Handheld N2PK VNA with color TFT display
I have made an attempt to develop integrated graphic interface for N2PK VNA. Both VNA and Graphic TFT display are driven by embedded PIC32 microcontroller.
As you have most possibly noticed, there are no buttons on front panel of the VNA. All settings of the instrument done on touchscreen. This facilitates dramatically mechanical design and allows to get rid of cumbersome hardware interrupts usually associated with keyboards.
Design of the instrument based on N2PK project. Dual detector PCB developed by Ivan, VE3IVM. The instrument measures most common transmission and reflection parameters. The main purpose of the instrument is outdoor measurements of antennas and finding matching networks. It is especially useful when doing antenna fine tuning on the top of the tower since all the settings could be done on touchscreen by one thumb. Though, the instrument may also be controlled in standard way by PC via USB port. The PC software called myVNA, developed by Dave, G8KBB.
When controlled by PC, instrument’s display is disabled and, in order, not to interfere with PC USB interface, microcontroller’s I/Os are set to high impedance mode. The VNA and TFT display are controlled by embedded PIC32MX460F512L microcontroller running at 80MHz. 8 Mbit Serial Flash Memory (M25P80) used to store calibration data and scan frequency range. Calibration of the instrument performed on whole range of frequencies (50kHz-64MHz) with 50 KHz step. Totally there are 1280 calibration points. So, you do not need to re-calibrate it every time you change fequency range.
Calibration algorithm and calculation of reflection parameters based on code developed by Lawrence, VE7IT.
Transmission parameters calculation formulae described by Paul, N2PK in his article. I managed to convert the formulae into C code. Paul also developed standalone VNA with Text display. The VNA also measures Transmission parameters.
In my instrument code for microcontroller written in C under Microchip MPLab -X development environment. C32 compiler was used to crunch complex numbers and to control TFT display. Microchip Graphic Library Application Notes were used to create graphic interface.
TFT Display, component side
TFT Display, front side Schematics of the instrument, prepared by Ivan, and pictures of the board with microcontroller’s connection points presented here. Below are some screenshots:
Menu of the instrument, LC trend
Gain of 5-el crystal filter:
SWR of the Antenna
HP 8753C vs N2PK TV RFI filer measurements During transmission measurement HP 8753C VNA displayed much higher noise level compare to N2PK VNA. The VNA is energized by 3×18650 Li-ion batteries which last for few hours. Field test of the instrument was made in our VE3YAA club when fine-tuning of 80M vertical. The picture below captured critical moment of erecting the antenna. The dog coordinated our activities.
Surprisingly, the instrument demonstrated very high level of immunity against RFI produced by local broadcasting stations running big KW located in vicinity, unlike some other of my tools based on AD8302 detector: On the picture below you can see an SWR graph of the antenna after some fine turning. We mostly work CW, so the antenna was tuned to have minimum SWR in CW part of the 80M band. We put a lot of efforts keeping our antennas in a good shape and the VNA helped us a lot.
It seems to me, this is most accurate handheld instrument built in amateur society.
Evgeny VE3SSR, member of VE3YAA contest club (My Audi proudly bears my call sign) Evgeny@lecont.net