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

NanoVNA-H4 – battery charge from discharged

This article documents the charge cycle of a NanoVNA-H4 from fully discharged to charged.

The DUT is probably a ‘standard’ H4, but with Chinese sourced produce, you never, never know.

The original battery fitted to the NanoVNA-H v4.3 is a 804050 (8.0x40x50mm) 2000mAh LiPo pouch cell (1S) with protection board.

The charger chip is a TP4056, and it would appear to be limited by Rprog to about 0.75A (which includes the current drawn by the working NanoVNA-H4) (though the circuit employed would appear to tweak that limit between VNA on and off conditions with R44). The TP4056 is simply a charger chip, it will not prevent over-discharge of the cell so it is wise to use a cell with protection board (as originally supplied on the DUT).

Above is a plot of the calibrated battery voltage reported by the NanoVNA-H4. Continue reading NanoVNA-H4 – battery charge from discharged

RF Power Meter 2 (RFPM2) – 40dB external attenuator calibration and integration

RF Power Meter 2 is a logging RF power meter based on AD8307 and ESP8266.

This article describes its calibration and use with a 40dB 50Ω 20W attenuator to make a 20W or 43dBm RF power meter.

Above is a pic of the system under test on a nominal 5W transmitter, indicating 37.4dBm, equivalent to 5.5W. Continue reading RF Power Meter 2 (RFPM2) – 40dB external attenuator calibration and integration

(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