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| Figure 1: The SAA-2N, a variant of the Version 2. As noted in the text I chose this version as it had N connectors and was well suited for my specific needs and has served well for several years. Click on the image for a larger version. |
The original NanoVNA - sometimes found for less than US$50 - works reasonably well up to about 250-ish MHz - the limit of the frequency range of its RF synthesizer (typically an Si5131). Using harmonics for the higher ranges allows the unit to "work" higher than this, but with the dropping gain of the mixers and lower power of the higher-order harmonics diminish its usefulness and it pretty much "runs out of steam" by the time one gets to around 1 GHz.
The "Version 2" of the NanoVNA (which includes many variants, such as the "H2") improved on this using a different detector (often the Analog Devices AD8342 which is rated to work above 3 GHz) and two RF synthesizers - the original Si5351 and another device (usually the Analog Devices ADF4350 or similar) that takes over from the '5351 at the higher frequencies (usually above around 140 MHz) up through 3+ GHz. The different architecture of the Version 2 necessitates greater complexity - which also implies increased vulnerability as we'll see.
Background
When I looked for an "upgrade" to the original NanoVNA I was looking for a device that would have durable connectors that would withstand hundreds of connect/disconnect cycles and be able to hang from the interconnect cables themselves. This "need" was due to its expected use: Being dragged around in the equipment box to repeater and radio sites, the houses of other amateurs and being used on the workbench and almost never above about 1.5 GHz - a frequency range for which I have other gear, anyway. Having friends with other NanoVNA V2 variants, I've heard how physically fragile these devices are - mostly related to the supplied cables and the SMA connectors themselves, and how they are physically attached to the circuit board. All of this ruled out anything with SMA connectors.
For this reason I got the SAA-2N and have used it enough that the nickle plating is starting to wear off the threads of the N connectors, indicative of a number of connect/disconnect cycles that would have trashed even the best-quality SMA connectors: Had anyone else made a device as physically rugged as this, I'd have considered it - but as of the time of this writing, no-one else has done so!
The problem
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| Figure 2: If the RF switch chip is has a "dead" short to ground, you may end up with chaos of lines like this after you do a "short-open-load" recalibration. Click on the image for a larger version. |
Specifically, the problem is the Maxscend MXD8641 RF switch found in several places on the "Version 2" NanoVNA variants (e.g. those that work past 3 GHz). This is a SP4T (4-way) RF switch used to route the signals that, among other things, is used to switch between the Si5351 oscillator and the ADF4350 (or similar) - but one of these devices may be found on the S11 port - and that's where the problem lies. (This device is not on the original NanoVNA, which seems to be more rugged in this respect.) As the list price of the MXD8641 is well under US$0.07 in any sort of production quantity, it's unlikely that so-called "cheap counterfeits" are being used.
While the data sheet for this device indicates that it has good ESD protection, it's clear from the NanoVNA forums - and my personal experience and that of friends who also have a V2 NanoVNA - that this protection is NOT necessarily adequate for in-the-field, casual use. If your NanoVNA never leaves the workbench, you may never have this sort of problem - but if you drag it out in the field like I do, you may well have run across situations where the NanoVNA becomes damaged.
As it happens, a good-quality coaxial cable is capable of storing quite a bit of energy in terms of voltage. With a typical 50 ohms coax having around 30pF per foot (100pF/meter) this, coupled with triboelectric effects - not to mention static on your person or on the antenna to which you might connect it - pose a hazard to sensitive electronics, and this is device is no exception.
On the forums I've seen people commenting on how they are suddenly getting nonsensical readings (see Figure 2) or the apparent inability of the unit to maintain calibration - and this is indicative of possible damage to the MXD8641 found on the S11 port, likely caused by voltage discharge: With a 0.047uF series coupling capacitor, it would not take much accidentally inject enough energy to damage that part!
My own NanoVNA V2 was, itself, damaged through discharge, apparently when connected to a fairly long length of coaxial cable - despite reasonable effort and care. I have also encountered others with V2 NanoVNA variants that have also experienced similar issues and in all of the cases where I have been able to get the unit into my hands, it's been due to damage of the MXD8641.
Diagnosis:
Having corresponded with several others who have had the misfortune of damaging their V2 NanoVNAs, there are several ways that the damage will manifest itself that may appear to be totally different, but are actually the same problem.
