Category: Measurement

NanoVNA – measuring Q of an inductor using s21 – fails?

There is a fashion of seeing s21 measurements as the answer to all things, and amongst the revelations is an explanation of measuring inductor Q using s21 shunt through configuration.

Let’s explore the use of s21 shunt through to directly find the half power bandwidth of a series tuned circuit and calculate the Q from that and the resonant frequency (as demonstrated by online posters).

To eliminate most of the uncertainties of measurement, let’s simulate it in Simsmith.

The simulation has a series tuned circuit resonated at 3400kHz, and the source and plot are set to calculate |s21| in dB. Though the model specifies Q independent of frequency, the D block adjusts Q for a constant equivalent series resistance (ESR) which simplifies discussion of resonance and Q. Continue reading NanoVNA – measuring Q of an inductor using s21 – fails?

Last update: 5th May, 2022, 1:00 PM

Sontheimer coupler – transformer issues

It is not uncommon that ham designs for Sontheimer coupers (aka Tandem coupler, Grebenkemper coupler) fall short in the design of the magnetic components resulting in one or both of:

  • high InsertionVSWR; and
  • high core loss.

The above circuit is from (Grebenkemper 1987) and is an embodiment of (Sontheimer 1966). In their various forms, this family of couplers have one or sometimes two transformers with their primary in shunt with the through line. Let’s focus on transformer T2. It samples the though line RF voltage, and its magnetising impedance and transformed load appear in shunt with the through line. T2’s load is usually insignificant, but its magnetising impedance is significant and is often a cause of: Continue reading Sontheimer coupler – transformer issues

Last update: 25th July, 2022, 1:19 PM

NanoVNA – Port 1 port extension

A VNA is usually calibrated by the user at some chosen reference plane using standard parts, commonly an open circuit, short circuit, and nominal (50Ω) load. As a result of this OSL calibration, the VNA is able to correct measured s11 to that reference plane, and display its results wrt that reference plane.

There are occasions where it is not possible, or not convenient to locate the DUT at the reference plane. This article discusses the problem created, and some solutions that might give acceptable accuracy for the application at hand.

The discussion assumes the VNA is calibrated for nominal 50+j0Ω.

Above is a diagram of a configuration where the unknown Zl is not located exactly at the reference plane, but at some extension. Continue reading NanoVNA – Port 1 port extension

Last update: 1st May, 2022, 11:37 AM

NanoVNA – DiSlord NanoVNA-D v1.1.00 & NanoVNA-App-v1.1.209-OD10 calibration

This article explains the interworking of DiSlord NanoVNA-D v1.1.00 firmware and NanoVNA-App-v1.1.209-OD10 with respect to calibration.

This applies to the specific combination of versions of firmware and software client, do not assume it applies to other combinations.

DiSlord NanoVNA-D v1.1.00 firmware supports a scan_bin command where bit 3 of the outmask field is used to request raw measurement data, ie uncorrected measurements.

NanoVNA-App-v1.1.209-OD10 supports exploitation of that capability when it recognises that firmware version and command support.

Above, NanoVNA-App-v1.1.209-OD10  has a dropdown list to choose calibration mode. Continue reading NanoVNA – DiSlord NanoVNA-D v1.1.00 & NanoVNA-App-v1.1.209-OD10 calibration

Last update: 2nd May, 2022, 10:59 AM

NanoVNA – trying the DiSlord built in cable length measurement feature

A recent discussion online on the use of this facility in some model or other NanoVNA/firmware combination quickly ran to over 100 posts, and you might think it is really difficult, or plain does not work.

Let me say I am leery of built in features that invite users to perform something they do not understand, and may misinterpret the outcome.

Lots of the discussion ran to explaining why measurement of a sample of coax  would be out by 5% or more, lots of pseudo tech discussion about age related, contamination related, quality related explanations for the measurement, things which might cause the measurer to condemn the sample, to discard it.

Well, you would want to be pretty confident in yourself to make that call, given that the explanation might well be measurement error.

I don’t use this facility, so I am quite unfamiliar with it, and there is no documentation, so one make make an informed guess as to how to use it.

Let’s measure… Continue reading NanoVNA – trying the DiSlord built in cable length measurement feature

Last update: 26th April, 2022, 10:13 AM

Ferrite cored RF chokes in Class-E RF power amplifiers – core material issues

At Ferrite cored RF chokes in Class-E RF power amplifiers a design was offered for a choke using a Fair-rite 2843000202 core (commonly sold as a BN43-202), and the point was made that some products sold as BN43-202 might be significantly different.

Let’s look at the calibrated model estimates of choke impedance and core loss, side by side. Continue reading Ferrite cored RF chokes in Class-E RF power amplifiers – core material issues

Last update: 19th April, 2022, 1:44 PM

NanoVNA phase confusion

One sees online discussions and videos where phase from a NanoVNA display is central to the subject, and more often than not, the use is quite confused.

Let’s look at some examples.

Example 1

A poster advising on how to measure inductance using a NanoVNA posted a .s1p file of his measurements of a SM inductor of nominally 4.7µH from 1-5MHz and discussed the use of phase in determining the inductance.

Above is a plot of the data in the VNWA PC client. Four values are plotted: Continue reading NanoVNA phase confusion

Last update: 18th April, 2022, 6:43 AM

NanoVNA-H4 – inductor challenge – part 6

This article is part of a series discussing inductors, their characteristics, and measurement, continuing from NanoVNA-H4 – inductor challenge – part 5. The previous articles have discussed these matters in the context of an air cored solenoid, this article moves on to inductors with a magnetic core.

A magnetic core increases the flux Φ due to a current flowing in the inductor, and since \(L \propto \phi\), the magnetic core increases inductance.

Magnetic core materials are not usually linear, they exhibit saturation and hysteresis (which brings core loss), and changing magnetic field induces eddy currents in the material which also brings core loss.

The B-H curve relates flux density to magnetising force, and as mentioned, the underlying material is non-linear and exhibits saturation (where at some point, B increases very little for increased H).

Above is a generic BH curve for magnetic core material. It shows saturation and hysteresis. Note that saturation (Bs) is total saturation of the core, but saturation begins at half that flux density in this case. Continue reading NanoVNA-H4 – inductor challenge – part 6

Last update: 17th April, 2022, 10:50 AM

Ferrite cored RF chokes in Class-E RF power amplifiers

Class-E RF power amplifiers have become quite fashionable in ham radio in the last decade or two.

This article discusses a common issue with the design of the RF choke providing DC to the Class-E stage.

Above is a circuit above is from (Sokal 2001) which explains the amplifier and gives guidance on selection of components. One key recommendation is that the usual choice of XL1 being 30 or more times the unadjusted value of XC1. This spells out that L1’s role is essentially an RF choke, it is intended to pass DC but to largely prevent RF current, it needs a high impedance at RF, and low DC resistance. Continue reading Ferrite cored RF chokes in Class-E RF power amplifiers

Last update: 25th July, 2022, 1:19 PM

Chinese attenuator board review

I purchased a little inexpensive attenuator board on Aliexpress.

At first use, it was clear that the connectors were weak and warranted testing in a controlled way.

Here is the result of trying to tighten them with two torque wrenches:

  • 0.6Nm (5.4 lb-in); and
  • 1.0Nm (8.9 lb-in).

The torque wrenches calibrations were checked before the test.

First pass was with the 0.6Nm, and all but one deformed (twisted on the PCB).

A second pass with the 1.0Nm broke three off the board, and twisted all but one further.

Above, only the front right connector was undamaged. Continue reading Chinese attenuator board review

Last update: 10th April, 2022, 6:02 PM