[SI-LIST] Re: ESD solution on antenna output (re-send)
- From: Ian Barrett <Ibarrett@xxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 4 Feb 2004 18:34:26 -0000
OK update. Thanks again for all the replies, it has certainly helped to
clarify things in my mind.
The device is a 400MHz transmitter that fits in the palm of your hand, and
the failure mechanism in nothing to do with the output stage - another part
of the circuit is going into a lock-up that requires a power cycle to reset.
Parallel work is looking at exactly how and why this is happening, but I am
looking at how the ESD is coupling into the circuit in the first place.
I have tried the experiment below - short the antenna to ground - and the
circuit locks-up. When I completely isolate the antenna (from the output
stage and any surrounding ground plane) it survives. Unfortunately,
insulating the antenna as someone suggested, is not an option.
Ian.
----- Original Message -----
From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, February 03, 2004 08:57 AM
Subject: Re: [SI-LIST] ESD solution on antenna output
What would happen if you (experimentally) replace the inductor with a short;
i.e., directly short the antenna to ground? Of course it wouldn't transmit
or receive, but it might help prove/disprove your hypothesis that the
failure mechanism involves the ESD getting into the ground system and then
doing some damage there.
If it is what you suspect, then we need to get into a different mindframe.
Most of the replies have assumed that the ESD problem is a voltage between
the signal and ground terminals, across the amp input or output. But if
it's a spike event IN the ground system, then you have to figure out where
it goes within your ground system and how it causes the damage. Maybe the
problem is that the inductor is connected to the wrong "ground" point!
An inductor to ground is used on many antenna terminals, but apparently not
everywhere. I think they are fairly common on HF/VHF/UHF antennas that are
not already self-grounding by having some sort of loop. (A base-fed
vertical antenna is not self-grounding, but a shunt-fed one is.) If the
antenna to electronics are capacitively coupled, a mobile or outdoor
antenna could build up several thousand volts of static charge from wind
and/or rain hitting it, and then you can have something like a blown
capacitor and electronics. Hence the inductor. It provides a DC and low
frequency current path to ground to bleed off this static build-up, as Ray
Anderson mentions.
But this assumes 2-terminal (1-port) thinking; i.e., that the enemy is
voltage ACROSS the terminals, not something within the electronics' ground
system.
If you think inductive kick might be worsening the problem, a few-hundred
ohm resistor might be better.
Another variation is to use transformer coupling to the antenna, which if
narrowband might reject more out-of-band energy while shunting low frequency
ESD energy to ground.
But, if you are fairly sure the problem is the ESD getting into your ground
plane and then causing damage, you need to think of different solutions.
Regards,
Andy
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