- Sudden high return loss everywhere. Sometimes the discharge will cause the '8641 to simply fail dead-shorted and the user will see a very high VSWR on everything and if one configures the unit for an S21 (e.g. "through") reading, the apparent insertion loss is very high. If one attempts to recalibrate (Open/Short/Load) a completely nonsensical display like that in Figure 2 is likely to result after doing so.
- Unit will not seem to "hold" calibration well. This seems to be the more common failure: The user will suddenly find that the unit is out of calibration for no obvious reason - and this is often worse at higher (VHF/UHF) frequencies than at HF (30 MHz or lower). Using the Open/Short/Load, the unit will seem to calibrate, but the noise floor in the "S21" mode will likely be higher - and the calibration will seem to drift all over the place over time and with temperature.
This second failure mode is a bit more sinister in that the user may, at first, presume that the loss of calibration was a result of user error. What has actually happened was that rather than fail shorted, the '8641 will be damaged, often putting between 30 and 80 ohms of resistance between the input port and ground - a value that will vary significantly with temperature and over time. This is insidious in that the unit may "seem" to be working - but it will likely give incorrect readings, even if it seems to calibrate properly. This causes two problems:
- Calibration drift. The NanoVNA's source impedance is nominally 50 ohms - but placing a varying resistance across this - possibly in the 30-80 ohm range - will lower this, causing calibration to seem to vary despite frequent recalibrations. As noted above, if an "S21" measurement is done (e.g. a "through insertion loss"), the shunting of the source signal (on CH0/Port 1) will reduce its amplitude - possibly significantly - and you may notice that the noise floor is higher than expected as the signal levels have been reduced.
- Misleading results. What's worse is that even though the unit will seem to calibrate correctly using the Open/Short/Load, the fact that it is no longer sourcing/loading 50 ohms can cause misleading readings if you are trying to sweep a filter, antenna, cable or other device that is expecting a source impedance in the area of 50 ohms. This may be proven by using a known-good 50 ohm cable that is significantly longer than the one with which you may have calibrated the unit and terminating it with a good 50 ohm load: Ideally, it should remain "flat"(low VSWR/return loss) but if the unit itself no longer sources/loads 50 ohms, this test will reveal something other than a flat response.
Figure 3:
The screwdriver tip is touching the "circuit" side of the
0.047 uF coupling cap. It is between this point and ground
that one would test with a digital Ohmmeter to determine if
the RF switch chip has been damaged.
Click on the image for a larger version.
A quick check with an Ohmmeter will reveal the problem and figure 3 shows where a measurement may be made, on the "circuit" side of the blocking capacitor on the CH0/Port1 terminal. This photo shows the SAA-2N, but your model may have a slightly different layout, but it should be pretty easy to locate this capacitor and do a similar test on other variants.
If all is well, a typical digital Ohmmeter will likely read 10k or higher with the unit powered down - but if you get anything lower - specially if it is in the hundreds of Ohms or lower - the MXD8641 is likely blown. (Do NOT use the "diode test" fucntion. Be sure to check your Ohmmeter in both directions to rule out a reading a protection diode on the chip.)
As mentioned earlier, the MXD8641 is a very inexpensive part - but it's difficult to find from U.S. suppliers - and for this reason I sourced a strip of the MXD8641 on EvilBay with each part costing well under US$1.00, including shipping. Replacing this part, however, is another matter as we'll see shortly.
A promising equivalent part would seem to be the Skyworks
13414-485LF (available from DigiKey) which is also a SP4T switch - in the same package and with
the same pin-out, but despite what look like identical specifications, parameters and
truth tables in the data sheet, the units that I got from DigiKey did not work in the 'V2 for reasons still unknown.
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| Figure 4: Highlighted section shows the installed TVS - the 30 volt 0603-package part mentioned in the text. It is laying on its side, which was the easiest orientation for mounting. Click on the image for a larger version. |
Unfortunately, none of the V2 variants that I've seen have usually included overt protection in addition to that intrinsic to the MXD8641 - and this would likely be in the form of a low-capacitance TVS diode. As it happens, there are a number of TVS diodes that have very low capacitance (0.5pF or less) that are specifically intended for protection of GHz-range devices (HDMI, USB3 and RF/antenna) which make the suitable candidates for use with a NanoVNA.
