Electronics > RF, Microwave, Ham Radio

Bias tee ESD protection

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e.sotillet:
Hi folks, I keep working in an old design trying to make it better, this time is related to the input of the amplifier. It has a bias tee to send power througt coaxial cable. The last version of the circuit include a GDT, a PTC and a slow diode (1N5401) as part of the circuit.

But this combination makes the input impedance close to 30 ohm -j80 (capacitive). I was thinking to move the PTC after the inductor and change the GDT for a RF TVS and eliminate the diode, what do you think?

Best regards

coppercone2:
when you have a GDT and diode together you need a impedance between them otherwise the diode always works first.
the PTC is your impedance so if you remove it the GDT will no longer function properly in the circuit. Your replacement protection system is probobly less robust then a GDT for overall damage, in a traditional circuit, but this has a series diode, not a diode clamp, so thats different.

Your power is protected by a G-R(PTC)-C circuit right now, so for signals going into the antenna they effectively are RC filtered into the power supply. By moving the PTC you maintain the RC circuit but the diode is no longer protected from surges, its a surge preventer.

You are making alot of changes so it really depends on what you want.

T3sl4co1l:
What's the diode for?  And what are you measuring impedance with respect to, and at what frequency?

I don't get what you'd need about a bias tee, other than the inductor and capacitor.  Maybe a clamp if you have potential nasties out at the far end (compare for example, the clamp diodes often added to a LISN), which would probably be put on the AC-coupled port.

Tim

e.sotillet:
Sorry, i forgot to mention the operating frequency: 172MHz.

I wanna move the PTC because it is physically larger, i think it adds up more capacitance, right?

I dont know the propuse of the diode, I also were intrigued about its roll because it is AC conected. The anode is "floating".

I want to protect the devices after the bias tee (inductor) from nasties from coaxial. 48VCD is injected and it is wired near industrial noise.

The impedace was measure in the conector (near the GDT). After the diode, i place a 50ohm load to dinulate the rest of the circuit (first is a RF switch and later a MMIC).

Best regards

T3sl4co1l:
So this is some existing device?  What's the rest of it look like?

Do you know that the device was not tested for ESD (or to a lower acceptance criteria than you are apparently looking for) originally?

Is the coax prone to ESD strikes to the inner conductor? -- if so, it sounds like a major wiring fault (in particular, defective shielding), more than an ESD problem necessarily.  For sure, not something you want to have, with 172MHz leaking everywhere.

Also, so this is just a receiver?  Well, switch, I suppose it could be both...

The PTC is in a strange place, but doesn't seem like it would necessarily have much effect.  If its resistance is low, it won't affect insertion loss much, and if it's a small chip component, the layout around it can be adjusted to give reasonably stable 50 ohms.  Even without such care, it should be less than a pF, nothing significant here.  The next most likely explanation is you were measuring the diode junction capacitance, at whatever bias your impedance meter's excitation drove it up to (which probably isn't much, that type is quite slow -- more of a PIN diode than a rectifier; which could be an explanation for its placement, but no bias resistors or other switching components are shown).

Tim

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