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When a VNA is connected to an antenna (often they are outdoors) there are some potential issues with electrostatic discharge (ESD.) In dry areas of the world ESD can be an issue even when working with objects that are purely indoors. I have seen issues when working on tables indoors. Other VNA devices are known to have issues. Does anyone know how sensitive the RF inputs to the nanovna are with regard to ESD? Is there any ESD protection built into the RF connections on the nanovna?
--
Bryan, WA5VAH

The CH0 input of the nanoVNA is basically an attenuator connecting to a SA612 mixer without any additional protection.

So the question is, what precautions can be taken to protect the NanoVNA ?

Prayer is not an option ;-)

73 de Andy

slide 11

Simple and cheap: Back to back diodes
? Protection not dependent upon configuration
? Diode type is not critical (except, don’t use PIN diodes)
? Limited to low input power levels => receive only applications
? +30 dBm = 1 watt max (when using ? watt diodes)
? If either diode fails open => receiver front end not protected
? Spurious signals in receiver can be a problem
? Some mfgs offer choices on spurious levels (DX Engineering RG-5000 series)

Erik..

Two things here confuse me.

1 - I've looked at the SA612 spec sheet. I can't see anything that tells me what the maximum input is before the device is destroyed.
That would determine what methodology and parameters to design any protection circuit for.

2 - Wouldn't a diode introduce capacitance across Ch0 ? Or inductive lead effects at VHF/UHF and ruin any measurements ?

73 de Andy

You could use something like a ESD0P4RFL. Will need some space on the board. And it's quite small, but very low capacitance.

Thanks Erik.
I read the slides from the link but did not notice mention of another protection method that I've seen using zener diodes.

Referring to the attached image, drawing A is mentioned in the slides.
However, I have also seen the use of back-to-back zener diodes in series as shown in drawing B that I think would provide a higher input range.
Lastly, I wonder how the use of a two-color LED as in drawing C would work, with its higher forward voltage. (and visual indication?)

The original aim of the diode protection was to limit input voltage to 0.6V at the radio's sensitive front-end.
The VNA uses a resistance bridge - how much signal can it tolerate before physical damage or actually modifying what you're trying to measure?

One other thought.

During a discussion yesterday on this group, measurments were taken as to what the OUTPUT levels were from Ch0.

They were 200mv peak to peak IIRC.

As such, I think I'd want to assume that this would be the maximum level that any of the ports could handle and would want
to account for.

0.6v (600mv) for a basic diode to start clamping sounds a bit risky to me.

If I'm missing something here then I'm willing to learn ;-)

73 de Andy

I can't see anything that tells me what the maximum input is before the device is destroyed.

Inputs appreciably outside power supply limits will be problematic.
ESD was most problematic for CMOS not internally protected.
50 Ohm bridge pretty much protects the active chips;
bridge resistors can of course be damaged
by exceeding power dissipation rating (V*V/R)

Wouldn't a diode introduce capacitance across Ch0 ?
Or inductive lead effects at VHF/UHF and ruin any measurements ?

Having variable amounts of built-in parasitic reactances,
each nanoVNA already needs "calibration".

Folks,

Thanks for all the comments. I notice that the diagram provided by erik does not indicate any protection. Thanks for that diagram. The 50 ohm bridge mentioned above does provide some help but only a small amount. ESD events are often in the thousands of volts so even a few dB of attenuation is not enough. I have previously blown up a VNAs input switch when I touched an antenna connected to the VNA in my workshop so this is a real concern for me. Yes, it was repairable. I got to learn a lot about soldering very tiny surface mount components in that case.

Back-to-back zener diodes are available for this very purpose with very low capacitance (0.06 pf) for example the PGB010603 made by Littlefuse. I have attached a data sheet for others to consider. The spec sheet says it is good up to about 9 kv for a contact discharge and about 15 kv for an air discharge. Furthermore, the response time is under 1 ns which is about 1 cycle at 900 MHz. I suggest the mixer in the nanovna won't handle a typical ESD event without some help. I purchased some of these devices for about $0.50 each so they are not a big investment. Please be careful interpreting the data sheet. You might conclude these diodes don't help, but they have in my case.

