Digital display for half wave detector with cubic spline interpolation – part 1

Digital display for QRP labs 20W dummy load – part 1 and the following articles discussed an approach to compensating the non-linear response of the half wave detector by finding a polynomial curve fit over a desired range. Unfortunately, the range for a good fit can be smaller than one desires.

This article discussed an alternative using cubic spline interpolation and might be applicable to extend the range or for responses that aren’t well approximated by a simple curve fit.

Introduction

Essentially, this technique applies a piecewise polynomial to fit the data points, and a relatively small number of data points may provide a very good approximation.

The graph above shows: Continue reading Digital display for half wave detector with cubic spline interpolation – part 1

Digital display for QRP labs 20W dummy load – part 2

Digital display for QRP labs 20W dummy load – part 1 laid out a initial study into the feasibility of an approach to the project.

A prototype has been built based on an Ardunio Nano (ATmega328P 5V 16MHz). The ‘328P is loaded with a custom build of Optiboot 8 supporting reading and writing EEPROM.

Above, another prototype using a 0.96″ 128×64 OLED display, an end to end test of a BAT46 prototype for function testing using an Arduino Nano and OLED display. This prototype is well within 5% accurate based solely on the LTSPICE model, assuming no error in the voltage divider, tracking well from 1W to 20W. When calibrated for the voltage divider and ADC Vref error, power displayed was within 2% of a proven power meter at several spots from 0.8W to 25W, on a spot check it is within 10% (0.4dB) at 1mW. Continue reading Digital display for QRP labs 20W dummy load – part 2

Digital display for QRP labs 20W dummy load – part 1

The QRP labs 50-ohm 20W QRP HF Dummy Load is an inexpensive kit for a low power dummy load.

The load comprises 20 x 1W resistors, time will tell what its continuous power rating is actually.

This article explores a possible design for a digital display of power using the provided pads for a half wave detector.

Dimensional

Above, the supplied connector fails a gauge test (the female part sticks out 0.4mm+ (0.015+”) too much… I should have gauged it before assembling the thing.

RF performance

Above is a ReturnLoss plot from 1-100MHz, ReturnLoss is good below 60MHz, very good below 30MHz. Continue reading Digital display for QRP labs 20W dummy load – part 1

Digital display for Revex W560 directional wattmeter – part 1

The Revex W560 is a dual range VSWR meter that was also sold under other brand names.

W560-01

The low frequency range is specified as 1.8-160MHz.

This project is for an external digital display to suit the low frequency band of the W560. Whilst this project is for a specific meter, the techniques are applicable more widely. Continue reading Digital display for Revex W560 directional wattmeter – part 1

How important is directional coupler Directivity?

How important is coupler Directivity?

Let’s discuss what the term means, and the uncertainty of measurement of DUT VSWR or ReturnLoss due to coupler Directivity.

Coupler performance parameters

 

Consider the above diagram, when terminated in a matched load, the key performance characteristics are: Continue reading How important is directional coupler Directivity?

AT-100 ATU Sontheimer coupler using a single binocular ferrite core

A reader of my recent article Sontheimer coupler – transformer issues and the follow up articles giving better transformer designs asked whether the coupler use in the popular AT-100 ATU wouldn’t be a good solution.

A quick Google search did not turn up any published design rationale or measurement data for the AT-100 coupler design.

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, and another which is in series with the through line to sense current. To achieve good Directivity, these transformers must be symmetric, nearly ideal, and they must be independent, ie no significant coupling between the transformers by magnetic or electric fields.

The AT-100 uses a Sontheimer coupler, they are very popular with ham users for perceived better performance, notably better Directivity over a wide frequency range. Continue reading AT-100 ATU Sontheimer coupler using a single binocular ferrite core

ISP programming of the (tr)uSDX – more on SPI

ISP programming of the (tr)uSDX (trusdx) showed that filtering on the MOSI pin in that kit distorted the MOSI signal significantly and suggested a workaround (reducing SCK rate) for reliable programming.

Some correspondence prompts a little more information on the nature of the ATmega328P ISCP signals.

The line protocol used is actually SPI, quite a common protocol.

ISP uses SPI MODE 0 (CPOL=0, CPHA=0), shift out on the falling edge of SCK, and capture input on the rising edge.

Let’s look at a three channel capture of SCK, MOSI and MISO of a AVRDUDE / USBasp driving an Arduino Nano.

The capture shows SCK at around 750kHz rate, the default (-B1) rate for AVRDUDE in this setup. Continue reading ISP programming of the (tr)uSDX – more on SPI

(tr)uSDX – review of the directional coupler ADC design

I noted some online discussions where some people had troubles with the displayed forward and reverse RF power  values, and the calculated SWR.

Some of the reports indicate non-zero RF power values displayed when the transmitter is off, symptoms which direct diagnosis in the first instance to review of the ADC input circuit.

This article reviews the hardware design based on documents as published at the date of this article.

ATmega328P datasheet

Let’s start by reviewing some relevant parts of the ATmega328P datasheet.

Above is a simplified schematic of the ADC pin input circuit. Note the current sources IIH and IIL. Continue reading (tr)uSDX – review of the directional coupler ADC design

Sontheimer coupler – transformer issues – an alternative design – FT37-43

Sontheimer coupler – transformer issues discussed problems with the Sontheimer coupler in a recently published QRP transceiver ((tr)uSDX / trusdx), suggesting that the core loss in transformer T2 was excessive.

This article presents an alternative design for the transformer for a coupler for a 5W transmitter.

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 – an alternative design – FT37-43