NanoVNA-H4 – inductor challenge – part 3

NanoVNA-H4 – inductor challenge – part 2 showed how to approximately undo the transmission line effects of the measurement fixture to improve accuracy of measurement of the coil end to end.

Let’s look at the impedance plot at the coil ends.

So, it is clear we have a device with multiple resonances… a resonator in broad terms and representing it as a fixed inductance in series with some small resistance is quite inadequate for the frequency range above. Continue reading NanoVNA-H4 – inductor challenge – part 3

NanoVNA-H4 – inductor challenge – part 2

Recall the fixture from NanoVNA-H4 – inductor challenge – part 1:

NanoVNA-H4 – inductor challenge – part 1 stated:

In fact, we have the underlying inductor connected by 35mm of 570Ω two wire transmission line, so there is a small amount of impedance transformation (which could be approximately corrected in this case by setting port extension to 20ps… but that is not done for this article).

Let’s explore that using Simsmith. Continue reading NanoVNA-H4 – inductor challenge – part 2

NanoVNA-H4 – inductor challenge -part 1

Let’s explore measurement of a test inductor with the NanoVNA.

Above is the test inductor, enamelled wire on an acrylic tube.

Let’s hook it up to the NanoVNA for an s11 reflection measurement of Z.

Above, one wire is plugged into the centre pin of the top / Port 1 connector. The other wire is clamped to the external male threads of the Port 2 connector using a plastic clothes peg. Note that this VNA is modified, it has the two coax outers bonded together.

In fact, we have the underlying inductor connected by 35mm of 570Ω two wire transmission line, so there is a small amount of impedance transformation (which could be approximately corrected in this case by setting port extension to 20ps… but that is not done for this article).

An ideal inductor of 20µH would have zero resistance and reactance proportional to frequency.

Let’s look at measured impedance from 1-5MHz. My fork of NanoVNA-App will by used.

Above, measured Z of this practical inductor looks a bit like that, very low R and X∝f.

Above, we can plot the equivalent series inductance from the measurement, and it looks like 20µH independent of frequency.

That all looks pretty good… but let’s measure Z of this practical inductor from 1-200MHz.

Above, this is nothing like zero resistance and reactance proportional to frequency.

What is going on?

Continued at NanoVNA-H4 – inductor challenge – part 2

NanoVNA-App – driver for NanoVNA firmware updates

NanoVNA-App contains a facility to upload / download NanoVNA flash memory using the DFU bootloader.

The appropriate Windows driver filename is STTub30.sys (or later?) from ST.

Above is a properties list from USBDView of the correct driver.

If you wish to use it, make sure that you have not replaced the bootloader driver with something else. The driver is packaged with ST’s DfuseDemo. Continue reading NanoVNA-App – driver for NanoVNA firmware updates

InsertionVSWR of Chinese 1:9 balun module #3 – correcting Z using e-delay

The article InsertionVSWR of Chinese 1:9 balun module #2 gave a plot of R,X where the reference plane was at the compensation capacitor on the module. Recall that the transformer was actually 1:4.

Ideally you might OSL calibrate the fixture at that point, or at the transformer terminals to obtain a good picture of the transformer response, but it is not convenient to be desoldering the transformer or cutting tracks, so there is a fairly good alternative at the frequencies being discussed (1-30MHz).

Above is an annotated pic of the test setup. The NanoVNA is calibrated at the Port 1 connector (in fact the outboard end of the SMA savers that are never removed), and the desired reference plane is some 32mm away of transmission line of segments of uncertain Zo and loss.

Firstly, the loss will be very low, so we can simply deal with the time delay that the fixture introduces.

The NanoVNA-H4 with DiSlord v1.1.0 firmware has a facility for port extension (though it is not called that, it is called e-delay), as does the NanoVNA-App software used. In both cases, the value of e-delay converted to phase is subtracted from the s11 measurement at each frequency, and the result is correction for the port extension delay.

Note that this:

  • does not compensate amplitude change, so it is only accurate where loss is very low; and
  • assumes that the port extension has Zo=50+j0Ω though mixed Zo does not cause significant errors if the transmission line sections are very short.

Let’s apply a temporary short circuit the the compensation capacitor at the reference point.

We want X=0, the rising X is due to the transmission line transformation within the test fixture, and that transformation will render any uncompensated impedance values incorrect (though the ReturnLoss and VSWR will be correct).

Lets look at the phase of s11 for the same case, remembering that the phase of the short circuit should be 180°.

s11 lags 180° by 4.6° @ 30MHz, we can calculate the time equivalent: \(t=\frac{\phi}{2 \pi f}=\frac{4.6 \pi}{360 \pi f}=\frac{4.6}{360 f}=426 \text{e-9s}\), so 426ns is a good starting point for e-delay.

Ok, returning to the R,X chart and adjusting e-delay around 426ns for best X plot…

Above, 430ns is pretty acceptable.

Having backed out the delay of the fixture, lets look at R,X.

Above, R,X with and without the e-delay adjustment. The difference is quite small in this case (the lower R and X @ 30MHz are the e-delay corrected values).