I hope that when we do an open calibration any stray capacitance such as 0.06 pf is measured and removed. 0.06 pf at 900 MHz is a reactance of about 2950 ohms which should not have a major impact on most measurements anyway.

I hope folks find this information useful.

--
Bryan, WA5VAH

Bryan,
The lab I worked at had spectrum analyzers and receivers that cost more than my first house. To protect the equipment our laboratory manager purchased a set of HP 11867A limiters ( I'm guessing you could buy 5 or 6 nanoVNA's for what he paid for one of those). I believe the limiter was just a couple of well matched, low capacitance, fast switching diodes packaged in a Type N adapter. It turned out that the limiters were a waste of money for our application because diodes are non-linear devices and sometimes would mix with the signal we were measuring and generate phantom signals. For tuning an antenna that extra noise might not be a concern, but for compliance testing it was a deal breaker. We eventually ordered some Type N adapter RF fuse holders that had less than a dB of loss up to 1.5 GHz. The holders had fusible links that could be replaced in-house without having to send the equipment out for expensive front end repairs. I was the beneficiary of their use more times than I care to admit.

Never tried either solution with our network analyzers as I certainly would have been fired on the spot for attempting to use an HP8753 VNA in the scenario you are envisioning. Makes you wonder what kind of front end protection expensive antenna analyzers use.

On Fri, 20 Sep 2019 at 13:57, Andy G0FTD via Groups.Io <punkbiscuit=
[email protected]> wrote:

One other thought.

During a discussion yesterday on this group, measurments were taken as to
what the OUTPUT levels were from Ch0.

They were 200mv peak to peak IIRC.

As such, I think I'd want to assume that this would be the maximum level
that any of the ports could handle and would want
to account for.

0.6v (600mv) for a basic diode to start clamping sounds a bit risky to me.

If I'm missing something here then I'm willing to learn ;-)

73 de Andy

The output level of a source is not an indication of the damage threshold.
My HP 8720D can output 10 dBm, but the damage threshold on the test ports
is I believe 30 dBm, so 100x higher.

--

Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...

Telephone 01621-680100./ +44 1621 680100

Registered in England & Wales.
Company number 08914892.
Registered office:
Stokes Hall Lodge,
Burnham Rd,
Althorne,
Chelmsford,
Essex,
CM3 6DT,
United Kingdom

On Fri, Sep 20, 2019 at 03:46 PM, Andy G0FTD wrote:

2 - Wouldn't a diode introduce capacitance across Ch0 ? Or inductive lead
effects at VHF/UHF and ruin any measurements ?

Yes, limiter diode will ruin measurement. There are two issues with diode:
- it add parasitic reactance
- it add non-linear distortions of the signal

There is no simple way to protect RF device with no deterioration of it's dynamic range.

On Sat, Sep 21, 2019 at 01:20 AM, <qrp.ddc@...> wrote:

Yes, limiter diode will ruin measurement. There are two issues with diode:
- it add parasitic reactance
- it add non-linear distortions of the signal

100% agree.

There is no simple way to protect RF device with no deterioration of it's
dynamic range.

Seems to be that way.

I had a brief research period last yesterday just to look up how others deal with this issue.

It seems they don't, and simply practice safe procedure.

Spectrum Analysers on the other hand have much higher input levels, and do have some
solutions, but are cannot be used in the case of a network analyser.

I remember my favourite instrument, a Hewlett Packard HP8594 SA having a blown front end and getting a quote for repair.

You could build a aircraft carrier at those prices, whence why the company was always known as High Price ;-)

In the end, I just used a buffer amplifier for general sniffing around as a precaution when it was suitable.

