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Design details.
20170403

I

This SWRsweeper version 1 is equipped with cheap (compatible)
 5Volt Arduino Pro Micro board
and a
5Volt (20 pins 9851 DDS board

This lttlle box enables SWR metering between abt. 0,8MHz and 89MHz. The used Arduino software is a by me adapted version of DG7EAO software. DC2WK Windows software in the PC/laptop gives nice graphic results. Sadly this program is not always stable.

The DAC output of a AD9851 DDS IC is a current source.

On the used DDS board isthe sinus current output pre-adjusted to 10mA peak by means of R6 (3k9) . I adjusted it to 20mA peak by paralleling R6 by another 3k9 resistor to enhance the RF voltages on the detector diodes. According to the 9851 datasheet, 30mA peak is allowable.

The output voltage from the DAC is generated by the RF current flowing through the total resistive load of the RF bridge. Due to the varying load of the antenna connected, this DAC voltage can vary between 0.66Vp and 2.0Vp during a frequency sweep. The "Ant" voltage can vary between 0Vp and 2.0Vp.

   Detection system.
As "ideal" RF detection diodes do not exist, something else had to be developed for detecting very low level RF voltages. After a lot of experiments, best results were achieved using "input voltage - amplification dependent - buffer stages"
(20x gain @ 0.8mV in) to (1,8x gain @ 168mV in).

The two LM358P buffer stages have variable negative feedback, due to D3 and D4 which should be paired to D1 and D2. The values of loading resistors R7 and R11 were experimentally found. In total it seems as D1 and D2 have a straight characteristic with very low threshold voltage.

REM : In SWRsweeper V2 another, more expensive solution is used with rms-to-dc voltage converters (30dB input range).
A double sided factory made PCBs is available for it.