35 MHz to 4 GHz ADF4351 Generator from China

Last updated June 6, 2021

Some time ago I found at Aliexpress.com a cheap module - a generator with ADF4351 synthetiser, claiming frequency range 35 MHz to 4 GHz. In fact the maximum frequency is about 4.4 GHz. I had some experience with this synthetiser type from the past (see Vrsicek2 project), so I bought one, just for fun.



The schema is quite standard - ADF4351 controlled by Atmel AVR AtMega8 MCU, standard HD4470 2 lines x 16 chars LCD. Interesting is the keyboard, realized by voltage divider with resistor values switched by button. The voltage is then processed by 10 bit ADC...



It works as expected - if you have no special requirement to signal purity, accept strong harmonics on frequencies bellow 2.2 GHz (there is a divider at the output) and agree with frequency stability corresponding to standard model of crystal oscillator, then it is fine (esp. for that price).
Here some pictures related to signal quality (spectrum analyzer not calibrated for level). The signal is the best when frequency is multiple of reference oscillator frequency (divider only in integer mode).

144 MHz harmonics


1296 MHz harmonics


2400 MHz harmonics


2400 MHz close spectrum (fractional mode)


4.4 GHz harmonics


4.4 GHz close spectrum (integer mode)


Anyway, I was not satisfied with the firmware - you can enter any frequency on keyboard but it is not always the frequency you get on the output (there is a 8 kHz synthetiser step in highest range), there is no output level control, and what I really hate - the display shows frequency in unit mHz (of course it should be MHz). So I decided to write own firmware...
Original binaries from the MCU are here, you can return to original FW at any time if you like.

Feature removed: frequency scanning
New features: true frequency display, output level control (off and -4 to 5 dBm), last frequency reload from EEPROM after power on, morse code beacon (since version 2), support for 25 MHz (original) and 10 MHz (XO replacement needed) reference frequencies.

Note: this is a very first version (not deeply tested). I would like to test other crystal oscillator type/frequency in order to increase the stability,...).
To change the reference oscillator frequency, beacon text and CW speed, modify the sources and recompile.

Just for fun - here's the downlink signal from QO100 satellite. Uplink is this generator @ 2.4GHz driving WiFi Luxul amplifier (about 0.5W into 80cm dish with helix feed).

The files:
Electrical schematic drawing
OK1DX firmware - sources and binaries, version 2.1 for 25 MHz reference

Enjoy it... 73 Pavel OK1DX