Category: Amateur Radio

NanoVNA – interpolation – part 2

NanoVNA – interpolation – part 1 introduced the principle on which VNA measurements are made and corrected based on a set of error terms derived from measurement of some known loads at the reference plane.

The technique of interpolation as a convenient means of increasing the utility and flexibility of a calibration data set was also introduced, and example raw (uncorrected) sweeps of an OC at the end of about 5m of RG58A/U were given to illustrate the challenge in interpolation with insufficient samples or control points.

A more common data flow is that shown above, where the correction terms are calculated for each of the frequencies in the calibration sweeps, and then those terms are interpolated to the frequencies actually used for a DUT measurement sweep. Continue reading NanoVNA – interpolation – part 2

Last update: 20th March, 2023, 4:40 AM

NanoVNA – interpolation – part 1

A simple two port VNA allows measurement of  S parameters s11 and s21 of a DUT. Port 1 contains a directional coupler to transmit a signal into the DUT, and to capture and measure the amplitude and phase of the reflected wave relative to the forward wave at Port 1 (s11). Port 2 simply measures the amplitude and phase of the signal at its input, the forward wave after it has passed through the DUT relative to the forward wave at Port 1 (s21).

This is typically done by stepping (sweeping) the source on Port 1 through a range of frequencies, specified for example by start and end frequency and the number of discrete steps.

There are several source of error in such a measurement, but by making a series of measurements of some known configurations (Short, Open and Load on Port 1, Isolation and Through to Port 2), those errors can be determined and corrected out of subsequent measurements. So, there is a calibration process to measure and save measurements on these known loads, and then a correction process to apply the calculated corrections to raw measurements.

Early VNAs invalidated the calibration data if sweep parameters were changed, and so corrections were applied to raw measurements at corrections measured and calculated at exactly the same frequency.

This was really inconvenient, especially where no facility was provided to permanently save and restore a set of calibrations.

Later VNAs included the facility to interpolate (but not extrapolate) the calibration / correction data to a new set of sweep parameters. This was really convenient, but introduced a new source of error, the interpolation error.

When all this is done under the covers, users with little understanding of what is going on under the covers can easily obtain invalid / worthless results.

Let’s focus on s11 measurement, though the same issues exist for s21 measurement.

Above is a plot of uncorrected or raw s11 sweeping a NanoVNA-H4 101 points from 1 to 900MHz with nothing on Port 1 (approximately an open circuit OC). Ideally s11 should be 1+j0, but the directional coupler circuitry and small distance to the connector means the amplitude and phase vary as shown in the plot. Continue reading NanoVNA – interpolation – part 1

Last update: 17th March, 2023, 10:31 AM

Can a diode be used to rectify signals smaller than its ‘threshold’ voltage?

Several articles on this site have used diode half wave detectors down to very low signal levels, well below the commonly perceived ‘threshold’ of the diodes, and it has prompted comments to the effect that this cannot work.

Really simple PN junction diode model

An ideal diode is a device that conducts in one direction with zero voltage drop, and does not conduct in the other direction.

Practical diodes typically have an IV characteristic with a knee at some small forward bias from about 0.2V to 0.6V depending on the nature of the PN junction.

An often used simple model of a practical diode is an ideal diode with a series battery of voltage equal to the offset of that knee, the ‘threshold’ if you like.

This model may be quite adequate when the applied voltage is much larger than the knee voltage, eg if you were rectifying 24V AC.

Practical diodes

Shockley’s diode equation

William Shockley modelled the IV characteristic of a diode as \(I_D=I_S(e^{\frac{V_D q}{n k_B T }}-1)\) where ID is the diode current, IS is the reverse-bias saturation current (or scale current), VD is the voltage across the diode, kB is Boltzman’s constant, T is absolute temperature, q is an elementary charge, and n is the ideality factor, also known as the quality factor or emission coefficient.

\(\frac{k_B T }{q}\) is often known as VT.

Shockley’s equation with n=1 is often known as Shockley’s ideal PN diode.

BAT46

Let’s look at the BAT46 Schottky diode, it has PIV=100V and is very suited to a lot of these higher voltage RF signal projects.

Above is the IV characteristic from a datasheet. They are often not very helpful at really low currents as used in some of these applications, but note the  great temperature sensitivity. Continue reading Can a diode be used to rectify signals smaller than its ‘threshold’ voltage?

Last update: 25th February, 2023, 6:10 PM

nanoVNA-H – can firmware be updated if it has a broken USB socket?

The usual method used for firmware upgrade is DFU (Direct Flash Update) using the USB interface and one of many PC clients to load the firmware.

Before attempting a firmware upgrade, be certain of the hardware you have, and the appropriate / compatible firmware file and format. Look for a label on the back, or on the silkscreen of the PCB (though sometimes hidden under the battery… doh!)… know what hardware you have to ensure you load compatible firmware.

