Elector TV modulator inductor values?

[neazoi]

Hello,
I wish to build this small video modulator
RF MODULATOR D.I.Y.

I would like to make a version with the inductors etched on the PCB instead of hand wind.

The old DOS version of appcad can calculate spiral inductor values, so I think I can manage to do it. The problem is that I do not know the values of L1/L2 and L4. The author gives only instructions of how to wind them but I do not have a so low inductance meter to measure them. But they are not too critical I believe...

Any guess about the inductors values???

Inductors:
L1, L2 = 3.5 turns of 0.2 mm (SWG 35 or 36) CuL on a ferrite bead of about 3.5 x 3.5 mm
L4 = 1 turn of 0.8. . .1 mm (SWG 19...21) CuL, air wound with a diameter of 8 mm

 

[srizbf]

for L1 and L2 , any data available for the ferrite bead (magnetic)?
for L4 , you can estimate .

 

[neazoi]

for L1 and L2 , any data available for the ferrite bead (magnetic)?
for L4 , you can estimate .

I have no data for these ferrite beads. In the prototype I used two random from the junk box. Their value is not critical I think, since the oscillator is a 27mhz only. But you cannot remove them, some inductance is needed at these points for the oscillator to work.

Any rough guess about their values?

About L4, I do not know if the inductance of a single air-core turn will be roughly the same as that of a PCB-core..

 

[srizbf]

for ferrite bead , if inner and outer dias and length of the bead is known , we can make an attempt to calculate its L vlaue.

for L4 of air core , there is a formula .
but you can use the following page to calculate it:

One-turn loop inductance calculation

---------- Post added at 14:14 ---------- Previous post was at 14:11 ----------

FOR L4 , once the inductance of air core is calculated , you can use it to to design that one for PCB inductance of track.

 

[neazoi]

the ferrite bead I used for the two coils was:

4mm length
3mm external diameter
2mm internal diameter (bead is 0.5mm thick)

 

[FvM]

Referring to your original question, L4 is the only inductor that applies for a PCB implementation. I guess, you'll realize, that 1 uH isn't very handy for a coreless PCB coil. L1 and L2 are rather chokes than inductors with a precise inductance requirement, but they'll also have uH order of magnitude, so the problem is basically the same.

In other words, it doesn't sound like a good idea.

 

[srizbf]

i have made a calculation without sufficient data of the bead and with given data .
it comes as : approx :30 microhenry.(for L1 and L2)
is it a possible value in the circuit given ?

 

[neazoi]

Thanks for your answers,
I was pretty sure they are in the uH scale since they are too small in size. But 30uH is too much different from 1uH or so, so I think the best solution for this random core coil is to build a more sensitive inductance meter (If I can find one) and perform an actual measurement on them.

are 1uH inductors so big to etch on the pcb really? considering this http://www.wa0itp.com/ns40manual.pdf they are not too big, just 4-turns on one side of the pcb and 4 on the other. But for greater values this definitelly should be big, so for these greater values this could be a good idea **broken link removed**

 

[FvM]

As I already mentioned, there's no need to implement exact inductance values or L1 and L2. For L1, you can derive from the resistance values, that it won't make much sense below 10 or 20 uH. But only high permeability ferrites would achieve similar inductances with 4 turns. so I'm not sure, if L1 serves a purpose at all. L2 can probably work with less inductance. In any case, I think a small fixed inductor will be the easier and less expensive variant.

You are right, that 1 uH can be achieved with an about 8 turns planar coil. Personally, I won't like the stray field and prefer a fixed (ferrite core) inductor as well.