Month: June 2022

Digital display for half wave detector with cubic spline interpolation – part 1 laid out the design concepts of a power meter display.

Whilst the preferred target was an Arduino Zero (SAMD21G) for its 32bit architecture, speed, and 12bit ADC, the code was developed to run on a Zero or a Arduino Nano (ATmega328P). Initially, my preferred approach of storing device calibration parameters in EEPROM was shelved because the SAMD21G does not have EEPROM, and it’s NVM alternative is not nearly as convenient.

Notwithstanding that, EEPROM support has been plumbed in and tested. For devices supporting Arduino EEPROM library, calibration coefficients can be supplied in EEPROM, or inline in source code.

The interpolation table is calculated separately in Excel (using a custom VBA function library), but could be done in any suitable tool.

Above is a screen shot  extract of the spreadsheet, the column on the right is C array initialisation code for pasting into the project source code. The same values are loaded into the EEPROM data structure if used. Continue reading Digital display for half wave detector with cubic spline interpolation – part 2

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 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

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