73 de Andy

On Sat, 21 Sep 2019 at 10:07, Andy G0FTD via Groups.Io <punkbiscuit=
[email protected]> wrote:

On Sat, Sep 21, 2019 at 01:20 AM, <qrp.ddc@...> wrote:

Yes, limiter diode will ruin measurement. There are two issues with

diode:

- it add parasitic reactance
- it add non-linear distortions of the signal

100% agree.

There is no simple way to protect RF device with no deterioration of it's
dynamic range.

Seems to be that way.

I had a brief research period last yesterday just to look up how others
deal with this issue.

It seems they don't, and simply practice safe procedure.

Spectrum Analysers on the other hand have much higher input levels, and do
have some
solutions, but are cannot be used in the case of a network analyser.

It obviously depends on individual instruments, but a quick check of mine
shows that the HP 8720D 20 GHz VNA I have, is *much* more tolerant of high
power & DC than my 22 GHz spectrum analyzer.

*8720D 50 MHz-20 GHz VNA*
* 40 V DC
* +30 dBm max
* No electrostatic discharge

*HP 70905A RF section of spectrum analyzer, 50 kHz to 22 GHz.*
* 0 V DC
* 15 to 30 dBm depending on the value of an internal attenuator.

Given that the attenuator in the SA is set by the the user from the
interface, I would say assuming it’s set to 0 dB is sensible, so the
maximum power is 15 dBm. That makes the power handling of the VNA 32 times
higher than the SA, and the VNA tolerating 40 V DC compared to 0 V of the
SA.

I remember my favourite instrument, a Hewlett Packard HP8594 SA having a
blown front end and getting a quote for repair.

73 de Andy

--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...

Telephone 01621-680100./ +44 1621 680100

Registered in England & Wales.
Company number 08914892.
Registered office:
Stokes Hall Lodge,
Burnham Rd,
Althorne,
Chelmsford,
Essex,
CM3 6DT,
United Kingdom

Er...one moment, please.

The gentleman who wrote that, and, might I say, the engineer or

academic who might favor that approach, does not have dirt under the
finger nails. The hard fact is that in Big City; saturated as it is by
truly stupendous microvolt per meter levels from a.m. broadcasting,
television broadcasting and no end of communication services clamoring
to get in; a non linear stage like that ahead of the receiver is a
mixer. That the diodes are not conducting does not mean that there is no
mixing going on.

Yes, those without practical experience of the elevated noise floor,

up and down the h.f. bands, that such an arrangement produces will be a
hard sell but the mixing effect is no less real.

John

at radio station VE7AOV? ?

On 2019-09-20 4:35 a.m., erik@... wrote:

slide 11

Simple and cheap: Back to back diodes
? Protection not dependent upon configuration
? Diode type is not critical (except, don’t use PIN diodes)
? Limited to low input power levels => receive only applications
? +30 dBm = 1 watt max (when using ? watt diodes)
? If either diode fails open => receiver front end not protected
? Spurious signals in receiver can be a problem
? Some mfgs offer choices on spurious levels (DX Engineering RG-5000 series)

--

Here are a few comments about diodes and ESD issues for your consideration.

I agree that limiter diodes will cause issues; however, the devices I suggested above are not typical limiter diodes. Limiter diodes do function as a mixing device, even at amplitudes below their limiting voltage. So, I would not recommend them either.

The device I referenced above (PGB010603) contains back-to-back zener diodes designed for ESD protection. They have a threshold voltage of about 24 volts not 0.7 volts like a typical diode. My understanding is that the diode only appears as a small capacitance at low-amplitude signals (< -9 dBm or about a little more than 0.1 volts pk in 50 ohms) which are normally present at the front-end of the nanovna. The leakage current , even at a 6 volts, is < 1 nanoAmp. These ESD diodes will not protect against high RF levels on the device; however, during an ESD event they will limit the voltage (and energy) during the ESD event. A plot of the resulting voltage spike during an ESD event is shown on page 2 of the data sheet I attached above. What really matters is that the total energy which gets past the diode is quite low by comparison to the full ESD event. This is what helps prevent damage to sensitive RF components.

