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Fried Ch0 S11 port #hardware

GM7TFP 2020/11/29 14:54

Hi all

I toasted the S11 port of my SAA2 v2.2 the other day with an un-scheduled DC input event. Bought a new one to replace it already (and a few Nooelec DC blocks!..ahh, hindsight), but I'm wondering if I can put the brick to any useful RF measurement purpose still with just S21 operational? If not, I'll salvage the screen, battery and SMA connectors for other use. Any suggestions?


Reinier Gerritsen 2020/11/30 08:52

There is a 10 uF series capacitor at both inputs, so DC is blocked. But
when connecting your device under test, the DC transient may still kill
your analyzer. The capacitor charge current path is through the internal
protection diodes in the input switch (port 1). Port 2 is much better
protected for DC transients since it has 14 dB of resistive attenuation.
I had a bunch of broken units, they came back to life when I replaced
the input switch U551. It is a MXD8641 which is sold on Aliexpress, 50
pcs for a few dollar.
How much DC did you apply?


Op 29-11-2020 om 23:54 schreef GM7TFP:

Stephen Laurence 2020/11/30 00:49

How did you acquire a “bunch of broken units” and do you know if it was esd or dc transient and what was the voltage connected to?
I presume they other peoples devices who asked you to fix them?
10uf is a huge input capacitance for the device, unless you are only interested in very low frequences. It would be a large enough capacitor for an audio amplifier input. Would it be worth changing to 1uf or 0.1uf?

Steve La

Reinier Gerritsen 2020/11/30 10:21

I bought 2 of them, that got stuk in transport due to the corona virus
situation. In the mean time I ordered some extra from different
suppliers to spread my chances. At the end, I had 5 units and a customer
on mine had 1. In one week, I managed to kill 2 units and my customer 1.
I swapped his unit for one of mine. No DC applied, but a lot of tweaking
with RFID (900 MHz) patch antennas. So most likely ESD.
For my work, even a 100 pF capacitor would do, but at low frequencies
you'll need the big one.
I added an Infineon ESD101B102ELE6327XTMA1 TVS diode to protect the
inputs. It adds 0.1 pF parallel capacitance which is neglectable up to
the instruments 4 GHz limit. The designer of this VNA is currently
looking into it and has some interesting findings. So I expect future
models to be improved and maybe we can retrofit existing models with
good protection.
It takes some skills to replace the switch because it is a QFN package
(ground pad under the package). Adding the protection diode is much
easier (0402 package).


Op 30-11-2020 om 09:49 schreef Stephen Laurence:

Stephen Laurence 2020/11/30 02:28

Thankyou. I am learning all the time (got to keep a 70yr old brain alive somehow).

I am getting some switches just in case. I have tools to (theoretically) replace them (controllable hot air gun, tweezer soldering iron, cheap Chinese microscope etc) and some dead Ipads to practice on.

However I will be careful. I am surprised rfid equipment could cause damage as it has to be safe for the public and equipment they might be carrying ( eg phones). Nevertheless, I might buy a stock of these diodes as well.

A stack of built v2 boards might be a good investment too, as most of the v2 variants (v2N, my favoutite) use it with additional bespoke boards for the niceties (display, switches etc).

Steve L

Reinier Gerritsen 2020/11/30 11:45

It's not the RFID reader that caused the problem, it's the antenna I'm
designing. And these antennas are just a stack of sheet metal and
spacers, all of the right dimensions of course. I need the VNA to tune
the antenna to the exact frequency and impedance I need. Adhesive copper
foil, a sharp knife and lots of experience is all you need.
I'm not sure what would happen if I put a 1W RFID transmitter directly
at the input, I would probably fry some tiny resistors ;). By the way,
RFID is of course well regulated for safety. The output power from the
antenna is limited to 2W ERP in Europe. Safe distance is around 25 cm.


Op 30-11-2020 om 11:28 schreef Stephen Laurence:

Stephen Laurence 2020/11/30 03:54

Using it without por1 functioning is a bit difficult as calibration is not possible.

You might find someone who can work on smd stuff to replace the blown switch for you, or try doing it yourself. You will need a bit of kit. It is a bit of a steep learning curve but you can practice on scrap phones, Ipads etc. I have the kit but not yet started doing it yet.

Actually, your local phone repairer might do it if you give him the replacement chip and explain what needs doing. However quite a few phone repairers are just screen replacement shops with no microsoldering ability.

Steve L

GM7TFP 2020/11/30 12:25

I'm setting up a QO-100 station and was doing a spot of testing. I had the Nano set up as a 2.4GHz CW source using S11, into an Axing in-line ~20dB amp. (, into a bias-T, then going to a R&S SA. The R&S was suitably DC protected and attenuated. My downfall was not thinking ahead and not twigging the fact that the in-line amp of course will pass DC (as it normally does to power a sat. dish LNA). A quick death for S11 at that point as soon as I applied DC to the bias-t. It was only 12V, so yes I would have thought the internal DC blocking caps would have been up to it, but nope.
I did have a look around for a replacement input switch as I have the necessary gear to replace. I'll now try Aliexpress, thanks for the hint.
Live & learn as they say :)

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