Contents
How did the 50 ohm impedance come about?
The standardization of fifty ohm impedance goes back to developing coax cables for kilowatt radio transmitters in the 1930s. A good explanation for the choice of fifty ohms is given in Microwave Tubes, by A. S. Gilmour, Jr. The quick answer is that 50 ohms is a great compromise between power handling and low loss, for air-dielectric coax.
Why are 50 ohms used in microwave tubes?
A good explanation for the choice of fifty ohms is given in Microwave Tubes, by A. S. Gilmour, Jr. The quick answer is that 50 ohms is a great compromise between power handling and low loss, for air-dielectric coax. Let’s look at the math that proves this, just for kicks.
What’s the minimum loss of 50 ohms for PTFE?
For the record, for solid PTFE (ER=2.2, yellow line) the minimum loss occurs near 52 ohms. So it’s serendipity that when we use 50 ohm semirigid coax cables with solid PTFE, they give nearly the lowest possible loss for ER=2.2!
How are 50 ohms used in a function generator?
From the function generator side. The function generator contains a 50 source resistor. The EVM contains a 49.9 ohm termination resistor, and therefore the output of the function generator will see (approximately) a 2:1 voltage divider. The function generator anticipates this, and scales its output accordingly.
What is the symbol for impedance matching in RF?
Considering how important impedance matching is in RF design, we shouldn’t be surprised to find that there is a specific parameter used to express the quality of a match. It is called the reflection coefficient; the symbol is Γ (the Greek capital letter gamma).
What is the minimum loss impedance for ER?
The opposing contributions of Er muddy the waters quite a bit. It turns out that the minimum loss impedance for ER=1.43 is around 64 ohms, as shown in the plot below (purple trace). For the record, for solid PTFE (ER=2.2, yellow line) the minimum loss occurs near 52 ohms.
What is the quality of a perfect impedance match?
A perfect match corresponds to Γ = 0, and a complete discontinuity (in which all the energy is reflected) corresponds to Γ = 1. Another way of quantifying the quality of an impedance match is the voltage standing wave ratio (VSWR).