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Hi Vojtek,
Would love to see the full document write up for the project. My guess is you are using a resistive absortion bridge for direct/reverse wave sampling and then a small microcontroller but I would love to see more info. TIA! 73, 72 de Juanjo, EC5ACA. EA-QRP #104, G-QRP #9742, QRP-L #1662, FP #899. Juanjo Pastor C/San Roque, 4-1? 46460 Silla SPAIN e-mail: ec5aca@... web: web del club: Tel.: +034 96 120 17 67 M¨®vil: 651 35 35 11 -----Mensaje original----- De: [email protected] [mailto:[email protected]] En nombre de Vojtech Bubnik Enviado el: lunes, 16 de septiembre de 2019 9:26 Para: [email protected] Asunto: Re: [qrp-tech] powering an SWR meter from transceiver output I have a prototype working, see the attached photos showing the infinite SWR with open antenna terminals by lighting up the last LED, and showing a SWR roughly 1:1.1 with a 50 Ohm load. The bridge is powered with around 1.5W input power at 40m. The six orange LEDs are interpolated by PWM, showing smooth transitions between levels. The SWR scale is following: 1st LED - 1:1 2nd LED - 1:1.5 3rd LED - 1:2 4th LED - 1:3 5th LED - 1:5 6th LED - 1:8 and higher therefore the 1st LED being illuminated significantly more than the 2nd LED indicates SWR somwhere between 1:1 and 1:125. On key up, the SWR meter is powered from a large 1000uF capacitor for around 1 second to blink the power detected on the 6 LED scale. I think that is all the functionality I can manage to squeeze into the 1kB of FLASH of this tiny $1 micro. The device works somehow, but it is far from optimal. The micro is powered by a full wave rectifier with an artificial ground in the middle of the resistive bridge. As long as the current drawn is low, such arrangement should not cause significant read out errors or harmonics generated. I plan to do simulations of harmonic content in ltspice. The SWR bridge electronics is grounded to the virtual ground with a low impedance wire, therefore it shall be physically as tiny as possible to produce as little as possible capacitance against the other poles of the bridge, and this capacitance shall be balanced. The other three connections from the electronics to the SWR bridge are high impedance (1kOhm for power, 39kOhm for the forward and reflected power read outs). I do not consider myself an expert in RF electronics, though I have a feeling that such a setup will be less than optimal above 20m due to the low impedance path between the electronics ground and the virtual ground at the bridge, but with today's tiny SMD circuitry the capacity may be lower than expected. There is a lot of work to be done on fine tuning the firmware and possibly the circuit. The firmware should utilize power saving features of the micro to a full extent, it should run the A/D conversion with all the processing disabled to lower the noise, it should use averaging to improve read out SNR, the diode detector read out shall be linearised. Currently the electronics draws around 2.5mA from the transceiver with 1mA flowing through the LEDs. To lower the micro current below the planned 1.5mA, as low as possible Vcc shall be used, likely around 2.2 to 2.5V. The micro would work from 1.8V, but a little bit higher voltag is required to have enough of current regulation on the resistors powering the low voltage red LEDs. The CPU shall be operated at as low as possible CPU frequency. It is an interesting engineering optimization task, it is a lot of fun, and it is manageable due to the simplicity of the circuit and the ATtiny13A micro. As always, I will be thankful for constructive criticism of my endeavors. Vojtech OK1IAK |
