For those interested in an open source design embodying these techniques, I have an open source hardware design here: https://github.com/russellmcc/dco with a blog write-up here: https://www.russellmcc.com/posts/2013-12-01-DCO.html .
Also of relevance is an open source hardware design for the later roland alpha juno digital oscillator design: https://github.com/russellmcc/alphaosc https://www.russellmcc.com/posts/2019-06-14-Alpha-DCO.html