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Re: SAA2N problem
Hi, the most likly reason, that the protection diode is not alredy present on the vna, is cost or possible problems against I good measure?No, cost is very low, jut a few cents. During the design phase ESD strikes were applied directly at the input, it passed the tests. However, after my report of broken units, the designer (Gabriel) tested again, now with at least 50 cm of cable connected and he was able to reproduce the problem. This happened to be my usual setup too. Gabriel concluded that the input voltage protection of the switch is sufficient, but that the current kills it. (current due to discharge of the cable capacitance). He suggested insering a small resistor of 5 Ohm to limit the current to the RF switch's protection diodes. A series resistor probably has some influence on the source impedance. A 0.1 pF protection diode does not. The non-linearity of the diode is a fraction of the 0.1pF. DC levels are zero anyway so you can consider the diode to be of no influence. I did not see any difference with or without diodes. |
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Re: NanoVNA RF Demo Kit connection
I couldn't make even the small number of connections suggested with those little connectors. They died before. I put in parallel with each connector a pair of male-male pins used with IC sockets (2.54mm) and another pair of male-female pins at one end of each test lead. They won't be 50 ohms but their length is too small to be very bad at the frequencies involved. And they allow you to connect and disconnect hundreds of times (and are easy to change later).
Regards |
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There are versions of V2 with N and SMA connectors.
V2 switches to si5351 for frequencies below 140MHz so that is correct. On Fri, 5 Feb 2021 at 16:05, bruiser419 via groups.io <bruiser419= [email protected]> wrote: Glad I caught this thread. I was thinking of getting a S-A-A-2 but if it |
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Re: SAA2N problem
Ken,
indeed you have to use the bidirectional type. Unidirectional ones conduct like any diode in the forward sense. They are pretty much like improved zener diodes. Bidirectional ones are two of them back-to-back in the same package. When selecting one you need to be very careful about capacitance. Those large enough to survive the discharge pulse that happens when you connect an antenna system that has a charge on it, usually have capacitances of several hundred pF, and thus are useless at RF. Small ones for RF use exist, with capacitances even lower than 1pF, but they have very limited pulse power handling ability and may fail when connecting a large antenna charged to a significant voltage. |
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Re: SAA2N problem
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.
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Reinier Op 4-2-2021 om 22:56 schreef Ken Sejkora: Greetings all, |
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Re: SAA2N problem
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 oneSoldering 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) |
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Re: SAA2N problem
Hi Dave
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The charge only accumulates if it is connected to an antenna that is not a DC short.? ?i.e.? long wire, gama matched Yagi, 1/4 wave vertical etc. Not a problem with a folded Dipole, loop, or the antenna has most balun designs. We use to connect an NE-2 neon lamp across the coax connector to a long wire antenna.? ? ? ?Takes about 80 volts to light up an NE-2.? ? ? ? ? ? ? ?Brisk breeze when the humidity was low and that neon light would flash every few seconds.? ?? This is why most preamps have a 10K resistor across their inputs to bleed off that change. OK, in the shack, but for outside antennas I would not connect my VNA's to an antenna that is not a DC short. Kent WA5VJB? ?Antenna Editor CQ Magazine. On Thursday, February 4, 2021, 02:56:03 PM CST, David Eckhardt <davearea51a@...> wrote:
A coaxial cable is a long cylindrical capacitor.? Being by design low loss, it can accumulate and store a charge for quite some time.? When connecting any longer piece of coaxial cable or other low-loss transmission line, I always make it a habit before connecting it to anything expensive or valued by pressing my finger across the end to discharge any accumulated charge. Sometimes, there is nothing, but the first time you get 'bit', you will become a believer.? Transmission charge is highly likely to accumulate during a thunder storm, wind-blown dust orsand, and even snow fall. Dave - W?LEV On Thu, Feb 4, 2021 at 1:27 PM schweppe <schweppe@...> wrote: When looking at the front end of the nanovna you see a 10?F capacitor in-- *Dave - W?LEV* *Just Let Darwin Work* |
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Re: SAA2N problem
A coaxial cable is a long cylindrical capacitor. Being by design low loss,
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it can accumulate and store a charge for quite some time. When connecting any longer piece of coaxial cable or other low-loss transmission line, I always make it a habit before connecting it to anything expensive or valued by pressing my finger across the end to discharge any accumulated charge. Sometimes, there is nothing, but the first time you get 'bit', you will become a believer. Transmission charge is highly likely to accumulate during a thunder storm, wind-blown dust orsand, and even snow fall. Dave - W?LEV On Thu, Feb 4, 2021 at 1:27 PM schweppe <schweppe@...> wrote:
When looking at the front end of the nanovna you see a 10?F capacitor in --
*Dave - W?LEV* *Just Let Darwin Work* |
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Re: SAA2N problem
I was measuring what turned out to be about 325 foot run of coax. About 200 feet of lmr400 and 125 feet of rg213. Think I will have to find some diodes to solder on the pcb. I don't think I have any surface mount stuff handy. I have mostly larger components on hand :-)
Thanks again! |
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Re: 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) |
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Re: SAA2N problem
On 2/4/21 12:00 PM, Thomas Kerns wrote:
Thanks for the help. I don't remember exactly what I was doing at the moment it quit, but I had been measuring a long run of coax, and I had also measured the swr of an outdoor antenna (a 160 meter dipole). That is a lot of wire in the air, with the potential to pick up static, I suppose. I wonder the best way to protect my vna in the future.It's a good practice in general to "permanently" have a leakage path to ground.? A 100k or 1 meg resistor in a Coax T is one way. If you get a big transient, it will probably fail, so that makes an ohmmeter a useful diagnostic tool <grin> Some people use a RF choke that has high Z at the operating frequency, but lower DC resistance to bleed the charge faster. ?Standard 0.405" coax (RG-8, RG-213) is about 40 pF/meter, so a 100 ft/30meter run is 1200pF. |
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Re: SAA2N problem
On 2/4/21 11:54 AM, 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. 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. |
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Re: SAA2N problem
Thanks for the help. I don't remember exactly what I was doing at the moment it quit, but I had been measuring a long run of coax, and I had also measured the swr of an outdoor antenna (a 160 meter dipole). That is a lot of wire in the air, with the potential to pick up static, I suppose. I wonder the best way to protect my vna in the future.
I see someone mentioned a drain resistor. Would this be something I would do only when testing? ie make a coax pigtail with a drain resistor across from the shield to the center? would that affect measurements? |
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Re: SAA2N problem
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.
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Reinier Op 4-2-2021 om 14:26 schreef schweppe: When looking at the front end of the nanovna you see a 10?F capacitor in series with the MXD8641¨CSP4T Switch. This switch can resist 100V in electrostatic discharge sensitivity testing (Machine Model). This test is done with a loaded 200pF capacitor and a 0.5 ?H inductor in series. |
Gab IU0 OQM
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