In a recent post I had my ADLAM Pluto transmitting using SDRangel on 1296.5MHz listening to it using an Icom R10 receiver. One thing that I did immediately notice was I had to tune the R10 down 15-18kHz to get both systems “on” frequency. One or both of them had to be off frequency, so which one was it ? After a few measurements with a spectrum analyser, I found the ADALM Pluto was the culprit, surprisingly the Icom R10 was within a 100Hz of where it was supposed to be.

So… the ADALM Pluto was at least 16.667kHz low using its internal 40MHz crystal reference, that is not going to cut the mustard on these higher bands.

I had always planned on phase locking the Pluto to an external 10MHz Rubidium reference. The only catch is the external reference input on the Pluto expects a 40MHz clock and the AD9361 transceiver within the Pluto, cannot be configured to use a reference clock much below 25MHz. So there was no way I was going to make this work with a 10MHz reference without adding something in between.

After some further research I decided I needed a 40MHz VCO that I could phase lock to the external 10MHz reference to solve my problem with the Pluto external clock input. A chance discussion at a conference in Tasmania at the end of 2024 lead me to laying my hands on a KK103c1 40MHz VCTXCO PCBA designed and built by David VK5KK.

David’s 40MHz VCTCXO design was published in Dubus Q3/2022. It was designed as a daughter board directly connected to an Arduino Nano, that it conveniently sits on top of. The specs for this board were perfect for what I was planning to do with the Pluto external clock input;

Ref In:     10MHz, -4dBm to +14dBm
Output:     40MHz ~1.0Vp-p (into 200R)
VCO:        Connor Winfield T604
PLL:        Analog Devices ADF4110
Micro:      Arduino Nano ATMEGA328p

Both the VCO and PLL are low phase noise varieties and the power supply decoupling is extensive. I was even given a basic Arduino sketch that I could use to test the board, that is provided I had an Arduino Nano. Oh well Nano ordered and should be here soon.

Certainly once I have this board running, I’ll be doing some phase noise measurements and experiments with my Rubidium 10MHz reference. It will be interesting to see what improvement I get in frequency accuracy, along with comparing the phase noise of the internal Pluto clock and seeing if there are any differences. However before this I’ll need to write some Arduino code to tailor this board for my purposes and perhaps a library to drive the ADF4110. That will of course land somewhere in my GitHub repository when ready.

I’ll give a shout out and my personal thanks to David VK5KK for gifting me one these boards and accelerating my ADLAM Pluto project. I certainly can’t wait to get to testing once the postman delivers a Arduino Nano. I know I’ve got some Nano’s somewhere, but for the life me of I can’t find that box.

More to come.