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When I redesigned the HackRF antenna protection, I removed the useless TVS, a bi-directional 15V type (that will only activate after everything in the HackRF is already dead), and instead used Skyworks Limiter Diodes SMP1330-085LF.

These are low-capacitance PIN diodes, but are designed with an unusual characteristic: after a forward pulse greater than 0.7v has turned them on, they briefly conduct in both directions with between 1 and 2 ohms impedance. The diode itself can absorb up to 30dBm safely, but the impedance discontinuity reflects up to 50dBm back toward the source, which protects the antenna input amplifiers.

In the case of the HackRF, I need to protect the LNA from any excursions above 13dBm, but the transmitter and mixer inputs are a bit more robust, so I used three of these diodes.

On Fri, Feb 5, 2021 at 03:30 PM, Clifford Heath wrote:

When I redesigned the HackRF antenna protection, I removed the useless TVS, a
bi-directional 15V type (that will only activate after everything in the
HackRF is already dead), and instead used Skyworks Limiter Diodes
SMP1330-085LF.

In the case of the HackRF, I need to protect the LNA from any excursions above
13dBm, but the transmitter and mixer inputs are a bit more robust, so I used
three of these diodes.

Clifford - Just took a look at your schematic.
--- I see you reverse biased one of the diodes to Vaa (3V) and the other to ground. Why not both to ground?
---- It look like you used 10 k resistors instead of RF chokes like the Skyworks app note. Any particular reason?
---- How much RF power can you now safely inject without damage using this new circuit.

Thanks - Roger

Has this been implemented for a fix to the issue of protection? I would like to update my unit with the fix.Thank you very much,BillW4MXT

In the HackRF, the majority of the circuit needs to be protected from excursions outside the 3.2v rail voltages. The two diodes near the input achieve this, allowing excursions from the mid-point (set by the 10K resistors) to below -0.7v or above 3.2+0.7, a peak excursion of 2.3v from the centre in either direction.

The LNA needs additional protection when it is switched into the circuit, which is what the diode to ground achieves.

This use of these limiter diodes is a bit off-label, more like you'd use traditional diode clamps. I couldn't use inductors because it's not possible to find inductors that actually act like inductors over the range from 1MHz-6GHz.

I did not have the budget to prototype this and destroy multiple HackRFs in pursuit of exact data on RF immunity, but there have been zero failure reports from any users of the first batch of 1000 units sold. Time will tell, but compared with a typical failure rate of over 30% LNAs being killed within the first year, things are going well so far.

thanks to Reiner info I bought the tvs diodes ESD101B102ELE6327XTMA1 with the idea of ??making me a protection for the nanovna with a pair of sma ports.
But when I got the tvs I felt lost and scared, I had never seen anything smaller to deal with.
However, I remembered that in one of my compulsive attacks I bought 35g of 60-40 paste and a portable hot air gun. On a pcb crumb I made a groove, two soldering pads, I tried to solder. A first tvs splashed somewhere in the lab while I was holding it in tweezers. With a second I was luckier and I think I soldered it. I assembled everything, on two sma ports, with also a 560 kohm resistor in parallel.
I tested the door with the vna to see how it felt with the measurements. I have tried it between 1 and 150 Mhz and I would say that it behaves well, moreover that I am currently interested in the 2 30 MHz range because I want to build a multiband octopus. I add some photos, hoping it will be useful (reference is calibration).
Ciao everybody.
gab

On Thu, Feb 4, 2021 at 09:54 PM, Reinier Gerritsen wrote:

I had 3 broken units a few months ago (the first version of the SAA). Replaced
the switches and they were back to life. On one unit I installed esd
protection diodes. But since I have my V2plus4, I don't use the old analyzers
anymore. I still have to modify all the other analyzers. TVS diode part
number: ESD101B102ELE6327XTMA1, a 5.5V TVS bi-directional diode from Infineon,
0.1 pF capacitance. Diodes are cheap. Switches are also cheap at Aliexpress.
Replacing the switch is not for the faint hearted. It is a tiny package with a
ground pad under the device. Hot air soldering is your only option. A good
stereo microscope helps.

Someone should make add on cards with those protection diodes. I wont solder them myself, 0603 are hard enough and sometimes even a 1206 between two SMA connector PCB pins.