Before discussing how to upgrade firmware if the USB interface is not functional, be sure that this problem is not driver related, that there is a real hardware problem. Continue reading nanoVNA-H – can firmware be updated if it has a broken USB socket?

Last update: 5th February, 2023, 8:23 PM

Motorola R2009D OCXO startup

This article documents the startup characteristic of the OCXO in a Motorola R2009D comms analyser.

Frequency error from cold start was logged using the FA-3-6G counter to make record of the time for reasonable stabilisation from cold. The R2009D had been calibrated to within 100mHz some days before this test.

This test was performed with power applied to the box, but the front panel switch in standby position.

Above is a plot of the startup.

At powerup, the oscillator is 514Hz high (error 5.14e-5), at 3min error is 2.8e-5, at 5min, error is 6.4e-6, at 20min error is -1.2e-7, improvement beyond this is very slow.

The front panel “Oven ready” LED is not a good indicator that the oscillator is very close to frequency, it comes on when OCXO heater current reduces, but the oscillator has not yet stabilised.

Calibration was carried out with the whole analyser powered and operating for 24h, the oscillator will not reach calibration frequency unless the whole box is operating. Experience is that setting the reference oscillator closer than 100mHz is wasted effort.

Last update: 22nd January, 2023, 4:14 PM

BG7TBL FA-3-6G frequency meter – initial impressions

A BG7TBL FA-3-6G  is a 6GHz frequency counter of the reciprocal counter type, well for Ch1, <=300MHz, at least.) This one was purchased on eBay for just under $300 delivered.

Above is the seller’s pic of the FA-3-6G frequency counter.

It was supplied with a non-compliant Chinese plug pack, it lacks rubber feet, and the nuts on all the BNC connectors were loose, not even hand tight. Continue reading BG7TBL FA-3-6G frequency meter – initial impressions

Last update: 29th January, 2023, 9:05 PM

Balanced ATUs – the Holy Grail?

It seems that the Holy Grail of many ham HF enthusiasts is a “true balanced ATU.”

The word “true” in there bodes poorly!

It seems that while there are plenty of online experts who have very strong opinions on common mode current, baluns and ATUs, it is very rare that we see quantitative evidence of their assertions, measurements even.

Less commonly does a “true balanced ATU” description include valid measurement of common mode current as evidence of operation.

A “true balanced ATU” project by LY1O in unusual in that it contains a probe purported to measure and display current balance.

Above is a schematic of LY1O’s measurement system, it has a pair of current transformers each with half wave diode detectors in each leg of ATU output. It is important to note that the detectors convert the RF AC wave into a DC value close to the peak value of the AC wave… so they respond to the magnitude only of the current in each leg. Continue reading Balanced ATUs – the Holy Grail?

Last update: 21st January, 2023, 5:09 PM

Comparing RF Power Amplifier Tube Performance Computer and Calculate initial load line of valve RF amplifier on the AL811H

A correspondent wrote to me about Ameritron AL811H tube selection. The article contained a table of performance figures derived from RF Power Amplifier Tube Performance Computer calibrated for measured supply voltages and power output in the various modes.

Above is the table, Table 1 `from the article. Continue reading Comparing RF Power Amplifier Tube Performance Computer and Calculate initial load line of valve RF amplifier on the AL811H

Last update: 23rd December, 2022, 5:51 AM

Another small efficient matching transformer for an EFHW – 2643251002 – #5 – improved Simsmith model (v1.03)

This article applies the improved model: An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03) to the design at Another small efficient matching transformer for an EFHW – 2643251002 – #1 – design workup.

Above is the prototype transformer wound with 14t of 0.71mm ECW tapped at 2t. The mm rule gives some scale. The turns are close wound, touching on the inner diameter of the core. Continue reading Another small efficient matching transformer for an EFHW – 2643251002 – #5 – improved Simsmith model (v1.03)

Last update: 29th January, 2023, 7:38 PM

A Simsmith model of a Ruthroff 1:4 voltage balun – 2843009902 (BN43-7051)

A correspondent asked whether I had a tool similar to An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03) to assist in the design of a ferrite cored Ruthroff 1:4 balun for HF.

In fact, the problem is the same as the one discussed in the article above, and the model is suited to application to the ferrite cored HF Ruthroff 1:4 balun case.

This analysis applies to a Fair-rite 2843009902 but may not apply to other manufacturer’s BN43-7051.

Above is a screenshot of the model calibrated against measurement. The magenta curve is the prediction and the blue curve is the measurement. Note that very small differences in measured value result in apparently large changes in InsertionVSWR, these two curves reconcile very well, especially considering the tolerances of ferrite material. Continue reading A Simsmith model of a Ruthroff 1:4 voltage balun – 2843009902 (BN43-7051)

Last update: 24th November, 2022, 3:32 PM