Based on some already existing designs, I have decided to developed my own antenna analyser to cover the HF, VHF and UHF. So far, I have reach the the HF range working, and still need to obtain the Impedance parameters and cover the VHF and UHF ranges. The current prototype can generate a tone from <1MHz up to 75MHz, far enough to cover the HF amateur band from 160 meters up to 6 meters. Following you have some description of the current prototype with some plots of measurements done on my Rigol Oscilloscope.
The design is based on the DDS generator from Analog Devices AD8951, also you can use the AD8950 with a lower maximum frequency generated. The signal is filtered using one of the application notes from Analog devices, and I use a Wheatstone bridge tuned for 50 Ohms for the impedance measurements. This circuit is connected to a prototype board that I have which includes a PIC32 Microcontroller and a TFT display among other peripherals. I use this board to generate the SPI signals to control the AD8951, and I will used the internal ADC to measure the Wheatstone Bridge voltages. The idea is to perform frequency sweeps and plot the measured SWR and impedance values among other parameters.
The PCB was milled with my CNC machine, here some pictures of the final results.
Measurements: RF Generator
A small code was implemented in the PIC32 to offer the possibility to change the output frequency by a slider in the TFT screen. The prototype board is power by a USB backup battery, so I avoid any type of noise from a PC, just a precaution !
In the development, I have a few issues that were related with the power supply generation. The RF is amplified by an operation amplifier AD8008 from Analog Devices which needs the negative and positive power supply. I have seen other designs that instead of using +-5v, uses +10v and move the RF signal to +5v and in the output place a capacitor to remove the DC and have only the RF signal. In my case, I power the AD8008 with +5v from the usb, and generate -5v using a DC-DC charge pump converter TPS60401. I have to solder a wire directly to the +5v USB power, so I don’t loose any mV and add two TPS circuits to supply enough power. After those changes, everything worked like a charm 😉
The following pictures show some of the measurements performed by my Rigol oscilloscope. For those not familiar with the Rigol Oscilloscopes, in the screen is divided in two, where inn the top part you can see the voltage vs. time signal and the lower part displays the FFT amplitude vs. frequency. Also in the bottom of the screen in yellow, there are some automatic measurements of the Vpp, Vrms, Vavg and Freq. In the top right corner within the signal display, you can see a frequency counter.
At high frequency, approximately above 60 MHz, you can start to see harmonics and the signal peak value decreases too. In general, I am very happy with the performance and I think there will be no problem because the upper frequencies are far from any amateur band in HF, the highest will be 6m at 50MHz.
The next steps are going to be dedicated to obtain the Wheatstone voltages and plot the frequency response in the TFT display, time for a cup of coffe and some typing to do … 😉