Someone pointed out this great blog post by Ryan Tolboom, where he digs deep into the “spread” measurement and calculations as done in the WSJTX program. Myself, I had only a hand-waving understanding until I read this:
Thank you Ryan!
I am having some QDX External Reference Boards being built, both the clock-multiplier and the direct interface versions. This should make it reasonably easy for anyone who wants to stabilize the QDX to connect an external 5, 10, or 25 MHz reference clock. I am also having some replacement QDX back-panels made, so we won’t have to drill a new hole in the existing panel.
I will have these back and tested within a week or two. If they work and fit as planned, they will cost under $20 for the adaptor / back panel set. Some assembly required!
If you are interested, contact me at Paul at [my callsign] dot com.
Direct Interface Board
(All 3D images done with KiCad)
This is a presentation that looks at frequency stability — history and practice — including why and how to modify the QDX transceiver to use an external clock reference.
Freq Stability (pdf)