Applying the calibration short circuit

In the experiment above, an X-ACTO #16 knife was used to bridge both sides of the capacitor mentioned. The same technique can be used to bond a track to an adjacent ground track (the knife will cut through the solder mask).

A selection of short circuit coax adapters, M & F, can be very handy when making measurements.

For example, to measure an antenna with a UHF female connector, a short cable with adapters SMA(M) for the VNA to UHF(M) for the antenna can be calibrated with a UHF(F) SC (made simply with a panel jack and the back side shorted with several copper conductors soldered as close to the insulator as possible).

Even if the place where you can conveniently apply the reference SC is not exactly where you want it, if the further extension is something you can characterise well (like a measurable length of known 50Ω line with known VF) you can further tweak the e-delay setting.

For example, to measure an antenna with a UHF female connector on a 400mm pigtail of Belden 8267 (RG213) from its actual feed point, a short cable with adapters SMA(M) for the VNA to UHF(M) for the antenna can be calibrated with a UHF(F) SC. Having found the appropriate e-delay to the applied short circuit, calculate the one-way delay of the coax pigtail. Using TLLC we get 2022ps, so the round trip delay is 4044ps. Now add that 4044ps to the e-delay to the applied short to get the appropriate e-delay to the actual feed point.

Summary

The NanoVNA-H4 with DiSlord v1.1.0 firmware has a facility for port extension (though it is not called that, it is called e-delay), as does the NanoVNA-App software used here. In both cases, the value of e-delay converted to phase is subtracted from the s11 measurement at each frequency, and the result is correction for the port extension delay.

Note that this:

  • does not compensate amplitude change, so it is only accurate where loss is very low; and
  • assumes that the port extension has Zo=50+j0Ω though mixed Zo does not cause significant errors if the transmission line sections are low loss, close to Zo=50+j0Ω, and very short.

The technique may provide a means of moving the reference plane with acceptable accuracy.

InsertionVSWR of Chinese 1:9 balun module #2

This article documents an InsertionVSWR test of another cheap Chinese 1-9 balun purchased for less than <$5 on eBay (shipped).

Above is the advertising pic of the 1-9 balun, it would seem to be a clone of the Noelec 1-9 balun. The balun is a compensated voltage balun with the secondary centre tap grounded for these measurements. Continue reading InsertionVSWR of Chinese 1:9 balun module #2

A 1:4 RF transformer for measurements – based on Noelec 1:9 balun assembly

The Noelec 1:9 balun (or perhaps Chinese knock off) is available quite cheaply on eBay (<$5) and provides a good hardware base for a 1:4 version.

Above is a modified device with the original transformer replaced with a Mini-circuits TC4-1TG2+ 1:4 transformer. The replacement is not exactly the same pads, but it is sufficiently compatible to install easily.

The loose parts are OSL calibration parts using 2.54mm pitch header pins with the middle pin removed. The resistor comprises two 100Ω 1% 1210 SMD resistors soldered back to back. (The 1% resistor code 1000 is for 100 * 10^0.)

The most notable departure from ideal of these small transformers is leakage inductance of 30nH give or take. Continue reading A 1:4 RF transformer for measurements – based on Noelec 1:9 balun assembly

NanoVNA-H4 v4.3 – broken SMA connector

It has been my experience over more than 50 years that accurate measurements using SMA connectors requires that they be torqued to a consistent and adequate torque. Specifications for brass threads commonly runs around 0.6Nm, and I have a torque wrench calibrated for that torque which I use whenever the connectors are to be properly tightened.

In my NanoVNA-H v3.3, I reinforced the SMA connectors because of a sense that to tighten them to 0.6Nm (5.3inlb) caused the board to flex and over time might crack the tracks (Strength of reinforcement of nanoVNA-H connectors).

Joe Q Smith on NanoVNA SMA connectors

Joe Q Smith has an interesting video where he tests some Chinese SMA connectors to destruction, worth watching: NanoVNA Torquing SMA Connectors . In his tests, he needs upwards of 2Nm to damage the cheap Chinese end launch PCB connectors, more than three times the torque I use.

The new NanoVNA-H4

About 6 weeks ago, I took delivery of a new NanoVNA-H4, which as far as I can tell, is a ‘genuine’ Hugyen product.

I did not reinforce the connectors on my new NanoVNA-H4 because there was not the needed clearance for a similar adequate brass bar inside the case.

So, having used a torque wrench at 0.6Nm in the few weeks that I have owned the NanoVNA-H4, the connector on Port 1 failed.

Above, the failed connector has rotated, shearing two of the support pins and tearing the other two off the tracks. Continue reading NanoVNA-H4 v4.3 – broken SMA connector

NanoVNA-H4 v4.3 – improving the SD card slot

One of the shortcomings of the NanoVNA-H4 v4.3 is that it is quite easy to drop the SD card inside the case when trying to insert it. Experience is that this is really easy to do in difficult field situations or poor lighting / visibility.

This could have been prevented by better design of the moulded case.

This article describes a simple modification to make it more difficult to miss the card slot.

Above, the modification is a small block of plastic that covers part of the aperture moulded into the case, it projects 1mm into the aperture. Continue reading NanoVNA-H4 v4.3 – improving the SD card slot