21-03-2018, 02:20 PM
The "rule of thumb" when selecting values for either bypass capacitors or isolation inductances is that the device must have a reactance greater than 10 times (or 1/10th for caps) the impedance to which it is connected.
That means that an isolation inductor has an inductive reactance > 10 times Zo, where Zo is 50 ohms when dealing with coax impedances of typically 50 ohms.
Conversely, bypass capacitor values must have a capacitive reactance < 10 times the circuit impedance to be effective. That means that the MAX value of capacitive reactance is 5 ohms at the lowest frequency involved. A better proposition to aim for is 1-2 ohms MAX. A 47nF capacitor has a reactance of 1.9 ohms at 1.8MHz so that would be the smallest value you should consider using so that it will provide effective bypassing at RF.
XC = 1/(2piFC) = 1 / (2 x 3.141592653 x 1.8 MHz x 47 nF)
= 1.88 ohms (to 3 significant digits)
= 1.9 ohms
Finally, remember that there are two inductors in parallel, one at each end, separated by coax and the effective value of inductive reactance is the composite value - and lower than just having one in circuit, probably half if two identical values are used.
The AD5X article tells you what was found in that case with those values but not why they were selected in the first place.
That means that an isolation inductor has an inductive reactance > 10 times Zo, where Zo is 50 ohms when dealing with coax impedances of typically 50 ohms.
Conversely, bypass capacitor values must have a capacitive reactance < 10 times the circuit impedance to be effective. That means that the MAX value of capacitive reactance is 5 ohms at the lowest frequency involved. A better proposition to aim for is 1-2 ohms MAX. A 47nF capacitor has a reactance of 1.9 ohms at 1.8MHz so that would be the smallest value you should consider using so that it will provide effective bypassing at RF.
XC = 1/(2piFC) = 1 / (2 x 3.141592653 x 1.8 MHz x 47 nF)
= 1.88 ohms (to 3 significant digits)
= 1.9 ohms
Finally, remember that there are two inductors in parallel, one at each end, separated by coax and the effective value of inductive reactance is the composite value - and lower than just having one in circuit, probably half if two identical values are used.
The AD5X article tells you what was found in that case with those values but not why they were selected in the first place.
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.