vna usb

  
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A N2PK VNA that it would be useful for antenna measurements and wanted to run it with a laptop PC. As most newer laptops do not have a parallel port on them I decided to explore running it off the USB port. USB has its attractions when it works properly and the idea of making the VNA plug and play appealed to me. My initial thoughts were to try using a standard
vna usb - schematic

USB to parallel port cable. The problem I found with one of these devices was that WinXP insisted on identifying it as a printer and set it up as a USB printer. No doubt there is a way round this if you know more about the operation of the USB port than I do. Having found that this approach was not going to work easily I bought one of the Elrasoft Cypress EZUSB modules described on Dave Robert`s, G8KBB, website. I followed the instructions to program the chip with the correct PID and VID for the VNA application having downloaded the EZUSB control panel from Cypress`s website. All that remained to do was to let the PC detect the USB board and wait for it to ask for the drivers etc for the VNA which I had already downloaded from Dave`s website. Once the installer program was pointed at the folders with the *. inf files all went smoothly. Having made the VNA communicate over the USB bus my thoughts turned to powering it from the bus. A USB port can provide 5 V at around 500 mA. A quick check with a multi-meter showed that the VNA only took about 200mA from its 5 V supply. The VNA also requires a nominal +12 V supply which is used to power a 7661 voltage inverter chip. This is used to generate a low powered -5 V supply on the VNA PCB. The 12 V is clearly not very critical and anywhere above about 7-8 volts seemed adequate. The problem was to find a way of producing 9-10 volts from the 5 V USB supply. The solution was to use...



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