Yes, it would be better to not have the ESD event in the first place by taking appropriate cautions. However, in some dry parts of the world where relative humidity can be less than 10%, ESD events are more common and difficult to prevent in all situations.

Here are a few suggestions from my experience. A good practice in a lab or garage environment is to use an ESD wrist strap and resistive mat to protect against the human introduced ESD event. Perhaps this approach has the least impact to RF measurements in the lab or office environment. However, when climbing around on a tower or trying to make outdoor measurements such practices are more difficult. I suggest making it a habit to put a temporary short on coaxes prior to connecting them to the nanovna to avoid charge buildup on any antennas being measured.

I hope these suggestions will prevent someone going through my ESD experience of blowing up a switch in a VNA.

I think that calibration after installation of such a device should readily compensate for an extra 0.06 pF of stray capacitance in the circuit. I suggest that manufacturing tolerances alone will have more variation in input capacitance than this. Am I wrong about in these assumptions?

--
Bryan, WA5VAH

On Sat, Sep 21, 2019 at 07:27 PM, <bryburns@...> wrote:

I think that calibration after installation of such a device should readily
compensate for an extra 0.06 pF of stray capacitance in the circuit. I suggest
that manufacturing tolerances alone will have more variation in input
capacitance than this. Am I wrong about in these assumptions?

yes, you're wrong with these assumptions. First, voltage suppressor diodes have about 10-15 pF. It works like low-pass filter with cut-off at about 400 MHz. VNA can compensate it by calibration, but dynamic range will be lost. Because noise floor is here and if you attenuate signal, you lose dynamic range.

The second issue with voltage suppressor diode is that they make non-linear distortions. You cannot calibrate VNA for non-linear distortions. It just ruins RF signal with no way to restore it.

qrp.ddc

Thanks for your reply. I agree it would be very detrimental to the our measurements to put a diode in place that has 10-15 pF of capacitance. I would never recommend that.

Apparently you did not look at the data sheet for the PGB1010603 device I attached above. I did not assume it was 0.06 pf. On page 1 of the data sheet the measured capacitance for the device is 0.06 pf when measured at 250 MHz. At that frequency this corresponds to a reactance of a little over 10,000 ohms. Surely this won't have much impact on measurements in a 50 ohm system at that frequency. Therefore, it appears that this device is substantially different from those with which you are familiar.

I agree we should not use typical voltage suppressor diodes to protect the input to VNA devices. They produce large non-linearities which can really screw up measurements.

Please follow my thinking for a minute. The first page of the data sheet for the device says that the "leakage" current through the device will be less than 1 nano amp at 6 volts. I would think that the current will be even less when we apply ~-10 dBm or 0.1 v pk signals to it. 0.1 v is approximately 35 dB smaller than 6 V. But, let's assume it is still 1 nano amp. This corresponds to a power level of about -100 dBw or -70 dBm peak power. This is approximately 60 dB below the signal being applied to the circuit we are testing. This still seems like a very low power in the diode for creating mixing products. I don't think any mixing products can be larger than this amplitude, in fact, I think they have to be smaller.

Perhaps I did not ask my question above regarding assumptions clearly. I am not an expert on how VNA calibration works. I hope you are. If there is variation in stray capacitance on the VNA input of a fraction of a pF for any reason (manufacturing tolerances, component variations, etc.), will the OSLT calibration we commonly use with devices such as nanovna compensate for it in our measurements after calibration? Is some other action required on my part? I thought this is one of the very purposes of OSLT calibration in all VNAs.

--
Bryan, WA5VAH

If you want to try out the GB1010603 devices, they are available on eBay as item?111468959425. The price for ten shipped in the USA is?$4.85. I am not associated with the seller.?They should be very effective in protecting sensitive RF devices and equipment. These devices are are surface mount, size 603.
Stuart K6YAZLos Angeles, USA