Friends of mine where searching the Internet for a Tracking Generator (used i.c.w. a Spectrum Analyzer). They found the website of Yanjun, BG6KHC who has built one (and also a spectrum analyzer based on the design of Scotty Sprowls, see my Spectrum Analyzer project pages). This generator was based on the Analog Devices ADF4350, a Wide band Synthesizer. The frequency range of this chip is from 137 to 4400 MHz! There is also a ADF4351 which has an even greater range: 35 to 4400 MHz. Both chips are pin compatible.
So I decided to use the ADF4351 chip to design a VFO and see how it works. Before I could start I asked Yanjun if he was selling his PCB’s and he did. After a week or two, the boards arrived and I start building the VFO and the PIC (18F452) control PCB to control the ADF4351.
In addition to that: SV1AFN is also selling a nice ADF4351 board.
Because the PIC board of Yanjun was in a experimental stadium, I had to made some modifications to it, e.g. let the PIC working on 3.3V in stead of 5V because the ADF4351 was running at 3.3Volt as well. For that reason I added a 3.3V regulator on the board. Also Yanjun has added the 3.3V regulator on his newly designed board.
I modified the software to get a VFO with a lot of extra (menu controlled) features such as:
– Changing frequency in different stepsizes: 100 MHz (separated encoder) and 10, 1 MHz, 100, 10, 1 KHz and 100Hz
Adding a menu by pressing the menu button on one of the encoders with the following functions:
– Select internal (25 MHz (new!) or 30MHz) or external (10 MHz) reference frequency. When selecting 10 MHz the 25 or 30 MHz oscillator is disabled (depending if an oscillator with Tri-state control is used)
– Calibrating option when using internal 30 or 25 MHz reference x-tal oscillator.
Note: When selecting this option, connect the VFO output to a counter and tune the VFO display to the measured frequency on the counter.
– Changing power output in 3dB steps from -4 to +5 dBm
– Low noise or low spur output (experimental state)
– Fixed frequency option (starting at 500 MHz and a fixed step size of 500 MHz)
After pressing the menu button again you will be returned to the last used frequency and settings.
A special note about the used encoders: There are different types of (cheap) encoders. The C (ground) connection is sometimes in the middle or at the right side of the 3 connections, looking at the backside. When you connect the encoder wrong, it’s not working (well).
Modification of the display lay-out (Februari 2017)
Because I missed the information about the actual output power of the VFO (set with the Menu function), I added the selected output power on the second line of the display. Software version is: Version 2.2.
Software modification: added a 25 MHz reference oscillator selection to the menu (June 2017)
On e-bay you can buy cheap ADF4351 modules but they are using 25 MHz as reference. For that reason I added a selection option for 25 MHz reference in the menu. I also repaired some small display bugs… Software version is: Version 2.3
Below some pictures of the VFO.