3 turn step up coaxial transformer cable impedance ?

[gbugh]

I'm trying to step up a class e amplifier to a 50ohm output. I only have room left on my circuit board for 3 turns.

If I use 3 turns of coaxial cable through ferrite to step up the voltage out and match 50 ohms, what cable impedance is best?

I've been trying to find design data but can't find much.

What determines the ideal coaxial cable impedance?

If it is not critical, is 50 ohm cable better since it won't arc between the shield and center conductor as soon as lower impedance cable would?

Thanks, George
73 de AF5IE

 

[D.A.(Tony)Stewart]

What is your purpose? max power? max voltage? max efficiency? Compatible with what load? What bandwidth? F?
Why coax?

 

[davenn]

What determines the ideal coaxial cable impedance?

The impedance of the RF coax cable is chiefly governed by the diameters of the inner and outer conductors. On top of this the dielectric constant of the material between the conductors of the RF coax cable has a bearing. The relationship needed to calculate the impedance is given simply by the formula:

The equation relating the coax cable impedance to the diameters of the inner and outer conductors and the dielectric constant of the medium between them

Where:
Zo = Characteristic impedance in Ω
εr = Relative permeability of the dielectric
D = Inner diameter of the outer conductor
d = Diameter of the inner conductor

Note: The units of the inner and outer diameters can be anything provided they are the same, because the equation uses a ratio.

Dave

 

[vfone]

Because you mentioned "step up to 50 ohms", means the PA output impedance is lower, and usually this kind of transformers use 25 ohms coax cables.

 

[gbugh]

Because you mentioned "step up to 50 ohms", means the PA output impedance is lower, and usually this kind of transformers use 25 ohms coax cables.

Thanks, I've seen 25 ohms used other places but a few places mention even lower and I can't find anything that defines why the cable impedance should be lower.

The class E output MOSFET can drive low impedances but the final output needs to match a 50 ohm antenna cable and antenna, so I'm stepping up the impedance to 50 ohms with a 3 turn transformer. The MOSFET will see 9 times less impedance which should be a 5.55ohm load.

Is 25 ohms chosen for the cable mainly because it is about half way between 5.55 and 50?

Unfortunately I've already fabricated a PC board with pads that will not mate well with 25 ohm center conductor and shield diameters.

I guess I was hoping someone would say it won't make much different in performance.

The total length of 3 turns is less than 18 inches and the amplifier will be used between maybe 6 meters at the upper end.

George

 

[vfone]

Yes, the 25 ohms is the best compromise for loss transformation in this situation.
For a high-power design (> 100W) using 50 ohms coax instead 25 ohms coax would make a difference, but probably in a low power design the circuit can handle a 50 ohms coax cable.
If is about a short 50 ohms coax cable, you can try to replace the central conductor with a thicker one (double in diameter). I've done successfully this operation while ago for a short RG-400 replacing the 1mm central conductor with a 2mm (#14 gauge) wire.

 

[D.A.(Tony)Stewart]

What is your purpose? max power? max voltage? max efficiency? Compatible with what load? What bandwidth? F?
Why coax?

You will have a lot of issues neglecting the nuances of EMI, stray coupling and impedance of switched LC circuits with multiple modes of resonance separate from the switching rates. The space left for the coil is a clue to the number of afterthoughts.

Where are your specs? How will you verify each stage without them?
How do you know your source impedance is 25 Ohms over all frequencies applied?

 

[gbugh]

Thanks , I'll try that.

Yes, the 25 ohms is the best compromise for loss transformation in this situation.
For a high-power design (> 100W) using 50 ohms coax instead 25 ohms coax would make a difference, but probably in a low power design the circuit can handle a 50 ohms coax cable.
If is about a short 50 ohms coax cable, you can try to replace the central conductor with a thicker one (double in diameter). I've done successfully this operation while ago for a short RG-400 replacing the 1mm central conductor with a 2mm (#14 gauge) wire.