Here is the introduction of my original GPSDO which is controlled by STM32F411CEU6 and using a Chinese GNSS module ATGM332D(or ATGM336H). Of course works with U-Blox M8N,M6 and MTK MT3333 modules which supports 1PPS output, too.
There are many GPSDO examples on the Internet and also on YouTube. But many bloggers and YouTubers are using surplus equipment such as Thunderbolt or commercial models like a BG7TBL’s GPSDO. And almost other original GPSDOs are FLL type, or PLL type are using 74HC4046 for a phase detector. Of course, I agree almost all of they have enough performances.
However, I just want to make a original designed GPSDO which is more simple and has a well reproducibility. I programmed in C/C++, not use Arduino.
I found STM32F411CEU6 development module on AliExpress which is called “Black Pill”. It works up to 100MHz system clock. It’s able to use an external accurate 10MHz clock as the system clock with internal PLL.
As you know the GPSDO needs VCO( Voltage Controlled Oscillator) which is controlled frequency by a voltage, called EFC(Electronic Frequency Control). Usually, DAC(Digital to Analog Converter) is used to control the frequency. Many MCUs have some embedded DACs or PWM outputs can use as DAC, but their resolutions are up to 12bits. 12bits, it isn’t enough to control the frequency, 1E-12( 0.00001Hz) orders. So I chose 32bits DAC PCM5102A which is designed for audio equipment, fortunately STM32F411CEU6 has some I2S I/F. I’m lucky, I can buy this PCM5102A as a modules and it’s more reasonable price than make it by myself. We can purchase it from AliExpress or Amazon.
The most important parts is VCO… I chose Morion MV89A Double oven controlled X’tal Oscillator, it’s very accurate, high stability and easily purchased in the surplus market.
And also I purchased a rubidium oscillator Symmetricom LPRO(LPRO-101) and made the another one. I bought these on eBay.
Here is the basic block diagram of the GPSDO.
And the basic schematic is here.
I measured Allan deviation by a easy way, this is just checking its performance. I will try to measure more accurate and details…
I wrote more details past of this GPSDO on this blogs. Sorry but their all written in Japanese. If you have a interested in this project. Please try to read using “Google translator” , instead… (^^;/