω resistor will do. In fact, the 1.2 Ω, 1/3 W space miser resistor, which fits very nicely into a 0.25-in. or 0.3-in. spacing, is a better choice than a wire jumper, because it provides some damping effect against ringing and oscillations.
When the interconnect distance exceeds the the limit, one must use a transmission line connection. Teflon insulated miniature coax such as the RG178 or the semi-rigid, 0.8mm coax CA50034 from Precision Tube provides adequate performance up to about 3.5 GHz.
Plug-in 50 Ω coaxial interconnect modules shown above are intended for use with 0.35- in. long 0.025-in. sq. stick pins. They are easy to connect and disconnect and are reusable. Effective bandwidth of better than 2.5 GHz is achievable with 4-in. to 8-in. segments.