Yes, that makes sense. The diode I used is a bi-directional type with a clamping voltage of 5.5V. Static charge build-up is limited to 5.5V which should be harmless. In my case I'm developing UHF RFID antennas (860-930 MHz). That involves lots of touching the antenna, adding or removing copper tape for frequency tuning and impedance matching. The risk of ESD is high, I blew up 2 units and a customer of mine also 1. For me this is the only serious flaw of the SAA2 analyzer series. Next models will get ESD protection according to the designer. By the way, I did not add a bleeder resistor, might do that to make it even more fool proof.
Reinier
Op 4-2-2021 om 22:56 schreef Ken Sejkora:
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Greetings all,
Interesting discussion. When I Googled TVS (Transient Voltage Suppression) diode, it appears there are two varieties, unidirectional and bidirectional. I would assume a ‘bidirectional’ TVS diode effectively does not have a polarity and would dissipate static charge in either direction. It make perfect sense that wind/dust/snow/etc. blowing across an antenna could induce a static charge across the ‘capacitor’ represented by a coax cable, so the “polarity” of the center-conductor versus the coax shield could change depending on the specific conditions. If a unidirectional TVS diode was connected “backwards” across the coax connector as referenced to the coax itself, wouldn’t it represent an ineffective drain of the static charge, and potentially result in damaging the NanoVNA? If TVS diode polarity is important, I would think a bidirectional TVS diode would be the preferred device, followed by a high-value resistor, to bleed off the static charge.
Is my logic flawed? Please enlighten me. Thanks.
Ken, WB?OCV
From: Reinier Gerritsen
Sent: Thursday, February 4, 2021 03:18 PM
To: [email protected]
Subject: Re: [nanovna-users] SAA2N problem
It's kind of expensive (because SMA connectors aren't free), but one
could probably make a little protection board with a SMA jack one end
and SMA plug on the other, with the TVS diode on the single
microstripline trace in between.? Sort of a dual "connector saver" and
"VNA saver".? The parasitics of the board would "calibrate out" for
the most part.? If you had a steady hand, you might be able to build
one out of just the two connectors, if you get the kind with the posts
- solder the posts together and somehow put the diode in between. I've
done this for making a T or for oddball loads, but it's not something
you'd be proud of.
Soldering the diodes directly on the pcb is easy and you can never
forget them...
See pictures (follow the trace from connector to series capacitor to TVS
diode to resistive pad (3 resistors). The other port has the TVS diode
directly at the input (protects the capacitor too)
