RF shielding and micros

[hamilton]

I have a project with a 27Mhz osc/amp and a microcontroller.

The 27Mhz is about 200Watts.

I have been seeing some of the RF in the micro boards power supplys.

What is the proper way to isolate the RF out of the power supply ?

Would putting all the wiring into a steel tube help shield everything ?

My micro pcb is currently only 2 layer, the next one will be four layer
with ground plains on the top and bottom layer.

Does any one have any good references on RF shilding ?

thanks

hamilton

 

[Jamie]

I have a project with a 27Mhz osc/amp and a microcontroller.

The 27Mhz is about 200Watts.

I have been seeing some of the RF in the micro boards power supplys.

What is the proper way to isolate the RF out of the power supply ?

Would putting all the wiring into a steel tube help shield everything ?

My micro pcb is currently only 2 layer, the next one will be four layer
with ground plains on the top and bottom layer.

Does any one have any good references on RF shilding ?

thanks

hamilton

And what would this project be on 27Mhz at 200 watts?

 

[hamilton]

And what would this project be on 27Mhz at 200 watts?

Inductive heating

 

[[email protected]]

I have a project with a 27Mhz osc/amp and a microcontroller.

The 27Mhz is about 200Watts.

I have been seeing some of the RF in the micro boards power supplys.

If you can see it, you have real problems.

What is the proper way to isolate the RF out of the power supply ?

Shield. Filter what you can't shield.

Would putting all the wiring into a steel tube help shield everything ?

Why steel? Unless you need magnetic shielding (simple steel won't work),
Aluminum is usually easier to work with. Copper or copper-clad FR4 is
sometimes easier for a one-off, because it can be soldered easily. Copper
tape is also very useful for prototypes.

My micro pcb is currently only 2 layer, the next one will be four layer
with ground plains on the top and bottom layer.

Why? This is almost always a *bad* plan. It gains nothing and is a royal
PITA. Internal planes work (one ground, one power) and still allow access to
the signals.

Does any one have any good references on RF shilding ?

It's pretty straight forward. Anything over 1/20 wavelength is too big. Are
you looking for a commercial solution?

 

[Paul Keinanen]

I have a project with a 27Mhz osc/amp and a microcontroller.

The 27Mhz is about 200Watts.

I have been seeing some of the RF in the micro boards power supplys.

If the power supply secondary is floating (no connection to case etc.)
use a sufficiently thick twisted pair and run it through a ferrite
(both wires) close to the micro board ?

What is the proper way to isolate the RF out of the power supply ?

Does the power supply suffer from shutdown or erratic output voltages
when the RF is present ? The RF is then entering through the DC output
terminals and into the voltage sense feedback path, driving the
differential amplifier into saturation etc. Some ferrites on the power
supply side of the DC cable will usually help and some chip capacitor
across the DC terminals as well. Of course the obvious place to filter
would be in the differential amplifier inputs, but this may cause loop
stability problems, unless you know what you are doing.

Would putting all the wiring into a steel tube help shield everything ?

In the presence of a strong altering magnetic field, you should avoid
any large wire loops, since the induced current is proportional to the
loop area. If you have a row of buttons on the front panel, do not run
a common ground wire to all buttons and separate wires to the button
hot side, since in the worst case, there will be a large loop, when
the button is pressed at the opposite side to the common ground wire.
Instead, run separate twisted pair wires to each button separately. A
chip capacitor at the exact point where the twisted pair enters the
PCB will also help.

In general, use differential signaling (RS-422/485 or 20 mA current
loop) when possible (instead of RS-232) etc. to simplify filtering.
Optoisolation also helps in avoiding loops and antenna effects
(capacitive coupling).

My micro pcb is currently only 2 layer, the next one will be four layer
with ground plains on the top and bottom layer.

More important is avoiding loops with large areas and reserving
sufficient space for filtering components on the perimeter of the PCB
to act as the first line of defense.

The frequency is quite low (wavelength 11 m), so this should not be
too hard to keep it out, but of course, you have to be careful about
selecting a suitable ferrite material, in order to achieve a
sufficiently large inductive reactance at that frequency.

 

[hamilton]

Do you have problems with the SW when the RF generator is on, or do

With the Inductive heating coil in open air, there seemed to be no problem.

When the coil was placed next to the material to be heated, the cpu resets.

Looking at the +12v supply rail, (which +5V was derived) we saw 2.1
Volts of 27 Mhz.

We did add extras caps (.1uf) to the +12 and +5 sides of the 7805
regulator and the resets stopped.

But there is still .2 V of 27Mhz in there.

I would guess that this will never pass EMI testing.

Thanks for all the comments, I am getting a better idea what I need to
do here.

hamilton

 

[Paul Keinanen]

With the Inductive heating coil in open air, there seemed to be no problem.

When the coil was placed next to the material to be heated, the cpu resets.

Check the reset circuit that it does not generate spurious resets due
to RF rectification.

Check that the MCU crystal oscillates at all times.