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Although I have not built such multiplier for low frequency, I found two such topics at h**p://users.otenet.gr/~athsam/frequency_multiplier.htm and on the last page of the paper at h**p://zebu.uoregon.edu/~rayfrey/431/lab5_431.ps ---there you may find some hints on the loop filter and the counter is a 4040 IC.
This same text in pdf format (and not postscript) can be downloaded from h**p://zebu.uoregon.edu/~rayfrey/431/lab5_431.pdf
By combining it with a preamplifier from the first url you may have success.