Optically transparent electrical isolation of analog signal

[ricko_uk]

Hi,
what is the best solution to transfer with highest accuracy an electrical signal (0 to 5V - up to 30KHz) from outside a faraday cage to insode it?

1) I was wondering if there are optically-transparent RF-shielding materials and then use some optocoupling means?
2) maybe there are already ready-made module solutions?
3) magnetically couple? but I am looking for a good fidelity signal transfer...

Any suggestions?

Many thanks

[mikeselectricstuff]

Fibre optic through a metal tube

[tggzzz]

Digitise to whatever standard suits you, then use optical fibre to transmit the digits, reconstruct on the outside.

Since you didn't include a latency specification, you could also transfer the digits on paper or magnetic tape

[ricko_uk]

thank you Mike,
nice idea about the optical fibre.
Any advantages using the tube over cutting a small round hole for the optical fibre into the enclosure? It would still leave that small area open, or am I missing something?

Thank you

[mikeselectricstuff]

The tube just gives more isolation than a simple hole. bear in mind the hole may need to be big enough to get a connector through

[ahbushnell]

thank you Mike,
nice idea about the optical fibre.
Any advantages using the tube over cutting a small round hole for the optical fibre into the enclosure? It would still leave that small area open, or am I missing something?

Thank you

Since you want infinite bandwidth on your shielding then you need a tube.  Normally on screen room you would use a tube that's a quarter wave length long of the highest frequency you want to shield.  go buy copper tubing at a hardware store.  and solder it to your shield where in penetrates. 

[ricko_uk]

Thank you Mike and Ahbushnell,

how does the length of the tube affect the frequencies passed/blocked?

lets say the tube diameter is 10mm ID so that everything above 120GHz (4 x 30GHz) is blocked... comparing that to a circular cutout in the Faraday cage of the same diameter, isn't the effect/result the same? Or does the tube have a different effect?

[NiHaoMike]

1) I was wondering if there are optically-transparent RF-shielding materials and then use some optocoupling means?

Metal mesh is just that - commonly available as window screen.

2) maybe there are already ready-made module solutions?

Cheap S/PDIF ADCs and DACs are a cheap way to convert to and from fiber. Keep in mind you'll want the ones that can do 96kHz sample rate or more in order to be able to pass 30kHz.

Then there are feedthrough filters that pass low frequency signals through without compromising the RF isolation.

[ricko_uk]

Thank you NiHaoMike

What type of feedthrough filters? The only ones I know are for the power lines (like general PCB components type). Could you suggest a random part number so I can understand the type?

Ref the S-PDIF, doesn't the S-PDIF inside the Faraday cage (receiver) need a clock to recover/re-convert the signal? That would introduce noise. Are there some that don't need a clock to reconstruct the signal on the receiver side?

Thank you

[ejeffrey]

Thank you Mike and Ahbushnell,

how does the length of the tube affect the frequencies passed/blocked?

lets say the tube diameter is 10mm ID so that everything above 120GHz (4 x 30GHz) is blocked... comparing that to a circular cutout in the Faraday cage of the same diameter, isn't the effect/result the same? Or does the tube have a different effect?

A tube is much better.  A waveguide below cutoff does not support travelling waves but does support evanescencent waves.  These die off exponentially with distance but if the tube is too short you can still get some signal through. 

[ahbushnell]

1) I was wondering if there are optically-transparent RF-shielding materials and then use some optocoupling means?

Metal mesh is just that - commonly available as window screen.

2) maybe there are already ready-made module solutions?

Cheap S/PDIF ADCs and DACs are a cheap way to convert to and from fiber. Keep in mind you'll want the ones that can do 96kHz sample rate or more in order to be able to pass 30kHz.

Then there are feedthrough filters that pass low frequency signals through without compromising the RF isolation.

It's like window screen but you want a good conductor that you can solder too.  So brass screen would be good.

 

[mikeselectricstuff]

Re. Spdif adc/dacs, bear in mind these are not specced for dc, so offset and drift performance,maybe also gain stability may not be good

[ricko_uk]

Thank you all,

A tube is much better.  A waveguide below cutoff does not support travelling waves but does support evanescencent waves.  These die off exponentially with distance but if the tube is too short you can still get some signal through.

How do I decide/calculate the minimum length of such tube?

Thank you