The device that I chose was the Inpaq EGA10603V12B0DG - a 12/30 volt, 0.2pF part in a 0603 SMD package (DigiKey P/N: 3526-EGA10603V12B0DGCT-ND). There are other devices with low capacitance with even lower voltages (which would have been preferred) but these were available only in 0402 (or smaller!) packages, making their handling very difficult: As it is, a 0603 package is about 1/4th of the size of a grain of rice!
With a fine tip and small-diameter solder, it's possible to add the TVS diode. Figure 4 shows how this might be done on the SAA-2N: Adjacent to the blocking capacitor, the green coating is scraped off the board to bare the ground plane and the TVS diode is installed - first, soldered to the "circuit" (and not the "connector") side of the capacitor and then the other end flowed on the bared ground. I find it a bit easier to set the TVS on its side, but your mileage may vary.
You will note that the TVS is placed on the "circuit" side of the blocking capacitor rather than on the antenna terminals. The reason for this is that if there is a voltage across the input (and thus across the capacitor) that is then shorted, the energy of the 0.047uF capacitor (plus any "ringing" from inductance/resonance) will be dumped into the circuitry. By placing the TVS on the "circuit" side, it will be able to dissipate at least some of the energy dumped by the capacitor, placing an absolute limit on the voltage peak.
How much effect does this TVS have on the readings? I've added one to a perfectly functioning Version 2 NanoVNA and then re-checked the calibration: Below several hundred MHz, there was no obvious effect of the device's small amount of capacitance - and even well into the 2+ GHz range, the effect was very minor.
As I rarely use my NanoVNA above about 1.5
GHz, this effect was acceptable - and the added protection against
damage (that is, finding your NanoVNA non-functional at a radio site - possibly disrupting the planned activities - perhaps resulting in a wasted trip!) is worth it. The 0.2pF capacitance of this device is probably less than stray capacitances in the circuit/layout, anyway.
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| Figure 5: Location of the failed chip (U551) - the first active component in the signal path on CH0/Port 1 of the VNA - and the one most likely to be damaged. Click on the image for a larger version. |
Note: I haven't put similar protection on CH1/Port 2 as it's less-commonly used - and it's typically used in conjunction with a device under test that is less likely to produce ESD. If you are measuring the loss of coaxial cable, however, it would be a good idea to dissipate any stored charge on it (shorting the center to the shield - or even touching across the two with your fingers) prior to connecting it to either terminal of the NanoVNA.
Replacing the MXD8641
If your 'VNA has already been damaged, it may be too late unless you have the equipment to do small surface-mount work. For such work, it's not so much skill that is needed - but the right gear (hot-air rework, ceramic tweezers, etc.) and good magnification is required: None of this is particularly expensive as a suitable hot-air rework station can be had new for well under US$100.
Using the hot air rework wand with a small-diameter blower tip (Figure 6) the chip is carefully heated and using the ceramic tweezers (which are fairly heat-insulative and non-conductive) the old, blown-up chip is lifted off - noting the orientation of the small dot on the original chip - which, on this board, corresponds with the "arrow" symbol seen at about the 7 o'clock position on U551 in Figure 5.
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| Figure 6: Heating the failed chip with a hot-air rework tool. Be sure to note the orientation of the tiny dot on the chip before you remove it! Click on the image for a larger version. |
Letting the unit cool - and verifying that its dot is oriented in the same manner as the original part - the unit may be reassembled and tested: I've had extremely good luck with this method and have never had a failure - and having replaced '8641's in several Version 2 NanoVNAs, I've been 100% successful - but then, I've been doing surface-mount work for quite some time.
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| Figure 7: The repaired NanoVNA, recalibrated. The flatness and level of the noise floor indicate that it's well matched and that the losses are low. Click on the image for a larger version. |
Without careful examination of your Version 2 NanoVNA it may not be possible to determine if it has the added protection of the TVS diode - although this may be noted in its documentation. If it does not, it's worth considering adding it.
Note: As much as I would like to help, I'm not going to get into the business of repairing/modifying NanoVNAs in the manner described, so please don't ask - sorry. If you have a "blowed up" NanoVNA and don't have the gear/skill to do SMD rework, find someone who can. At the very least, you now know why this issue might happen and it may prevent you from damaging other devices.
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This page stolen from ka7oei.blogspot.com
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